Commit 1b000b90c9

mlugg <mlugg@mlugg.co.uk>
2024-08-30 21:29:27
Air: direct representation of ranges in switch cases
This commit modifies the representation of the AIR `switch_br` instruction to represent ranges in cases. Previously, Sema emitted different AIR in the case of a range, where the `else` branch of the `switch_br` contained a simple `cond_br` for each such case which did a simple range check (`x > a and x < b`). Not only does this add complexity to Sema, which we would like to minimize, but it also gets in the way of the implementation of #8220. That proposal turns certain `switch` statements into a looping construct, and for optimization purposes, we want to lower this to AIR fairly directly (i.e. without involving a `loop` instruction). That means we would ideally like a single instruction to represent the entire `switch` statement, so that we can dispatch back to it with a different operand as in #8220. This is not really possible to do correctly under the status quo system. This commit implements lowering of this new `switch_br` usage in the LLVM and C backends. The C backend just turns any case containing ranges entirely into conditionals, as before. The LLVM backend is a little smarter, and puts scalar items into the `switch` instruction, only using conditionals for the range cases (which direct to the same bb). All remaining self-hosted backends are temporarily regressed in the presence of switch range cases. This functionality will be restored for at least the x86_64 backend before merge.
1 parent 49ad51b
src/Air/types_resolved.zig
@@ -386,6 +386,10 @@ fn checkBody(air: Air, body: []const Air.Inst.Index, zcu: *Zcu) bool {
                 var it = switch_br.iterateCases();
                 while (it.next()) |case| {
                     for (case.items) |item| if (!checkRef(item, zcu)) return false;
+                    for (case.ranges) |range| {
+                        if (!checkRef(range[0], zcu)) return false;
+                        if (!checkRef(range[1], zcu)) return false;
+                    }
                     if (!checkBody(air, case.body, zcu)) return false;
                 }
                 if (!checkBody(air, it.elseBody(), zcu)) return false;
src/arch/aarch64/CodeGen.zig
@@ -5105,6 +5105,8 @@ fn airSwitch(self: *Self, inst: Air.Inst.Index) !void {
 
     var it = switch_br.iterateCases();
     while (it.next()) |case| {
+        if (case.ranges.len > 0) return self.fail("TODO: switch with ranges", .{});
+
         // For every item, we compare it to condition and branch into
         // the prong if they are equal. After we compared to all
         // items, we branch into the next prong (or if no other prongs
src/arch/arm/CodeGen.zig
@@ -5053,6 +5053,7 @@ fn airSwitch(self: *Self, inst: Air.Inst.Index) !void {
 
     var it = switch_br.iterateCases();
     while (it.next()) |case| {
+        if (case.ranges.len > 0) return self.fail("TODO: switch with ranges", .{});
         // For every item, we compare it to condition and branch into
         // the prong if they are equal. After we compared to all
         // items, we branch into the next prong (or if no other prongs
src/arch/riscv64/CodeGen.zig
@@ -5681,6 +5681,8 @@ fn airSwitchBr(func: *Func, inst: Air.Inst.Index) !void {
 
     var it = switch_br.iterateCases();
     while (it.next()) |case| {
+        if (case.ranges.len > 0) return func.fail("TODO: switch with ranges", .{});
+
         var relocs = try func.gpa.alloc(Mir.Inst.Index, case.items.len);
         defer func.gpa.free(relocs);
 
src/arch/wasm/CodeGen.zig
@@ -4064,6 +4064,8 @@ fn airSwitchBr(func: *CodeGen, inst: Air.Inst.Index) InnerError!void {
 
     var it = switch_br.iterateCases();
     while (it.next()) |case| {
+        if (case.ranges.len > 0) return func.fail("TODO: switch with ranges", .{});
+
         const values = try func.gpa.alloc(CaseValue, case.items.len);
         errdefer func.gpa.free(values);
 
src/arch/x86_64/CodeGen.zig
@@ -13695,6 +13695,8 @@ fn airSwitchBr(self: *Self, inst: Air.Inst.Index) !void {
 
     var it = switch_br.iterateCases();
     while (it.next()) |case| {
+        if (case.ranges.len > 0) return self.fail("TODO: switch with ranges", .{});
+
         var relocs = try self.gpa.alloc(Mir.Inst.Index, case.items.len);
         defer self.gpa.free(relocs);
 
src/codegen/c.zig
@@ -5017,12 +5017,13 @@ fn airSwitchBr(f: *Function, inst: Air.Inst.Index) !CValue {
     const liveness = try f.liveness.getSwitchBr(gpa, inst, switch_br.cases_len + 1);
     defer gpa.free(liveness.deaths);
 
-    // On the final iteration we do not need to fix any state. This is because, like in the `else`
-    // branch of a `cond_br`, our parent has to do it for this entire body anyway.
-    const last_case_i = switch_br.cases_len - @intFromBool(switch_br.else_body_len == 0);
-
+    var any_range_cases = false;
     var it = switch_br.iterateCases();
     while (it.next()) |case| {
+        if (case.ranges.len > 0) {
+            any_range_cases = true;
+            continue;
+        }
         for (case.items) |item| {
             try f.object.indent_writer.insertNewline();
             try writer.writeAll("case ");
@@ -5041,29 +5042,56 @@ fn airSwitchBr(f: *Function, inst: Air.Inst.Index) !CValue {
         }
         try writer.writeByte(' ');
 
-        if (case.idx != last_case_i) {
-            try genBodyResolveState(f, inst, liveness.deaths[case.idx], case.body, false);
-        } else {
-            for (liveness.deaths[case.idx]) |death| {
-                try die(f, inst, death.toRef());
-            }
-            try genBody(f, case.body);
-        }
+        try genBodyResolveState(f, inst, liveness.deaths[case.idx], case.body, false);
 
         // The case body must be noreturn so we don't need to insert a break.
     }
 
     const else_body = it.elseBody();
     try f.object.indent_writer.insertNewline();
+
+    try writer.writeAll("default: ");
+    if (any_range_cases) {
+        // We will iterate the cases again to handle those with ranges, and generate
+        // code using conditions rather than switch cases for such cases.
+        it = switch_br.iterateCases();
+        while (it.next()) |case| {
+            if (case.ranges.len == 0) continue; // handled above
+
+            try writer.writeAll("if (");
+            for (case.items, 0..) |item, item_i| {
+                if (item_i != 0) try writer.writeAll(" || ");
+                try f.writeCValue(writer, condition, .Other);
+                try writer.writeAll(" == ");
+                try f.object.dg.renderValue(writer, (try f.air.value(item, pt)).?, .Other);
+            }
+            for (case.ranges, 0..) |range, range_i| {
+                if (case.items.len != 0 or range_i != 0) try writer.writeAll(" || ");
+                // "(x >= lower && x <= upper)"
+                try writer.writeByte('(');
+                try f.writeCValue(writer, condition, .Other);
+                try writer.writeAll(" >= ");
+                try f.object.dg.renderValue(writer, (try f.air.value(range[0], pt)).?, .Other);
+                try writer.writeAll(" && ");
+                try f.writeCValue(writer, condition, .Other);
+                try writer.writeAll(" <= ");
+                try f.object.dg.renderValue(writer, (try f.air.value(range[1], pt)).?, .Other);
+                try writer.writeByte(')');
+            }
+            try writer.writeAll(") ");
+            try genBodyResolveState(f, inst, liveness.deaths[case.idx], case.body, false);
+        }
+    }
+
     if (else_body.len > 0) {
-        // Note that this must be the last case (i.e. the `last_case_i` case was not hit above)
+        // Note that this must be the last case, so we do not need to use `genBodyResolveState` since
+        // the parent block will do it (because the case body is noreturn).
         for (liveness.deaths[liveness.deaths.len - 1]) |death| {
             try die(f, inst, death.toRef());
         }
-        try writer.writeAll("default: ");
         try genBody(f, else_body);
     } else {
-        try writer.writeAll("default: zig_unreachable();");
+        try writer.writeAll("zig_unreachable();");
     }
     try f.object.indent_writer.insertNewline();
 
src/codegen/llvm.zig
@@ -6230,7 +6230,15 @@ pub const FuncGen = struct {
 
         const cond = try self.resolveInst(switch_br.operand);
 
-        const else_block = try self.wip.block(1, "Default");
+        // This is not necessarily the actual `else` prong; it first contains conditionals
+        // for any range cases. It's just the `else` of the LLVM switch.
+        const llvm_else_block = try self.wip.block(1, "Default");
+
+        const case_blocks = try self.gpa.alloc(Builder.Function.Block.Index, switch_br.cases_len);
+        defer self.gpa.free(case_blocks);
+        // We set incoming as 0 for now, and increment it as we construct the switch.
+        for (case_blocks) |*b| b.* = try self.wip.block(0, "Case");
+
         const llvm_usize = try o.lowerType(Type.usize);
         const cond_int = if (cond.typeOfWip(&self.wip).isPointer(&o.builder))
             try self.wip.cast(.ptrtoint, cond, llvm_usize, "")
@@ -6294,12 +6302,17 @@ pub const FuncGen = struct {
             break :weights @enumFromInt(@intFromEnum(tuple));
         };
 
-        var wip_switch = try self.wip.@"switch"(cond_int, else_block, llvm_cases_len, weights);
+        var wip_switch = try self.wip.@"switch"(cond_int, llvm_else_block, llvm_cases_len, weights);
         defer wip_switch.finish(&self.wip);
 
         var it = switch_br.iterateCases();
+        var any_ranges = false;
         while (it.next()) |case| {
-            const case_block = try self.wip.block(@intCast(case.items.len), "Case");
+            if (case.ranges.len > 0) any_ranges = true;
+            const case_block = case_blocks[case.idx];
+            case_block.ptr(&self.wip).incoming += @intCast(case.items.len);
+            // Handle scalar items, and generate the block.
+            // We'll generate conditionals for the ranges later on.
             for (case.items) |item| {
                 const llvm_item = (try self.resolveInst(item)).toConst().?;
                 const llvm_int_item = if (llvm_item.typeOf(&o.builder).isPointer(&o.builder))
@@ -6314,7 +6327,42 @@ pub const FuncGen = struct {
         }
 
         const else_body = it.elseBody();
-        self.wip.cursor = .{ .block = else_block };
+        self.wip.cursor = .{ .block = llvm_else_block };
+        if (any_ranges) {
+            const cond_ty = self.typeOf(switch_br.operand);
+            // Add conditionals for the ranges, directing to the relevant bb.
+            // We don't need to consider `cold` branch hints since that information is stored
+            // in the target bb body, but we do care about likely/unlikely/unpredictable.
+            it = switch_br.iterateCases();
+            while (it.next()) |case| {
+                if (case.ranges.len == 0) continue;
+                const case_block = case_blocks[case.idx];
+                const hint = switch_br.getHint(case.idx);
+                case_block.ptr(&self.wip).incoming += 1;
+                const next_else_block = try self.wip.block(1, "Default");
+                var range_cond: ?Builder.Value = null;
+                for (case.ranges) |range| {
+                    const llvm_min = try self.resolveInst(range[0]);
+                    const llvm_max = try self.resolveInst(range[1]);
+                    const cond_part = try self.wip.bin(
+                        .@"and",
+                        try self.cmp(.normal, .gte, cond_ty, cond, llvm_min),
+                        try self.cmp(.normal, .lte, cond_ty, cond, llvm_max),
+                        "",
+                    );
+                    if (range_cond) |prev| {
+                        range_cond = try self.wip.bin(.@"or", prev, cond_part, "");
+                    } else range_cond = cond_part;
+                }
+                _ = try self.wip.brCond(range_cond.?, case_block, next_else_block, switch (hint) {
+                    .none, .cold => .none,
+                    .unpredictable => .unpredictable,
+                    .likely => .then_likely,
+                    .unlikely => .else_likely,
+                });
+                self.wip.cursor = .{ .block = next_else_block };
+            }
+        }
         if (switch_br.getElseHint() == .cold) _ = try self.wip.callIntrinsicAssumeCold();
         if (else_body.len != 0) {
             try self.genBodyDebugScope(null, else_body, .poi);
src/codegen/spirv.zig
@@ -6211,6 +6211,7 @@ const NavGen = struct {
             var num_conditions: u32 = 0;
             var it = switch_br.iterateCases();
             while (it.next()) |case| {
+                if (case.ranges.len > 0) return self.todo("switch with ranges", .{});
                 num_conditions += @intCast(case.items.len);
             }
             break :blk num_conditions;
src/Air.zig
@@ -1143,10 +1143,12 @@ pub const SwitchBr = struct {
     else_body_len: u32,
 
     /// Trailing:
-    /// * item: Inst.Ref // for each `items_len`.
-    /// * instruction index for each `body_len`.
+    /// * item: Inst.Ref // for each `items_len`
+    /// * { range_start: Inst.Ref, range_end: Inst.Ref } // for each `ranges_len`
+    /// * body_inst: Inst.Index // for each `body_len`
     pub const Case = struct {
         items_len: u32,
+        ranges_len: u32,
         body_len: u32,
     };
 };
@@ -1862,6 +1864,10 @@ pub const UnwrappedSwitch = struct {
             var extra_index = extra.end;
             const items: []const Inst.Ref = @ptrCast(it.air.extra[extra_index..][0..extra.data.items_len]);
             extra_index += items.len;
+            // TODO: ptrcast from []const Inst.Ref to []const [2]Inst.Ref when supported
+            const ranges_ptr: [*]const [2]Inst.Ref = @ptrCast(it.air.extra[extra_index..]);
+            const ranges: []const [2]Inst.Ref = ranges_ptr[0..extra.data.ranges_len];
+            extra_index += ranges.len * 2;
             const body: []const Inst.Index = @ptrCast(it.air.extra[extra_index..][0..extra.data.body_len]);
             extra_index += body.len;
             it.extra_index = @intCast(extra_index);
@@ -1869,6 +1875,7 @@ pub const UnwrappedSwitch = struct {
             return .{
                 .idx = idx,
                 .items = items,
+                .ranges = ranges,
                 .body = body,
             };
         }
@@ -1881,6 +1888,7 @@ pub const UnwrappedSwitch = struct {
         pub const Case = struct {
             idx: u32,
             items: []const Inst.Ref,
+            ranges: []const [2]Inst.Ref,
             body: []const Inst.Index,
         };
     };
src/print_air.zig
@@ -864,6 +864,12 @@ const Writer = struct {
                 if (item_i != 0) try s.writeAll(", ");
                 try w.writeInstRef(s, item, false);
             }
+            for (case.ranges, 0..) |range, range_i| {
+                if (range_i != 0 or case.items.len != 0) try s.writeAll(", ");
+                try w.writeInstRef(s, range[0], false);
+                try s.writeAll("...");
+                try w.writeInstRef(s, range[1], false);
+            }
             try s.writeAll("] ");
             const hint = switch_br.getHint(case.idx);
             if (hint != .none) {
src/Sema.zig
@@ -11353,9 +11353,14 @@ const SwitchProngAnalysis = struct {
                         const coerced = try sema.coerce(&coerce_block, capture_ty, uncoerced, case_src);
                         _ = try coerce_block.addBr(capture_block_inst, coerced);
 
-                        try cases_extra.ensureUnusedCapacity(3 + coerce_block.instructions.items.len);
-                        cases_extra.appendAssumeCapacity(1); // items_len
-                        cases_extra.appendAssumeCapacity(@intCast(coerce_block.instructions.items.len)); // body_len
+                        try cases_extra.ensureUnusedCapacity(@typeInfo(Air.SwitchBr.Case).@"struct".fields.len +
+                            1 + // `item`, no ranges
+                            coerce_block.instructions.items.len);
+                        cases_extra.appendSliceAssumeCapacity(&payloadToExtraItems(Air.SwitchBr.Case{
+                            .items_len = 1,
+                            .ranges_len = 0,
+                            .body_len = @intCast(coerce_block.instructions.items.len),
+                        }));
                         cases_extra.appendAssumeCapacity(@intFromEnum(case_vals[idx])); // item
                         cases_extra.appendSliceAssumeCapacity(@ptrCast(coerce_block.instructions.items)); // body
                     }
@@ -12578,21 +12583,18 @@ fn analyzeSwitchRuntimeBlock(
         };
 
         try branch_hints.append(gpa, prong_hint);
-        try cases_extra.ensureUnusedCapacity(gpa, 3 + case_block.instructions.items.len);
-        cases_extra.appendAssumeCapacity(1); // items_len
-        cases_extra.appendAssumeCapacity(@intCast(case_block.instructions.items.len));
+        try cases_extra.ensureUnusedCapacity(gpa, @typeInfo(Air.SwitchBr.Case).@"struct".fields.len +
+            1 + // `item`, no ranges
+            case_block.instructions.items.len);
+        cases_extra.appendSliceAssumeCapacity(&payloadToExtraItems(Air.SwitchBr.Case{
+            .items_len = 1,
+            .ranges_len = 0,
+            .body_len = @intCast(case_block.instructions.items.len),
+        }));
         cases_extra.appendAssumeCapacity(@intFromEnum(item));
         cases_extra.appendSliceAssumeCapacity(@ptrCast(case_block.instructions.items));
     }
 
-    var is_first = true;
-    var prev_cond_br: Air.Inst.Index = undefined;
-    var prev_hint: std.builtin.BranchHint = undefined;
-    var first_else_body: []const Air.Inst.Index = &.{};
-    defer gpa.free(first_else_body);
-    var prev_then_body: []const Air.Inst.Index = &.{};
-    defer gpa.free(prev_then_body);
-
     var cases_len = scalar_cases_len;
     var case_val_idx: usize = scalar_cases_len;
     var multi_i: u32 = 0;
@@ -12602,31 +12604,27 @@ fn analyzeSwitchRuntimeBlock(
         const ranges_len = sema.code.extra[extra_index];
         extra_index += 1;
         const info: Zir.Inst.SwitchBlock.ProngInfo = @bitCast(sema.code.extra[extra_index]);
-        extra_index += 1 + items_len;
+        extra_index += 1 + items_len + 2 * ranges_len;
 
         const items = case_vals.items[case_val_idx..][0..items_len];
         case_val_idx += items_len;
+        // TODO: @ptrCast slice once Sema supports it
+        const ranges: []const [2]Air.Inst.Ref = @as([*]const [2]Air.Inst.Ref, @ptrCast(case_vals.items[case_val_idx..]))[0..ranges_len];
+        case_val_idx += ranges_len * 2;
+
+        const body = sema.code.bodySlice(extra_index, info.body_len);
+        extra_index += info.body_len;
 
         case_block.instructions.shrinkRetainingCapacity(0);
         case_block.error_return_trace_index = child_block.error_return_trace_index;
 
         // Generate all possible cases as scalar prongs.
         if (info.is_inline) {
-            const body_start = extra_index + 2 * ranges_len;
-            const body = sema.code.bodySlice(body_start, info.body_len);
             var emit_bb = false;
 
-            var range_i: u32 = 0;
-            while (range_i < ranges_len) : (range_i += 1) {
-                const range_items = case_vals.items[case_val_idx..][0..2];
-                extra_index += 2;
-                case_val_idx += 2;
-
-                const item_first_ref = range_items[0];
-                const item_last_ref = range_items[1];
-
-                var item = sema.resolveConstDefinedValue(block, LazySrcLoc.unneeded, item_first_ref, undefined) catch unreachable;
-                const item_last = sema.resolveConstDefinedValue(block, LazySrcLoc.unneeded, item_last_ref, undefined) catch unreachable;
+            for (ranges, 0..) |range_items, range_i| {
+                var item = sema.resolveConstDefinedValue(block, LazySrcLoc.unneeded, range_items[0], undefined) catch unreachable;
+                const item_last = sema.resolveConstDefinedValue(block, LazySrcLoc.unneeded, range_items[1], undefined) catch unreachable;
 
                 while (item.compareScalar(.lte, item_last, operand_ty, zcu)) : ({
                     // Previous validation has resolved any possible lazy values.
@@ -12664,9 +12662,14 @@ fn analyzeSwitchRuntimeBlock(
                     );
                     try branch_hints.append(gpa, prong_hint);
 
-                    try cases_extra.ensureUnusedCapacity(gpa, 3 + case_block.instructions.items.len);
-                    cases_extra.appendAssumeCapacity(1); // items_len
-                    cases_extra.appendAssumeCapacity(@intCast(case_block.instructions.items.len));
+                    try cases_extra.ensureUnusedCapacity(gpa, @typeInfo(Air.SwitchBr.Case).@"struct".fields.len +
+                        1 + // `item`, no ranges
+                        case_block.instructions.items.len);
+                    cases_extra.appendSliceAssumeCapacity(&payloadToExtraItems(Air.SwitchBr.Case{
+                        .items_len = 1,
+                        .ranges_len = 0,
+                        .body_len = @intCast(case_block.instructions.items.len),
+                    }));
                     cases_extra.appendAssumeCapacity(@intFromEnum(item_ref));
                     cases_extra.appendSliceAssumeCapacity(@ptrCast(case_block.instructions.items));
 
@@ -12713,134 +12716,39 @@ fn analyzeSwitchRuntimeBlock(
                 };
                 try branch_hints.append(gpa, prong_hint);
 
-                try cases_extra.ensureUnusedCapacity(gpa, 3 + case_block.instructions.items.len);
-                cases_extra.appendAssumeCapacity(1); // items_len
-                cases_extra.appendAssumeCapacity(@intCast(case_block.instructions.items.len));
+                try cases_extra.ensureUnusedCapacity(gpa, @typeInfo(Air.SwitchBr.Case).@"struct".fields.len +
+                    1 + // `item`, no ranges
+                    case_block.instructions.items.len);
+                cases_extra.appendSliceAssumeCapacity(&payloadToExtraItems(Air.SwitchBr.Case{
+                    .items_len = 1,
+                    .ranges_len = 0,
+                    .body_len = @intCast(case_block.instructions.items.len),
+                }));
                 cases_extra.appendAssumeCapacity(@intFromEnum(item));
                 cases_extra.appendSliceAssumeCapacity(@ptrCast(case_block.instructions.items));
             }
 
-            extra_index += info.body_len;
             continue;
         }
 
-        var any_ok: Air.Inst.Ref = .none;
-
-        // If there are any ranges, we have to put all the items into the
-        // else prong. Otherwise, we can take advantage of multiple items
-        // mapping to the same body.
-        if (ranges_len == 0) {
-            cases_len += 1;
-
-            const analyze_body = if (union_originally)
-                for (items) |item| {
-                    const item_val = sema.resolveConstDefinedValue(block, LazySrcLoc.unneeded, item, undefined) catch unreachable;
-                    const field_ty = maybe_union_ty.unionFieldType(item_val, zcu).?;
-                    if (field_ty.zigTypeTag(zcu) != .noreturn) break true;
-                } else false
-            else
-                true;
+        cases_len += 1;
 
-            const body = sema.code.bodySlice(extra_index, info.body_len);
-            extra_index += info.body_len;
-            const prong_hint: std.builtin.BranchHint = if (err_set and
-                try sema.maybeErrorUnwrap(&case_block, body, operand, operand_src, allow_err_code_unwrap))
-            h: {
-                // nothing to do here. weight against error branch
-                break :h .unlikely;
-            } else if (analyze_body) h: {
-                break :h try spa.analyzeProngRuntime(
-                    &case_block,
-                    .normal,
-                    body,
-                    info.capture,
-                    child_block.src(.{ .switch_capture = .{
-                        .switch_node_offset = switch_node_offset,
-                        .case_idx = .{ .kind = .multi, .index = @intCast(multi_i) },
-                    } }),
-                    items,
-                    .none,
-                    false,
-                );
-            } else h: {
-                _ = try case_block.addNoOp(.unreach);
-                break :h .none;
-            };
-
-            try branch_hints.append(gpa, prong_hint);
-            try cases_extra.ensureUnusedCapacity(gpa, 2 + items.len +
-                case_block.instructions.items.len);
-
-            cases_extra.appendAssumeCapacity(@intCast(items.len));
-            cases_extra.appendAssumeCapacity(@intCast(case_block.instructions.items.len));
-
-            for (items) |item| {
-                cases_extra.appendAssumeCapacity(@intFromEnum(item));
-            }
-
-            cases_extra.appendSliceAssumeCapacity(@ptrCast(case_block.instructions.items));
-        } else {
+        const analyze_body = if (union_originally)
             for (items) |item| {
-                const cmp_ok = try case_block.addBinOp(if (case_block.float_mode == .optimized) .cmp_eq_optimized else .cmp_eq, operand, item);
-                if (any_ok != .none) {
-                    any_ok = try case_block.addBinOp(.bool_or, any_ok, cmp_ok);
-                } else {
-                    any_ok = cmp_ok;
-                }
-            }
-
-            var range_i: usize = 0;
-            while (range_i < ranges_len) : (range_i += 1) {
-                const range_items = case_vals.items[case_val_idx..][0..2];
-                extra_index += 2;
-                case_val_idx += 2;
-
-                const item_first = range_items[0];
-                const item_last = range_items[1];
-
-                // operand >= first and operand <= last
-                const range_first_ok = try case_block.addBinOp(
-                    if (case_block.float_mode == .optimized) .cmp_gte_optimized else .cmp_gte,
-                    operand,
-                    item_first,
-                );
-                const range_last_ok = try case_block.addBinOp(
-                    if (case_block.float_mode == .optimized) .cmp_lte_optimized else .cmp_lte,
-                    operand,
-                    item_last,
-                );
-                const range_ok = try case_block.addBinOp(
-                    .bool_and,
-                    range_first_ok,
-                    range_last_ok,
-                );
-                if (any_ok != .none) {
-                    any_ok = try case_block.addBinOp(.bool_or, any_ok, range_ok);
-                } else {
-                    any_ok = range_ok;
-                }
-            }
-
-            const new_cond_br = try case_block.addInstAsIndex(.{ .tag = .cond_br, .data = .{
-                .pl_op = .{
-                    .operand = any_ok,
-                    .payload = undefined,
-                },
-            } });
-            var cond_body = try case_block.instructions.toOwnedSlice(gpa);
-            defer gpa.free(cond_body);
-
-            case_block.instructions.shrinkRetainingCapacity(0);
-            case_block.error_return_trace_index = child_block.error_return_trace_index;
+                const item_val = sema.resolveConstDefinedValue(block, LazySrcLoc.unneeded, item, undefined) catch unreachable;
+                const field_ty = maybe_union_ty.unionFieldType(item_val, zcu).?;
+                if (field_ty.zigTypeTag(zcu) != .noreturn) break true;
+            } else false
+        else
+            true;
 
-            const body = sema.code.bodySlice(extra_index, info.body_len);
-            extra_index += info.body_len;
-            const prong_hint: std.builtin.BranchHint = if (err_set and
-                try sema.maybeErrorUnwrap(&case_block, body, operand, operand_src, allow_err_code_unwrap))
-            h: {
-                // nothing to do here. weight against error branch
-                break :h .unlikely;
-            } else try spa.analyzeProngRuntime(
+        const prong_hint: std.builtin.BranchHint = if (err_set and
+            try sema.maybeErrorUnwrap(&case_block, body, operand, operand_src, allow_err_code_unwrap))
+        h: {
+            // nothing to do here. weight against error branch
+            break :h .unlikely;
+        } else if (analyze_body) h: {
+            break :h try spa.analyzeProngRuntime(
                 &case_block,
                 .normal,
                 body,
@@ -12853,40 +12761,36 @@ fn analyzeSwitchRuntimeBlock(
                 .none,
                 false,
             );
+        } else h: {
+            _ = try case_block.addNoOp(.unreach);
+            break :h .none;
+        };
 
-            if (is_first) {
-                is_first = false;
-                first_else_body = cond_body;
-                cond_body = &.{};
-            } else {
-                try sema.air_extra.ensureUnusedCapacity(
-                    gpa,
-                    @typeInfo(Air.CondBr).@"struct".fields.len + prev_then_body.len + cond_body.len,
-                );
+        try branch_hints.append(gpa, prong_hint);
 
-                sema.air_instructions.items(.data)[@intFromEnum(prev_cond_br)].pl_op.payload = sema.addExtraAssumeCapacity(Air.CondBr{
-                    .then_body_len = @intCast(prev_then_body.len),
-                    .else_body_len = @intCast(cond_body.len),
-                    .branch_hints = .{
-                        .true = prev_hint,
-                        .false = .none,
-                        // Code coverage is desired for error handling.
-                        .then_cov = .poi,
-                        .else_cov = .poi,
-                    },
-                });
-                sema.air_extra.appendSliceAssumeCapacity(@ptrCast(prev_then_body));
-                sema.air_extra.appendSliceAssumeCapacity(@ptrCast(cond_body));
-            }
-            gpa.free(prev_then_body);
-            prev_then_body = try case_block.instructions.toOwnedSlice(gpa);
-            prev_cond_br = new_cond_br;
-            prev_hint = prong_hint;
+        try cases_extra.ensureUnusedCapacity(gpa, @typeInfo(Air.SwitchBr.Case).@"struct".fields.len +
+            items.len + 2 * ranges_len +
+            case_block.instructions.items.len);
+        cases_extra.appendSliceAssumeCapacity(&payloadToExtraItems(Air.SwitchBr.Case{
+            .items_len = @intCast(items.len),
+            .ranges_len = @intCast(ranges_len),
+            .body_len = @intCast(case_block.instructions.items.len),
+        }));
+
+        for (items) |item| {
+            cases_extra.appendAssumeCapacity(@intFromEnum(item));
         }
+        for (ranges) |range| {
+            cases_extra.appendSliceAssumeCapacity(&.{
+                @intFromEnum(range[0]),
+                @intFromEnum(range[1]),
+            });
+        }
+
+        cases_extra.appendSliceAssumeCapacity(@ptrCast(case_block.instructions.items));
     }
 
-    var final_else_body: []const Air.Inst.Index = &.{};
-    if (special.body.len != 0 or !is_first or case_block.wantSafety()) {
+    const else_body: []const Air.Inst.Index = if (special.body.len != 0 or case_block.wantSafety()) else_body: {
         var emit_bb = false;
         if (special.is_inline) switch (operand_ty.zigTypeTag(zcu)) {
             .@"enum" => {
@@ -12933,9 +12837,14 @@ fn analyzeSwitchRuntimeBlock(
                     };
                     try branch_hints.append(gpa, prong_hint);
 
-                    try cases_extra.ensureUnusedCapacity(gpa, 3 + case_block.instructions.items.len);
-                    cases_extra.appendAssumeCapacity(1); // items_len
-                    cases_extra.appendAssumeCapacity(@intCast(case_block.instructions.items.len));
+                    try cases_extra.ensureUnusedCapacity(gpa, @typeInfo(Air.SwitchBr.Case).@"struct".fields.len +
+                        1 + // `item`, no ranges
+                        case_block.instructions.items.len);
+                    cases_extra.appendSliceAssumeCapacity(&payloadToExtraItems(Air.SwitchBr.Case{
+                        .items_len = 1,
+                        .ranges_len = 0,
+                        .body_len = @intCast(case_block.instructions.items.len),
+                    }));
                     cases_extra.appendAssumeCapacity(@intFromEnum(item_ref));
                     cases_extra.appendSliceAssumeCapacity(@ptrCast(case_block.instructions.items));
                 }
@@ -12979,9 +12888,14 @@ fn analyzeSwitchRuntimeBlock(
                     );
                     try branch_hints.append(gpa, prong_hint);
 
-                    try cases_extra.ensureUnusedCapacity(gpa, 3 + case_block.instructions.items.len);
-                    cases_extra.appendAssumeCapacity(1); // items_len
-                    cases_extra.appendAssumeCapacity(@intCast(case_block.instructions.items.len));
+                    try cases_extra.ensureUnusedCapacity(gpa, @typeInfo(Air.SwitchBr.Case).@"struct".fields.len +
+                        1 + // `item`, no ranges
+                        case_block.instructions.items.len);
+                    cases_extra.appendSliceAssumeCapacity(&payloadToExtraItems(Air.SwitchBr.Case{
+                        .items_len = 1,
+                        .ranges_len = 0,
+                        .body_len = @intCast(case_block.instructions.items.len),
+                    }));
                     cases_extra.appendAssumeCapacity(@intFromEnum(item_ref));
                     cases_extra.appendSliceAssumeCapacity(@ptrCast(case_block.instructions.items));
                 }
@@ -13014,9 +12928,14 @@ fn analyzeSwitchRuntimeBlock(
                     );
                     try branch_hints.append(gpa, prong_hint);
 
-                    try cases_extra.ensureUnusedCapacity(gpa, 3 + case_block.instructions.items.len);
-                    cases_extra.appendAssumeCapacity(1); // items_len
-                    cases_extra.appendAssumeCapacity(@intCast(case_block.instructions.items.len));
+                    try cases_extra.ensureUnusedCapacity(gpa, @typeInfo(Air.SwitchBr.Case).@"struct".fields.len +
+                        1 + // `item`, no ranges
+                        case_block.instructions.items.len);
+                    cases_extra.appendSliceAssumeCapacity(&payloadToExtraItems(Air.SwitchBr.Case{
+                        .items_len = 1,
+                        .ranges_len = 0,
+                        .body_len = @intCast(case_block.instructions.items.len),
+                    }));
                     cases_extra.appendAssumeCapacity(@intFromEnum(item_ref));
                     cases_extra.appendSliceAssumeCapacity(@ptrCast(case_block.instructions.items));
                 }
@@ -13046,9 +12965,14 @@ fn analyzeSwitchRuntimeBlock(
                     );
                     try branch_hints.append(gpa, prong_hint);
 
-                    try cases_extra.ensureUnusedCapacity(gpa, 3 + case_block.instructions.items.len);
-                    cases_extra.appendAssumeCapacity(1); // items_len
-                    cases_extra.appendAssumeCapacity(@intCast(case_block.instructions.items.len));
+                    try cases_extra.ensureUnusedCapacity(gpa, @typeInfo(Air.SwitchBr.Case).@"struct".fields.len +
+                        1 + // `item`, no ranges
+                        case_block.instructions.items.len);
+                    cases_extra.appendSliceAssumeCapacity(&payloadToExtraItems(Air.SwitchBr.Case{
+                        .items_len = 1,
+                        .ranges_len = 0,
+                        .body_len = @intCast(case_block.instructions.items.len),
+                    }));
                     cases_extra.appendAssumeCapacity(@intFromEnum(Air.Inst.Ref.bool_true));
                     cases_extra.appendSliceAssumeCapacity(@ptrCast(case_block.instructions.items));
                 }
@@ -13076,9 +13000,14 @@ fn analyzeSwitchRuntimeBlock(
                     );
                     try branch_hints.append(gpa, prong_hint);
 
-                    try cases_extra.ensureUnusedCapacity(gpa, 3 + case_block.instructions.items.len);
-                    cases_extra.appendAssumeCapacity(1); // items_len
-                    cases_extra.appendAssumeCapacity(@intCast(case_block.instructions.items.len));
+                    try cases_extra.ensureUnusedCapacity(gpa, @typeInfo(Air.SwitchBr.Case).@"struct".fields.len +
+                        1 + // `item`, no ranges
+                        case_block.instructions.items.len);
+                    cases_extra.appendSliceAssumeCapacity(&payloadToExtraItems(Air.SwitchBr.Case{
+                        .items_len = 1,
+                        .ranges_len = 0,
+                        .body_len = @intCast(case_block.instructions.items.len),
+                    }));
                     cases_extra.appendAssumeCapacity(@intFromEnum(Air.Inst.Ref.bool_false));
                     cases_extra.appendSliceAssumeCapacity(@ptrCast(case_block.instructions.items));
                 }
@@ -13142,41 +13071,22 @@ fn analyzeSwitchRuntimeBlock(
             break :h .cold;
         };
 
-        if (is_first) {
-            try branch_hints.append(gpa, else_hint);
-            final_else_body = case_block.instructions.items;
-        } else {
-            try branch_hints.append(gpa, .none); // we have the range conditionals first
-            try sema.air_extra.ensureUnusedCapacity(gpa, prev_then_body.len +
-                @typeInfo(Air.CondBr).@"struct".fields.len + case_block.instructions.items.len);
-
-            sema.air_instructions.items(.data)[@intFromEnum(prev_cond_br)].pl_op.payload = sema.addExtraAssumeCapacity(Air.CondBr{
-                .then_body_len = @intCast(prev_then_body.len),
-                .else_body_len = @intCast(case_block.instructions.items.len),
-                .branch_hints = .{
-                    .true = prev_hint,
-                    .false = else_hint,
-                    .then_cov = .poi,
-                    .else_cov = .poi,
-                },
-            });
-            sema.air_extra.appendSliceAssumeCapacity(@ptrCast(prev_then_body));
-            sema.air_extra.appendSliceAssumeCapacity(@ptrCast(case_block.instructions.items));
-            final_else_body = first_else_body;
-        }
-    } else {
+        try branch_hints.append(gpa, else_hint);
+        break :else_body case_block.instructions.items;
+    } else else_body: {
         try branch_hints.append(gpa, .none);
-    }
+        break :else_body &.{};
+    };
 
     assert(branch_hints.items.len == cases_len + 1);
 
     try sema.air_extra.ensureUnusedCapacity(gpa, @typeInfo(Air.SwitchBr).@"struct".fields.len +
-        cases_extra.items.len + final_else_body.len +
+        cases_extra.items.len + else_body.len +
         (std.math.divCeil(usize, branch_hints.items.len, 10) catch unreachable)); // branch hints
 
     const payload_index = sema.addExtraAssumeCapacity(Air.SwitchBr{
         .cases_len = @intCast(cases_len),
-        .else_body_len = @intCast(final_else_body.len),
+        .else_body_len = @intCast(else_body.len),
     });
 
     {
@@ -13195,7 +13105,7 @@ fn analyzeSwitchRuntimeBlock(
         }
     }
     sema.air_extra.appendSliceAssumeCapacity(@ptrCast(cases_extra.items));
-    sema.air_extra.appendSliceAssumeCapacity(@ptrCast(final_else_body));
+    sema.air_extra.appendSliceAssumeCapacity(@ptrCast(else_body));
 
     return try child_block.addInst(.{
         .tag = .switch_br,
@@ -37386,15 +37296,21 @@ pub fn addExtra(sema: *Sema, extra: anytype) Allocator.Error!u32 {
 }
 
 pub fn addExtraAssumeCapacity(sema: *Sema, extra: anytype) u32 {
-    const fields = std.meta.fields(@TypeOf(extra));
     const result: u32 = @intCast(sema.air_extra.items.len);
-    inline for (fields) |field| {
-        sema.air_extra.appendAssumeCapacity(switch (field.type) {
-            u32 => @field(extra, field.name),
-            i32, Air.CondBr.BranchHints => @bitCast(@field(extra, field.name)),
-            Air.Inst.Ref, InternPool.Index => @intFromEnum(@field(extra, field.name)),
+    sema.air_extra.appendSliceAssumeCapacity(&payloadToExtraItems(extra));
+    return result;
+}
+
+fn payloadToExtraItems(data: anytype) [@typeInfo(@TypeOf(data)).@"struct".fields.len]u32 {
+    const fields = @typeInfo(@TypeOf(data)).@"struct".fields;
+    var result: [fields.len]u32 = undefined;
+    inline for (&result, fields) |*val, field| {
+        val.* = switch (field.type) {
+            u32 => @field(data, field.name),
+            i32, Air.CondBr.BranchHints => @bitCast(@field(data, field.name)),
+            Air.Inst.Ref, InternPool.Index => @intFromEnum(@field(data, field.name)),
             else => @compileError("bad field type: " ++ @typeName(field.type)),
-        });
+        };
     }
     return result;
 }