Commit `9c20449cc5`

Robin Voetter <robin@voetter.nl>

2023-10-14 12:52:31

wasm: lower min/max for floats to compiler_rt

The min and max builtins in Zig have some intricate behavior related to floats, that is not replicated with the min and max wasm instructions or using simple select operations. By lowering these instructions to compiler_rt, handling around NaNs is done correctly. See also https://github.com/WebAssembly/design/issues/214

master

1 parent 2fe16e0

Changed files (2)

src

arch

wasm

CodeGen.zig

test

behavior

maximum_minimum.zig

@@ -6253,8 +6253,10 @@ fn airMulWithOverflow(func: *CodeGen, inst: Air.Inst.Index) InnerError!void {
     func.finishAir(inst, result_ptr, &.{ extra.lhs, extra.rhs });
 }
 
-fn airMaxMin(func: *CodeGen, inst: Air.Inst.Index, op: enum { max, min }) InnerError!void {
+fn airMaxMin(func: *CodeGen, inst: Air.Inst.Index, op: Op) InnerError!void {
+    assert(op == .max or op == .min);
     const mod = func.bin_file.base.options.module.?;
+    const target = mod.getTarget();
     const bin_op = func.air.instructions.items(.data)[inst].bin_op;
 
     const ty = func.typeOfIndex(inst);
@@ -6269,13 +6271,25 @@ fn airMaxMin(func: *CodeGen, inst: Air.Inst.Index, op: enum { max, min }) InnerE
     const lhs = try func.resolveInst(bin_op.lhs);
     const rhs = try func.resolveInst(bin_op.rhs);
 
-    // operands to select from
-    try func.lowerToStack(lhs);
-    try func.lowerToStack(rhs);
-    _ = try func.cmp(lhs, rhs, ty, if (op == .max) .gt else .lt);
+    if (ty.zigTypeTag(mod) == .Float) {
+        var fn_name_buf: [64]u8 = undefined;
+        const float_bits = ty.floatBits(target);
+        const fn_name = std.fmt.bufPrint(&fn_name_buf, "{s}f{s}{s}", .{
+            target_util.libcFloatPrefix(float_bits),
+            @tagName(op),
+            target_util.libcFloatSuffix(float_bits),
+        }) catch unreachable;
+        const result = try func.callIntrinsic(fn_name, &.{ ty.ip_index, ty.ip_index }, ty, &.{ lhs, rhs });
+        try func.lowerToStack(result);
+    } else {
+        // operands to select from
+        try func.lowerToStack(lhs);
+        try func.lowerToStack(rhs);
+        _ = try func.cmp(lhs, rhs, ty, if (op == .max) .gt else .lt);
 
-    // based on the result from comparison, return operand 0 or 1.
-    try func.addTag(.select);
+        // based on the result from comparison, return operand 0 or 1.
+        try func.addTag(.select);
+    }
 
     // store result in local
     const result_ty = if (isByRef(ty, mod)) Type.u32 else ty;

@@ -123,6 +123,12 @@ test "@min/max for floats" {
             try expectEqual(x, @min(y, x));
             try expectEqual(y, @max(x, y));
             try expectEqual(y, @max(y, x));
+
+            if (T != comptime_float) {
+                var nan: T = std.math.nan(T);
+                try expectEqual(y, @max(nan, y));
+                try expectEqual(y, @max(y, nan));
+            }
         }
     };

Commit 9c20449cc5

Commit `9c20449cc5`