Commit `15f111a085`

Andrew Kelley <andrew@ziglang.org>

2022-07-03 22:07:23

LLVM: update lowering of saturating shift-left

LLVM 14 makes it so that a RHS of saturating shift left produces a poison value if the value is greater than the number of bits of the LHS. Zig now emits code that will check if this is the case and select a saturated LHS value in such case, matching Zig semantics.

master

1 parent b1873f2

Changed files (2)

src

codegen

llvm.zig

stage1

codegen.cpp

@@ -6817,15 +6817,37 @@ pub const FuncGen = struct {
         const rhs_ty = self.air.typeOf(bin_op.rhs);
         const lhs_scalar_ty = lhs_ty.scalarType();
         const rhs_scalar_ty = rhs_ty.scalarType();
-
         const tg = self.dg.module.getTarget();
+        const lhs_bits = lhs_scalar_ty.bitSize(tg);
 
-        const casted_rhs = if (rhs_scalar_ty.bitSize(tg) < lhs_scalar_ty.bitSize(tg))
-            self.builder.buildZExt(rhs, try self.dg.lowerType(lhs_ty), "")
+        const casted_rhs = if (rhs_scalar_ty.bitSize(tg) < lhs_bits)
+            self.builder.buildZExt(rhs, lhs.typeOf(), "")
         else
             rhs;
-        if (lhs_scalar_ty.isSignedInt()) return self.builder.buildSShlSat(lhs, casted_rhs, "");
-        return self.builder.buildUShlSat(lhs, casted_rhs, "");
+
+        const result = if (lhs_scalar_ty.isSignedInt())
+            self.builder.buildSShlSat(lhs, casted_rhs, "")
+        else
+            self.builder.buildUShlSat(lhs, casted_rhs, "");
+
+        // LLVM langref says "If b is (statically or dynamically) equal to or
+        // larger than the integer bit width of the arguments, the result is a
+        // poison value."
+        // However Zig semantics says that saturating shift left can never produce
+        // undefined; instead it saturates.
+        const lhs_scalar_llvm_ty = try self.dg.lowerType(lhs_scalar_ty);
+        const bits = lhs_scalar_llvm_ty.constInt(lhs_bits, .False);
+        const lhs_max = lhs_scalar_llvm_ty.constAllOnes();
+        if (rhs_ty.zigTypeTag() == .Vector) {
+            const vec_len = rhs_ty.vectorLen();
+            const bits_vec = self.builder.buildVectorSplat(vec_len, bits, "");
+            const lhs_max_vec = self.builder.buildVectorSplat(vec_len, lhs_max, "");
+            const in_range = self.builder.buildICmp(.ULT, rhs, bits_vec, "");
+            return self.builder.buildSelect(in_range, result, lhs_max_vec, "");
+        } else {
+            const in_range = self.builder.buildICmp(.ULT, rhs, bits, "");
+            return self.builder.buildSelect(in_range, result, lhs_max, "");
+        }
     }
 
     fn airShr(self: *FuncGen, inst: Air.Inst.Index, is_exact: bool) !?*const llvm.Value {

@@ -3868,16 +3868,33 @@ static LLVMValueRef ir_render_bin_op(CodeGen *g, Stage1Air *executable,
             } else {
                 zig_unreachable();
             }
-        case IrBinOpShlSat:
-            if (scalar_type->id == ZigTypeIdInt) {
-                if (scalar_type->data.integral.is_signed) {
-                    return ZigLLVMBuildSShlSat(g->builder, op1_value, op2_value, "");
-                } else {
-                    return ZigLLVMBuildUShlSat(g->builder, op1_value, op2_value, "");
-                }
-            } else {
+        case IrBinOpShlSat: {
+            if (scalar_type->id != ZigTypeIdInt) {
                 zig_unreachable();
             }
+            LLVMValueRef result = scalar_type->data.integral.is_signed ?
+                ZigLLVMBuildSShlSat(g->builder, op1_value, op2_value, "") :
+                ZigLLVMBuildUShlSat(g->builder, op1_value, op2_value, "");
+            // LLVM langref says "If b is (statically or dynamically) equal to or
+            // larger than the integer bit width of the arguments, the result is a
+            // poison value."
+            // However Zig semantics says that saturating shift left can never produce
+            // undefined; instead it saturates.
+            LLVMTypeRef lhs_scalar_llvm_ty = get_llvm_type(g, scalar_type);
+            LLVMValueRef bits = LLVMConstInt(lhs_scalar_llvm_ty,
+                    scalar_type->data.integral.bit_count, false);
+            LLVMValueRef lhs_max = LLVMConstAllOnes(lhs_scalar_llvm_ty);
+            if (operand_type->id == ZigTypeIdVector) {
+                uint64_t vec_len = operand_type->data.vector.len;
+                LLVMValueRef bits_vec = LLVMBuildVectorSplat(g->builder, vec_len, bits, "");
+                LLVMValueRef lhs_max_vec = LLVMBuildVectorSplat(g->builder, vec_len, lhs_max, "");
+                LLVMValueRef in_range = LLVMBuildICmp(g->builder, LLVMIntULT, op2_value, bits_vec, "");
+                return LLVMBuildSelect(g->builder, in_range, result, lhs_max_vec, "");
+            } else {
+                LLVMValueRef in_range = LLVMBuildICmp(g->builder, LLVMIntULT, op2_value, bits, "");
+                return LLVMBuildSelect(g->builder, in_range, result, lhs_max, "");
+            }
+        }
     }
     zig_unreachable();
 }

Commit 15f111a085

Commit `15f111a085`