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LLVM: update lowering of saturating shift-left

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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.
This commit is contained in:
Andrew Kelley 2022-07-03 13:07:23 -07:00
parent b1873f2074
commit 15f111a085
2 changed files with 52 additions and 13 deletions
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32
src/codegen/llvm.zig
View File

@ -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 {

33
src/stage1/codegen.cpp
View File

@ -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();
}