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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
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@@ -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 {