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stage2: better codegen for byte-aligned packed struct fields

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* Sema: handle overaligned packed struct field pointers
 * LLVM: handle byte-aligned packed struct field pointers
This commit is contained in:
Andrew Kelley
2022-06-07 20:51:14 -07:00
parent 70dc910086
commit 3e30ba3f20
4 changed files with 136 additions and 13 deletions
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40
src/codegen/llvm.zig
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@@ -8311,23 +8311,41 @@ pub const FuncGen = struct {
field_index: u32,
) !?*const llvm.Value {
if (self.liveness.isUnused(inst)) return null;
const target = self.dg.object.target;
const struct_ty = struct_ptr_ty.childType();
switch (struct_ty.zigTypeTag()) {
.Struct => switch (struct_ty.containerLayout()) {
.Packed => {
// From LLVM's perspective, a pointer to a packed struct and a pointer
// to a field of a packed struct are the same. The difference is in the
// Zig pointer type which provides information for how to mask and shift
// out the relevant bits when accessing the pointee.
// Here we perform a bitcast because we want to use the host_size
// as the llvm pointer element type.
const result_llvm_ty = try self.dg.lowerType(self.air.typeOfIndex(inst));
// TODO this can be removed if we change host_size to be bits instead
// of bytes.
return self.builder.buildBitCast(struct_ptr, result_llvm_ty, "");
const result_ty = self.air.typeOfIndex(inst);
const result_ty_info = result_ty.ptrInfo().data;
const result_llvm_ty = try self.dg.lowerType(result_ty);
if (result_ty_info.host_size != 0) {
// From LLVM's perspective, a pointer to a packed struct and a pointer
// to a field of a packed struct are the same. The difference is in the
// Zig pointer type which provides information for how to mask and shift
// out the relevant bits when accessing the pointee.
// Here we perform a bitcast because we want to use the host_size
// as the llvm pointer element type.
return self.builder.buildBitCast(struct_ptr, result_llvm_ty, "");
}
// We have a pointer to a packed struct field that happens to be byte-aligned.
// Offset our operand pointer by the correct number of bytes.
const byte_offset = struct_ty.packedStructFieldByteOffset(field_index, target);
if (byte_offset == 0) {
return self.builder.buildBitCast(struct_ptr, result_llvm_ty, "");
}
const llvm_bytes_ptr_ty = self.context.intType(8).pointerType(0);
const ptr_as_bytes = self.builder.buildBitCast(struct_ptr, llvm_bytes_ptr_ty, "");
const llvm_usize = try self.dg.lowerType(Type.usize);
const llvm_index = llvm_usize.constInt(byte_offset, .False);
const indices: [1]*const llvm.Value = .{llvm_index};
const new_ptr = self.builder.buildInBoundsGEP(ptr_as_bytes, &indices, indices.len, "");
return self.builder.buildBitCast(new_ptr, result_llvm_ty, "");
},
else => {
const target = self.dg.module.getTarget();
var ty_buf: Type.Payload.Pointer = undefined;
if (llvmFieldIndex(struct_ty, field_index, target, &ty_buf)) |llvm_field_index| {
return self.builder.buildStructGEP(struct_ptr, llvm_field_index, "");