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llvm: lower tagged union payload as byte array #21354

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llvm: lower tagged union payload as byte array #21354

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59 changes: 19 additions & 40 deletions src/codegen/llvm.zig
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Original file line number Diff line number Diff line change
Expand Up @@ -3537,22 +3537,10 @@ pub const Object = struct {
return enum_tag_ty;
}

const aligned_field_ty = Type.fromInterned(union_obj.field_types.get(ip)[layout.most_aligned_field]);
const aligned_field_llvm_ty = try o.lowerType(aligned_field_ty);

const payload_ty = ty: {
if (layout.most_aligned_field_size == layout.payload_size) {
break :ty aligned_field_llvm_ty;
}
const padding_len = if (layout.tag_size == 0)
layout.abi_size - layout.most_aligned_field_size
else
layout.payload_size - layout.most_aligned_field_size;
break :ty try o.builder.structType(.@"packed", &.{
aligned_field_llvm_ty,
try o.builder.arrayType(padding_len, .i8),
});
};
const payload_ty = try o.builder.arrayType(
if (layout.tag_size == 0) layout.abi_size else layout.payload_size,
.i8,
);

if (layout.tag_size == 0) {
const ty = try o.builder.opaqueType(try o.builder.string(t.containerTypeName(ip).toSlice(ip)));
Expand Down Expand Up @@ -4271,33 +4259,28 @@ pub const Object = struct {
const union_obj = zcu.typeToUnion(ty).?;
const container_layout = union_obj.flagsUnordered(ip).layout;

var need_unnamed = false;
// LLVM does not necessarily support bitcasting our payload struct to the true union
// payload type (represented as an opaque byte array). Instead, we lower the payload
// as an unnamed struct, and every reference to the global must pointer cast to the
// expected type before access. The only exception is packed unions, which have early
// returns paths below.

const payload = if (un.tag != .none) p: {
const field_index = zcu.unionTagFieldIndex(union_obj, Value.fromInterned(un.tag)).?;
const field_ty = Type.fromInterned(union_obj.field_types.get(ip)[field_index]);
if (container_layout == .@"packed") {
if (!field_ty.hasRuntimeBits(zcu)) return o.builder.intConst(union_ty, 0);
const bits = ty.bitSize(zcu);
const llvm_int_ty = try o.builder.intType(@intCast(bits));

return o.lowerValueToInt(llvm_int_ty, arg_val);
}

// Sometimes we must make an unnamed struct because LLVM does
// not support bitcasting our payload struct to the true union payload type.
// Instead we use an unnamed struct and every reference to the global
// must pointer cast to the expected type before accessing the union.
need_unnamed = layout.most_aligned_field != field_index;

if (!field_ty.hasRuntimeBitsIgnoreComptime(zcu)) {
const padding_len = layout.payload_size;
break :p try o.builder.undefConst(try o.builder.arrayType(padding_len, .i8));
}
const payload = try o.lowerValue(un.val);
const payload_ty = payload.typeOf(&o.builder);
if (payload_ty != union_ty.structFields(&o.builder)[
@intFromBool(layout.tag_align.compare(.gte, layout.payload_align))
]) need_unnamed = true;
const field_size = field_ty.abiSize(zcu);
if (field_size == layout.payload_size) break :p payload;
const padding_len = layout.payload_size - field_size;
Expand All @@ -4311,20 +4294,16 @@ pub const Object = struct {
if (container_layout == .@"packed") {
const bits = ty.bitSize(zcu);
const llvm_int_ty = try o.builder.intType(@intCast(bits));

return o.lowerValueToInt(llvm_int_ty, arg_val);
}

const union_val = try o.lowerValue(un.val);
need_unnamed = true;
break :p union_val;
break :p try o.lowerValue(un.val);
};

const payload_ty = payload.typeOf(&o.builder);
if (layout.tag_size == 0) return o.builder.structConst(if (need_unnamed)
try o.builder.structType(union_ty.structKind(&o.builder), &.{payload_ty})
else
union_ty, &.{payload});
if (layout.tag_size == 0) return o.builder.structConst(
try o.builder.structType(union_ty.structKind(&o.builder), &.{payload_ty}),
&.{payload},
);
const tag = try o.lowerValue(un.tag);
const tag_ty = tag.typeOf(&o.builder);
var fields: [3]Builder.Type = undefined;
Expand All @@ -4342,10 +4321,10 @@ pub const Object = struct {
vals[2] = try o.builder.undefConst(fields[2]);
len = 3;
}
return o.builder.structConst(if (need_unnamed)
try o.builder.structType(union_ty.structKind(&o.builder), fields[0..len])
else
union_ty, vals[0..len]);
return o.builder.structConst(
try o.builder.structType(union_ty.structKind(&o.builder), fields[0..len]),
vals[0..len],
);
},
.memoized_call => unreachable,
};
Expand Down
22 changes: 22 additions & 0 deletions test/behavior/union.zig
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Original file line number Diff line number Diff line change
Expand Up @@ -2339,3 +2339,25 @@ test "signed enum tag with negative value" {

try expect(e.a == i);
}

test "init tagged union with underaligned payload" {
if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest; // TODO
if (builtin.zig_backend == .stage2_x86) return error.SkipZigTest; // TODO
if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
if (builtin.zig_backend == .stage2_spirv64) return error.SkipZigTest;
if (builtin.zig_backend == .stage2_riscv64) return error.SkipZigTest;

const U = union(enum) {
a: u64 align(1),
b: void,
};

var x: u32 = undefined;
x = 123;

const u: U = .{ .a = x };

try expect(u == .a);
try expect(u.a == 123);
}
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