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Rollup merge of #136895 - maurer:fix-enum-discr, r=nikic

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debuginfo: Set bitwidth appropriately in enum variant tags

Previously, we unconditionally set the bitwidth to 128-bits, the largest an enum would possibly be. Then, LLVM would cut down the constant by chopping off leading zeroes before emitting the DWARF. LLVM only supported 64-bit enumerators, so this would also have occasionally resulted in truncated data.

LLVM added support for 128-bit enumerators in llvm/llvm-project#125578

That patchset trusts the constant to describe how wide the variant tag is, so the high 64-bits of zeros are considered potentially load-bearing.

As a result, we went from emitting tags that looked like:
DW_AT_discr_value     (0xfe)

(because `dwarf::BestForm` selected `data1`)

to emitting tags that looked like:
DW_AT_discr_value	(<0x10> fe ff ff ff 00 00 00 00 00 00 00 00 00 00 00 00 )

This makes the `DW_AT_discr_value` encode at the bitwidth of the tag, which:
1. Is probably closer to our intentions in terms of describing the data.
2. Doesn't invoke the 128-bit support which may not be supported by all debuggers / downstream tools.
3. Will result in smaller debug information.
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Jubilee 2025-02-13 17:46:08 -08:00 • committed by GitHub
commit 864eba9fb1
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2 changed files with 9 additions and 3 deletions
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8
compiler/rustc_codegen_llvm/src/debuginfo/metadata/enums/native.rs
View file

@ -437,6 +437,12 @@ fn build_enum_variant_member_di_node<'ll, 'tcx>(
.source_info
.unwrap_or_else(|| (unknown_file_metadata(cx), UNKNOWN_LINE_NUMBER));
let discr = discr_value.opt_single_val().map(|value| {
let tag_base_type = tag_base_type(cx.tcx, enum_type_and_layout);
let size = cx.size_of(tag_base_type);
cx.const_uint_big(cx.type_ix(size.bits()), value)
});
unsafe {
llvm::LLVMRustDIBuilderCreateVariantMemberType(
DIB(cx),
@ -448,7 +454,7 @@ fn build_enum_variant_member_di_node<'ll, 'tcx>(
enum_type_and_layout.size.bits(),
enum_type_and_layout.align.abi.bits() as u32,
Size::ZERO.bits(),
discr_value.opt_single_val().map(|value| cx.const_u128(value)),
discr,
DIFlags::FlagZero,
variant_member_info.variant_struct_type_di_node,
)

4
tests/codegen/enum/enum-debug-niche-2.rs
View file

@ -5,8 +5,8 @@
//@ ignore-msvc
//
// CHECK: {{.*}}DICompositeType{{.*}}tag: DW_TAG_variant_part,{{.*}}size: 32,{{.*}}
// CHECK: {{.*}}DIDerivedType{{.*}}tag: DW_TAG_member,{{.*}}name: "Placeholder",{{.*}}extraData: i128 4294967295{{[,)].*}}
// CHECK: {{.*}}DIDerivedType{{.*}}tag: DW_TAG_member,{{.*}}name: "Error",{{.*}}extraData: i128 0{{[,)].*}}
// CHECK: {{.*}}DIDerivedType{{.*}}tag: DW_TAG_member,{{.*}}name: "Placeholder",{{.*}}extraData: i32 -1{{[,)].*}}
// CHECK: {{.*}}DIDerivedType{{.*}}tag: DW_TAG_member,{{.*}}name: "Error",{{.*}}extraData: i32 0{{[,)].*}}
#![feature(never_type)]
#[derive(Copy, Clone)]