https://github.com/phoebewang created https://github.com/llvm/llvm-project/pull/217847
The x86-64 SysV classifier ignored all unnamed bit-fields as padding. A non-zero-width unnamed bit-field is real storage, though, so an eightbyte occupied only by one was left NO_CLASS. When a struct holds a run of __int128/_BitInt(128) bit-fields, record lowering merges the run into a single i128 access unit that spans both eightbytes, but classification then marked only one of them INTEGER. The i128 was therefore queried at offset 8 (tripping assert(IROffset == 0) in GetINTEGERTypeAtOffset) or returned as the low part while the high eightbyte was NO_CLASS (tripping assert(Hi == Integer) in the callers); without assertions it was passed in a register pair the ABI never assigned. Classify a non-zero-width unnamed bit-field like a named one so both eightbytes of such a struct are INTEGER. The value is then passed and returned as an i128 in two integer registers, matching GCC, and the existing i128 asserts hold. Zero-length bit-fields are still ignored, as they occupy no storage. Fixes #202205. >From 5ed88c38f8e20ebec5bacd58f18e78a26da8826e Mon Sep 17 00:00:00 2001 From: Phoebe Wang <[email protected]> Date: Fri, 21 Aug 2026 01:27:59 -0700 Subject: [PATCH] [Clang][CodeGen][X86] Classify non-zero-width unnamed bit-fields as INTEGER The x86-64 SysV classifier ignored all unnamed bit-fields as padding. A non-zero-width unnamed bit-field is real storage, though, so an eightbyte occupied only by one was left NO_CLASS. When a struct holds a run of __int128/_BitInt(128) bit-fields, record lowering merges the run into a single i128 access unit that spans both eightbytes, but classification then marked only one of them INTEGER. The i128 was therefore queried at offset 8 (tripping assert(IROffset == 0) in GetINTEGERTypeAtOffset) or returned as the low part while the high eightbyte was NO_CLASS (tripping assert(Hi == Integer) in the callers); without assertions it was passed in a register pair the ABI never assigned. Classify a non-zero-width unnamed bit-field like a named one so both eightbytes of such a struct are INTEGER. The value is then passed and returned as an i128 in two integer registers, matching GCC, and the existing i128 asserts hold. Zero-length bit-fields are still ignored, as they occupy no storage. Fixes #202205. Co-Authored-By: Claude Opus 4.8 <[email protected]> --- clang/docs/ReleaseNotes.md | 7 ++++++ clang/lib/CodeGen/Targets/X86.cpp | 12 +++++++--- clang/test/CodeGen/X86/x86_64-arguments.c | 29 +++++++++++++++++++++++ 3 files changed, 45 insertions(+), 3 deletions(-) diff --git a/clang/docs/ReleaseNotes.md b/clang/docs/ReleaseNotes.md index 8c9467ca7b742..48a8cc31f2618 100644 --- a/clang/docs/ReleaseNotes.md +++ b/clang/docs/ReleaseNotes.md @@ -96,6 +96,13 @@ features cannot lower the translation-unit ABI level; - On MIPS N32/N64, an `__int128` now correctly start in an even-numbered register or 16-byte aligned stack slot, matching GCC. +- On x86-64 System V, an eightbyte occupied only by a non-zero-width unnamed + bit-field is now classified INTEGER rather than NO_CLASS, matching GCC. As a + result a struct holding a run of `__int128`/`_BitInt(128)` bit-fields (which + record lowering merges into a single 128-bit access unit) is now passed and + returned in the two integer registers the ABI assigns it, instead of one. + This also fixes a crash when such a struct was passed or returned. (#GH202205) + ### AST Dumping Potentially Breaking Changes ### Clang Frontend Potentially Breaking Changes diff --git a/clang/lib/CodeGen/Targets/X86.cpp b/clang/lib/CodeGen/Targets/X86.cpp index f0d108f3279fd..78b732bb0be6b 100644 --- a/clang/lib/CodeGen/Targets/X86.cpp +++ b/clang/lib/CodeGen/Targets/X86.cpp @@ -2204,8 +2204,13 @@ void X86_64ABIInfo::classify(QualType Ty, uint64_t OffsetBase, Class &Lo, uint64_t Offset = OffsetBase + Layout.getFieldOffset(idx); bool BitField = i->isBitField(); - // Ignore padding bit-fields. - if (BitField && i->isUnnamedBitField()) + // Ignore zero-length bit-fields: they occupy no storage and only affect + // alignment. A non-zero-width unnamed bit-field, however, is real storage + // and its eightbyte(s) must be classified INTEGER, matching GCC. Skipping + // it would leave an eightbyte NO_CLASS even though record lowering merged + // the run into a single i128 access unit, which later trips the i128 + // handling in classify{Return,Argument}Type / GetINTEGERTypeAtOffset. + if (BitField && i->isUnnamedBitField() && i->isZeroLengthBitField()) continue; // AMD64-ABI 3.2.3p2: Rule 1. If the size of an object is larger than @@ -2246,7 +2251,8 @@ void X86_64ABIInfo::classify(QualType Ty, uint64_t OffsetBase, Class &Lo, // structure to be passed in memory even if unaligned, and // therefore they can straddle an eightbyte. if (BitField) { - assert(!i->isUnnamedBitField()); + // Named and non-zero-width unnamed bit-fields reach here; zero-length + // bit-fields were skipped above. uint64_t Offset = OffsetBase + Layout.getFieldOffset(idx); uint64_t Size = i->getBitWidthValue(); diff --git a/clang/test/CodeGen/X86/x86_64-arguments.c b/clang/test/CodeGen/X86/x86_64-arguments.c index 580f9487395d3..e156a759d23c9 100644 --- a/clang/test/CodeGen/X86/x86_64-arguments.c +++ b/clang/test/CodeGen/X86/x86_64-arguments.c @@ -590,6 +590,35 @@ _BitInt(128) f74(__uint128_t b, __uint128_t c, __uint128_t d, long e, _BitInt(12 return a; } +// A run of (u)int128_t bit-fields lowers to a single i128 access unit that +// spans both eightbytes. A non-zero-width unnamed bit-field is real storage, +// so both eightbytes classify INTEGER and the value is passed/returned as an +// i128 (two integer registers), matching GCC -- regardless of whether the +// named field sits in the low or high eightbyte. +struct s75 { + __uint128_t : 124; + __uint128_t a : 4; +}; +// CHECK-LABEL: define{{.*}} i128 @f75() +struct s75 f75(void) { + return (struct s75){0}; +} +// CHECK-LABEL: define{{.*}} void @f76(i128 %a.coerce) +void f76(struct s75 a) { +} + +struct s77 { + __uint128_t a : 4; + __uint128_t : 124; +}; +// CHECK-LABEL: define{{.*}} i128 @f77() +struct s77 f77(void) { + return (struct s77){0}; +} +// CHECK-LABEL: define{{.*}} void @f78(i128 %a.coerce) +void f78(struct s77 a) { +} + /// The synthesized __va_list_tag does not have file/line fields. // CHECK: = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "__va_list_tag", // CHECK-NOT: file: _______________________________________________ cfe-commits mailing list [email protected] https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
