https://github.com/pranavk updated https://github.com/llvm/llvm-project/pull/120670
>From 2c049c1a377e0b9142d4dcb8682ea27837750810 Mon Sep 17 00:00:00 2001 From: Pranav Kant <p...@google.com> Date: Fri, 20 Dec 2024 02:17:23 +0000 Subject: [PATCH 1/2] [clang] Return larger CXX records in memory We incorrectly return CXX records in AVX registers when they should be returned in memory. This is violation of x86-64 psABI. Detailed discussion is here: https://groups.google.com/g/x86-64-abi/c/BjOOyihHuqg/m/KurXdUcWAgAJ --- clang/docs/ReleaseNotes.rst | 2 ++ clang/include/clang/Basic/LangOptions.h | 5 +++++ clang/lib/CodeGen/Targets/X86.cpp | 17 +++++++++++++++++ clang/test/CodeGen/X86/avx-cxx-record.cpp | 23 +++++++++++++++++++++++ 4 files changed, 47 insertions(+) create mode 100644 clang/test/CodeGen/X86/avx-cxx-record.cpp diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst index 7fafe2807bd3883..d5469d96d615c41 100644 --- a/clang/docs/ReleaseNotes.rst +++ b/clang/docs/ReleaseNotes.rst @@ -43,6 +43,8 @@ C++ Specific Potentially Breaking Changes ABI Changes in This Version --------------------------- +- Return larger CXX records in memory instead of using AVX registers. Code compiled with older clang will be incompatible with newer version of the clang unless -fclang-abi-compat=20 is provided. (#GH120670) + AST Dumping Potentially Breaking Changes ---------------------------------------- diff --git a/clang/include/clang/Basic/LangOptions.h b/clang/include/clang/Basic/LangOptions.h index 114a5d34a008bd7..d8311dbdfdb43c6 100644 --- a/clang/include/clang/Basic/LangOptions.h +++ b/clang/include/clang/Basic/LangOptions.h @@ -248,6 +248,11 @@ class LangOptionsBase { /// function templates. Ver19, + /// Attempt to be ABI-compatible with code generated by Clang 20.0.x. + /// This causes clang to: + /// - Incorrectly return C++ records in AVX registers on x86_64. + Ver20, + /// Conform to the underlying platform's C and C++ ABIs as closely /// as we can. Latest diff --git a/clang/lib/CodeGen/Targets/X86.cpp b/clang/lib/CodeGen/Targets/X86.cpp index 5ee5179dd0f3e8f..3224362169bc418 100644 --- a/clang/lib/CodeGen/Targets/X86.cpp +++ b/clang/lib/CodeGen/Targets/X86.cpp @@ -1334,6 +1334,16 @@ class X86_64ABIInfo : public ABIInfo { return T.isOSLinux() || T.isOSNetBSD(); } + bool returnCXXRecordGreaterThan128InMem() const { + // Clang <= 20.0 did not do this. + if (getContext().getLangOpts().getClangABICompat() <= + LangOptions::ClangABI::Ver20) + return false; + + const llvm::Triple &T = getTarget().getTriple(); + return T.isOSLinux() || T.isOSNetBSD(); + } + X86AVXABILevel AVXLevel; // Some ABIs (e.g. X32 ABI and Native Client OS) use 32 bit pointers on // 64-bit hardware. @@ -2067,6 +2077,13 @@ void X86_64ABIInfo::classify(QualType Ty, uint64_t OffsetBase, Class &Lo, classify(I.getType(), Offset, FieldLo, FieldHi, isNamedArg); Lo = merge(Lo, FieldLo); Hi = merge(Hi, FieldHi); + if (returnCXXRecordGreaterThan128InMem() && + (Size > 128 && (Size != getContext().getTypeSize(I.getType()) || + Size > getNativeVectorSizeForAVXABI(AVXLevel)))) { + // The only case a 256(or 512)-bit wide vector could be used to return + // is when CXX record contains a single 256(or 512)-bit element. + Lo = Memory; + } if (Lo == Memory || Hi == Memory) { postMerge(Size, Lo, Hi); return; diff --git a/clang/test/CodeGen/X86/avx-cxx-record.cpp b/clang/test/CodeGen/X86/avx-cxx-record.cpp new file mode 100644 index 000000000000000..d8863ca4e45f9e5 --- /dev/null +++ b/clang/test/CodeGen/X86/avx-cxx-record.cpp @@ -0,0 +1,23 @@ +// RUN: %clang %s -S --target=x86_64-unknown-linux-gnu -emit-llvm -O2 -march=x86-64-v3 -o - | FileCheck %s + +using UInt64x2 = unsigned long long __attribute__((__vector_size__(16), may_alias)); + +template<int id> +struct XMM1 { + UInt64x2 x; +}; + +struct XMM2 : XMM1<0>, XMM1<1> { +}; + +// CHECK: define{{.*}} @_Z3foov({{.*}} [[ARG:%.*]]){{.*}} +// CHECK-NEXT: entry: +// CHECK-NEXT: store {{.*}}, ptr [[ARG]]{{.*}} +// CHECK-NEXT: [[TMP1:%.*]] = getelementptr {{.*}}, ptr [[ARG]]{{.*}} +// CHECK-NEXT: store {{.*}}, ptr [[TMP1]]{{.*}} +XMM2 foo() { + XMM2 result; + ((XMM1<0>*)&result)->x = UInt64x2{1, 2}; + ((XMM1<1>*)&result)->x = UInt64x2{3, 4}; + return result; +} >From 4fe3dda872bb6dca0599b979c1561fc92aac32f8 Mon Sep 17 00:00:00 2001 From: Pranav Kant <p...@google.com> Date: Mon, 3 Feb 2025 22:34:39 +0000 Subject: [PATCH 2/2] ABI change for all targets --- clang/lib/CodeGen/Targets/X86.cpp | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-) diff --git a/clang/lib/CodeGen/Targets/X86.cpp b/clang/lib/CodeGen/Targets/X86.cpp index 3224362169bc418..7e470abe078f595 100644 --- a/clang/lib/CodeGen/Targets/X86.cpp +++ b/clang/lib/CodeGen/Targets/X86.cpp @@ -1340,8 +1340,7 @@ class X86_64ABIInfo : public ABIInfo { LangOptions::ClangABI::Ver20) return false; - const llvm::Triple &T = getTarget().getTriple(); - return T.isOSLinux() || T.isOSNetBSD(); + return true; } X86AVXABILevel AVXLevel; _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
