[clang] 8ecec45 - [clang] Fix x86_64-windows-msvc over- and under-alignment (#196505)
via cfe-commits
cfe-commits at lists.llvm.org
Mon May 11 22:55:40 PDT 2026
Author: Hans Wennborg
Date: 2026-05-12T07:55:36+02:00
New Revision: 8ecec455183fd42fc191089d061f80bdf7b158fc
URL: https://github.com/llvm/llvm-project/commit/8ecec455183fd42fc191089d061f80bdf7b158fc
DIFF: https://github.com/llvm/llvm-project/commit/8ecec455183fd42fc191089d061f80bdf7b158fc.diff
LOG: [clang] Fix x86_64-windows-msvc over- and under-alignment (#196505)
This fixes two issues where Clang was both over- and under-aligning
variables:
1) We were applying the x86_64 Sys V psABI "large array" alignment increase
(default when inheriting from X86_64TargetInfo), but MSVC doesn't follow
that ABI.
2) MSVC implements a similar scheme though, where it increases the
alignment of large objects. This is documented for ARM64 [1] and was
implemented in Clang b7c6d95af5e295c560d1445e7090e31eb9289932, but it
also applies to x86_64. ([2] says "MSVC does size (total size, not
element size) based alignment for global symbols on ARM64 *which is
copied from AMD64*").
This patch stops doing 1) and implements 2) for x86_64-windows-msvc.
[1]
https://learn.microsoft.com/en-us/cpp/build/arm64-windows-abi-conventions?view=msvc-170#alignment
[2] https://github.com/llvm/llvm-project/issues/40851
Fixes https://github.com/llvm/llvm-project/issues/196071
Fixes https://github.com/llvm/llvm-project/issues/171855
Added:
clang/test/CodeGen/microsoft-64bit-struct-align.cpp
Modified:
clang/include/clang/Basic/TargetInfo.h
clang/lib/Basic/TargetInfo.cpp
clang/lib/Basic/Targets/AArch64.cpp
clang/lib/Basic/Targets/X86.cpp
clang/lib/Basic/Targets/X86.h
clang/test/CodeGen/align-x68_64.c
clang/test/CodeGen/asan-strings.c
clang/test/CodeGenCXX/ms-constexpr-static-data-member.cpp
clang/test/CodeGenObjC/encode-test-6.m
clang/test/CodeGenSYCL/kernel-caller-entry-point.cpp
Removed:
clang/test/CodeGen/arm64-microsoft-struct-align.cpp
################################################################################
diff --git a/clang/include/clang/Basic/TargetInfo.h b/clang/include/clang/Basic/TargetInfo.h
index 9f7d2a17a0f8a..e21155b2e4fd4 100644
--- a/clang/include/clang/Basic/TargetInfo.h
+++ b/clang/include/clang/Basic/TargetInfo.h
@@ -1968,6 +1968,8 @@ class TargetInfo : public TransferrableTargetInfo,
void CheckFixedPointBits() const;
};
+unsigned Microsoft64BitMinGlobalAlign(uint64_t TypeSize);
+
namespace targets {
std::unique_ptr<clang::TargetInfo>
AllocateTarget(const llvm::Triple &Triple, const clang::TargetOptions &Opts);
diff --git a/clang/lib/Basic/TargetInfo.cpp b/clang/lib/Basic/TargetInfo.cpp
index e6ae89e0948c5..ad083242d9e3e 100644
--- a/clang/lib/Basic/TargetInfo.cpp
+++ b/clang/lib/Basic/TargetInfo.cpp
@@ -1114,3 +1114,20 @@ TargetInfo::simplifyConstraint(StringRef Constraint,
}
return Result;
}
+
+unsigned clang::Microsoft64BitMinGlobalAlign(uint64_t TypeSize) {
+ // MSVC does size based alignment for arm64 based on alignment section in
+ // below document. Replicate that to keep alignment consistent with object
+ // files compiled by MSVC.
+ // https://docs.microsoft.com/en-us/cpp/build/arm64-windows-abi-conventions
+ // The same is done for x64, but not documented.
+
+ if (TypeSize >= 512) // TypeSize >= 64 bytes
+ return 128; // align type at least 16 bytes
+ if (TypeSize >= 64) // TypeSize >= 8 bytes
+ return 64; // align type at least 8 bytes
+ if (TypeSize >= 16) // TypeSize >= 2 bytes
+ return 32; // align type at least 4 bytes
+
+ return 0;
+}
diff --git a/clang/lib/Basic/Targets/AArch64.cpp b/clang/lib/Basic/Targets/AArch64.cpp
index 9b951e69cce33..9afe6cb10729d 100644
--- a/clang/lib/Basic/Targets/AArch64.cpp
+++ b/clang/lib/Basic/Targets/AArch64.cpp
@@ -1815,18 +1815,7 @@ unsigned MicrosoftARM64TargetInfo::getMinGlobalAlign(uint64_t TypeSize,
unsigned Align =
WindowsARM64TargetInfo::getMinGlobalAlign(TypeSize, HasNonWeakDef);
- // MSVC does size based alignment for arm64 based on alignment section in
- // below document, replicate that to keep alignment consistent with object
- // files compiled by MSVC.
- // https://docs.microsoft.com/en-us/cpp/build/arm64-windows-abi-conventions
- if (TypeSize >= 512) { // TypeSize >= 64 bytes
- Align = std::max(Align, 128u); // align type at least 16 bytes
- } else if (TypeSize >= 64) { // TypeSize >= 8 bytes
- Align = std::max(Align, 64u); // align type at least 8 butes
- } else if (TypeSize >= 16) { // TypeSize >= 2 bytes
- Align = std::max(Align, 32u); // align type at least 4 bytes
- }
- return Align;
+ return std::max(Align, Microsoft64BitMinGlobalAlign(TypeSize));
}
MinGWARM64TargetInfo::MinGWARM64TargetInfo(const llvm::Triple &Triple,
diff --git a/clang/lib/Basic/Targets/X86.cpp b/clang/lib/Basic/Targets/X86.cpp
index cb941c94c84a7..60c001a826078 100644
--- a/clang/lib/Basic/Targets/X86.cpp
+++ b/clang/lib/Basic/Targets/X86.cpp
@@ -1852,3 +1852,12 @@ X86_64TargetInfo::getTargetBuiltins() const {
"__builtin_ia32_"},
};
}
+
+unsigned
+MicrosoftX86_64TargetInfo::getMinGlobalAlign(uint64_t TypeSize,
+ bool HasNonWeakDef) const {
+ unsigned Align =
+ WindowsX86_64TargetInfo::getMinGlobalAlign(TypeSize, HasNonWeakDef);
+
+ return std::max(Align, Microsoft64BitMinGlobalAlign(TypeSize));
+}
diff --git a/clang/lib/Basic/Targets/X86.h b/clang/lib/Basic/Targets/X86.h
index c7afcc7c86053..c8c5d280754b4 100644
--- a/clang/lib/Basic/Targets/X86.h
+++ b/clang/lib/Basic/Targets/X86.h
@@ -968,6 +968,8 @@ class LLVM_LIBRARY_VISIBILITY MicrosoftX86_64TargetInfo
: WindowsX86_64TargetInfo(Triple, Opts) {
LongDoubleWidth = LongDoubleAlign = 64;
LongDoubleFormat = &llvm::APFloat::IEEEdouble();
+ LargeArrayMinWidth = 0;
+ LargeArrayAlign = 0;
}
void getTargetDefines(const LangOptions &Opts,
@@ -981,6 +983,9 @@ class LLVM_LIBRARY_VISIBILITY MicrosoftX86_64TargetInfo
getCallingConvKind(bool ClangABICompat4) const override {
return CCK_MicrosoftWin64;
}
+
+ unsigned getMinGlobalAlign(uint64_t TypeSize,
+ bool HasNonWeakDef) const override;
};
// x86-64 MinGW target
diff --git a/clang/test/CodeGen/align-x68_64.c b/clang/test/CodeGen/align-x68_64.c
index cf128b43433ea..91f5fac199136 100644
--- a/clang/test/CodeGen/align-x68_64.c
+++ b/clang/test/CodeGen/align-x68_64.c
@@ -1,11 +1,21 @@
// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -emit-llvm %s -o - | FileCheck %s
+// RUN: %clang_cc1 -triple x86_64-windows-gnu -emit-llvm %s -o - | FileCheck %s
+// RUN: %clang_cc1 -triple x86_64-windows-msvc -emit-llvm %s -o - | FileCheck --check-prefix=MSVC %s
// PR5599
+char arr[16];
+
void test1_f(void *);
void test1_g(void) {
float x[4];
test1_f(x);
}
+// CHECK: @arr = {{.*}} align 16
// CHECK: @test1_g
// CHECK: alloca [4 x float], align 16
+
+// The "large array" alignment increase does not apply on windows-msvc.
+// MSVC: @arr = {{.*}} align 8
+// MSVC: @test1_g
+// MSVC: alloca [4 x float], align 4
diff --git a/clang/test/CodeGen/asan-strings.c b/clang/test/CodeGen/asan-strings.c
index 0c7420034f89e..e71445eaf7fe4 100644
--- a/clang/test/CodeGen/asan-strings.c
+++ b/clang/test/CodeGen/asan-strings.c
@@ -12,6 +12,6 @@ const char *foo(void) { return "asdf"; }
// LINUX: @.str = private unnamed_addr constant [5 x i8] c"asdf\00", align 1
-// WINDOWS: @"??_C at _04JIHMPGLA@asdf?$AA@" = linkonce_odr dso_local unnamed_addr constant [5 x i8] c"asdf\00", comdat, align 1
+// WINDOWS: @"??_C at _04JIHMPGLA@asdf?$AA@" = linkonce_odr dso_local unnamed_addr constant [5 x i8] c"asdf\00", comdat, align 4
-// WINWRITE: @.str = private unnamed_addr global [5 x i8] c"asdf\00", align 1
+// WINWRITE: @.str = private unnamed_addr global [5 x i8] c"asdf\00", align 4
diff --git a/clang/test/CodeGen/arm64-microsoft-struct-align.cpp b/clang/test/CodeGen/microsoft-64bit-struct-align.cpp
similarity index 80%
rename from clang/test/CodeGen/arm64-microsoft-struct-align.cpp
rename to clang/test/CodeGen/microsoft-64bit-struct-align.cpp
index 4076c3ca34ad7..c4e0f2f6dfc88 100644
--- a/clang/test/CodeGen/arm64-microsoft-struct-align.cpp
+++ b/clang/test/CodeGen/microsoft-64bit-struct-align.cpp
@@ -1,9 +1,9 @@
-// RUN: %clang_cc1 -triple aarch64-windows -ffreestanding -emit-llvm -O0 \
-// RUN: -x c++ -o - %s | FileCheck %s
+// RUN: %clang_cc1 -triple aarch64-windows-msvc -emit-llvm -o - %s | FileCheck %s
+// RUN: %clang_cc1 -triple x86_64-windows-msvc -emit-llvm -o - %s | FileCheck %s
struct size1 { char str[1]; };
struct size2 { char str[2]; };
-struct size7 { char str[4]; };
+struct size7 { char str[7]; };
struct size8 { char str[8]; };
struct size63 { char str[63]; };
struct size64 { char str[64]; };
diff --git a/clang/test/CodeGenCXX/ms-constexpr-static-data-member.cpp b/clang/test/CodeGenCXX/ms-constexpr-static-data-member.cpp
index 604a49fefbacb..4b191fd472c20 100644
--- a/clang/test/CodeGenCXX/ms-constexpr-static-data-member.cpp
+++ b/clang/test/CodeGenCXX/ms-constexpr-static-data-member.cpp
@@ -19,8 +19,8 @@ void usethem() {
useptr(&S::sdm_udt);
}
-// CHECK-DAG: @"?sdm_char_array at S@@2QBDB" = linkonce_odr dso_local constant [5 x i8] c"asdf\00", comdat, align 1
+// CHECK-DAG: @"?sdm_char_array at S@@2QBDB" = linkonce_odr dso_local constant [5 x i8] c"asdf\00", comdat, align 4
// CHECK-DAG: @"?sdm_char_ptr at S@@2QEBDEB" = linkonce_odr dso_local constant ptr @"??_C at _04JIHMPGLA@asdf?$AA@", comdat, align 8
-// CHECK-DAG: @"?sdm_udt at S@@2UFoo@@B" = linkonce_odr dso_local constant %struct.Foo { i32 1, i32 2 }, comdat, align 4
+// CHECK-DAG: @"?sdm_udt at S@@2UFoo@@B" = linkonce_odr dso_local constant %struct.Foo { i32 1, i32 2 }, comdat, align 8
diff --git a/clang/test/CodeGenObjC/encode-test-6.m b/clang/test/CodeGenObjC/encode-test-6.m
index c32f8f24c0009..dd770f523e2f3 100644
--- a/clang/test/CodeGenObjC/encode-test-6.m
+++ b/clang/test/CodeGenObjC/encode-test-6.m
@@ -80,6 +80,6 @@ @implementation SCNCamera
// CHECK-DWARF: define{{.*}} ptr @Test()
// CHECK-DWARF: ret ptr @e
-// CHECK-MSVC: @e = dso_local global [2 x i8] c"i\00", align 1
+// CHECK-MSVC: @e = dso_local global [2 x i8] c"i\00", align 4
// CHECK-MINGW: @e = dso_local global [2 x i8] c"i\00", align 1
// CHECK-ELF: @e = global [2 x i8] c"i\00", align 1
diff --git a/clang/test/CodeGenSYCL/kernel-caller-entry-point.cpp b/clang/test/CodeGenSYCL/kernel-caller-entry-point.cpp
index 410988e16acdc..528d27f85e54b 100644
--- a/clang/test/CodeGenSYCL/kernel-caller-entry-point.cpp
+++ b/clang/test/CodeGenSYCL/kernel-caller-entry-point.cpp
@@ -148,14 +148,14 @@ int main() {
// CHECK-HOST-LINUX @.str.5 = private unnamed_addr constant [30 x i8] c"_ZTS23fwd_ref_arg_kernel_name\00", align 1
// CHECK-HOST-LINUX: @.str.6 = private unnamed_addr constant [35 x i8] c"_ZTS28fwd_ref_arg_kernel_name_move\00", align 1
// CHECK-HOST-LINUX: @.str.7 = private unnamed_addr constant [33 x i8] c"_ZTS26rvalue_ref_arg_kernel_name\00", align 1
-// CHECK-HOST-WINDOWS: @"??_C at _0CB@KFIJOMLB at _ZTS26single_purpose_kernel_name@" = linkonce_odr dso_local unnamed_addr constant [33 x i8] c"_ZTS26single_purpose_kernel_name\00", comdat, align 1
-// CHECK-HOST-WINDOWS: @"??_C at _0BC@NHCDOLAA at _ZTSZ4mainEUlT_E_?$AA@" = linkonce_odr dso_local unnamed_addr constant [18 x i8] c"_ZTSZ4mainEUlT_E_\00", comdat, align 1
-// CHECK-HOST-WINDOWS: @"??_C at _0M@BCGAEMBE at _ZTS6?N?$LE?O?$IE?O?$IH?$AA@" = linkonce_odr dso_local unnamed_addr constant [12 x i8] c"_ZTS6\CE\B4\CF\84\CF\87\00", comdat, align 1
-// CHECK-HOST-WINDOWS: @"??_C at _0P@DLGHPODL at _ZTSZ4mainE2KN?$AA@" = linkonce_odr dso_local unnamed_addr constant [15 x i8] c"_ZTSZ4mainE2KN\00", comdat, align 1
-// CHECK-HOST-WINDOWS: @"??_C at _0BK@PPDJPOBM at _ZTS19ref_arg_kernel_name?$AA@" = linkonce_odr dso_local unnamed_addr constant [26 x i8] c"_ZTS19ref_arg_kernel_name\00", comdat, align 1
-// CHECK-HOST-WINDOWS: @"??_C at _0BO@KEIBIHKH at _ZTS23fwd_ref_arg_kernel_name?$AA@" = linkonce_odr dso_local unnamed_addr constant [30 x i8] c"_ZTS23fwd_ref_arg_kernel_name\00", comdat, align 1
-// CHECK-HOST-WINDOWS: @"??_C at _0CD@FDALJLMM at _ZTS28fwd_ref_arg_kernel_name_mo@" = linkonce_odr dso_local unnamed_addr constant [35 x i8] c"_ZTS28fwd_ref_arg_kernel_name_move\00", comdat, align 1
-// CHECK-HOST-WINDOWS: @"??_C at _0CB@HCPMABHM at _ZTS26rvalue_ref_arg_kernel_name@" = linkonce_odr dso_local unnamed_addr constant [33 x i8] c"_ZTS26rvalue_ref_arg_kernel_name\00", comdat, align 1
+// CHECK-HOST-WINDOWS: @"??_C at _0CB@KFIJOMLB at _ZTS26single_purpose_kernel_name@" = linkonce_odr dso_local unnamed_addr constant [33 x i8] c"_ZTS26single_purpose_kernel_name\00", comdat
+// CHECK-HOST-WINDOWS: @"??_C at _0BC@NHCDOLAA at _ZTSZ4mainEUlT_E_?$AA@" = linkonce_odr dso_local unnamed_addr constant [18 x i8] c"_ZTSZ4mainEUlT_E_\00", comdat
+// CHECK-HOST-WINDOWS: @"??_C at _0M@BCGAEMBE at _ZTS6?N?$LE?O?$IE?O?$IH?$AA@" = linkonce_odr dso_local unnamed_addr constant [12 x i8] c"_ZTS6\CE\B4\CF\84\CF\87\00", comdat
+// CHECK-HOST-WINDOWS: @"??_C at _0P@DLGHPODL at _ZTSZ4mainE2KN?$AA@" = linkonce_odr dso_local unnamed_addr constant [15 x i8] c"_ZTSZ4mainE2KN\00", comdat
+// CHECK-HOST-WINDOWS: @"??_C at _0BK@PPDJPOBM at _ZTS19ref_arg_kernel_name?$AA@" = linkonce_odr dso_local unnamed_addr constant [26 x i8] c"_ZTS19ref_arg_kernel_name\00", comdat
+// CHECK-HOST-WINDOWS: @"??_C at _0BO@KEIBIHKH at _ZTS23fwd_ref_arg_kernel_name?$AA@" = linkonce_odr dso_local unnamed_addr constant [30 x i8] c"_ZTS23fwd_ref_arg_kernel_name\00", comdat
+// CHECK-HOST-WINDOWS: @"??_C at _0CD@FDALJLMM at _ZTS28fwd_ref_arg_kernel_name_mo@" = linkonce_odr dso_local unnamed_addr constant [35 x i8] c"_ZTS28fwd_ref_arg_kernel_name_move\00", comdat
+// CHECK-HOST-WINDOWS: @"??_C at _0CB@HCPMABHM at _ZTS26rvalue_ref_arg_kernel_name@" = linkonce_odr dso_local unnamed_addr constant [33 x i8] c"_ZTS26rvalue_ref_arg_kernel_name\00", comdat
//
// CHECK-HOST-LINUX: define dso_local void @_Z26single_purpose_kernel_task21single_purpose_kernel() #{{[0-9]+}} {
// CHECK-HOST-LINUX-NEXT: entry:
More information about the cfe-commits
mailing list