[llvm-branch-commits] [clang] release/23.x: [Clang][X86] Introduce Clang ABI Gate for MSVC alignment (#210305) (PR #212122)

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<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: llvmbot

<details>
<summary>Changes</summary>

Backport cb383a37440d27238f8a01eee05228910d65d63e

Requested by: @<!-- -->zwuis

---
Full diff: https://github.com/llvm/llvm-project/pull/212122.diff


5 Files Affected:

- (modified) clang/include/clang/Basic/ABIVersions.def (+7-2) 
- (modified) clang/lib/Basic/Targets/X86.cpp (+19) 
- (modified) clang/lib/Basic/Targets/X86.h (+8) 
- (modified) clang/test/CodeGen/align-x68_64.c (+2) 
- (modified) clang/test/CodeGenCXX/ms-constexpr-static-data-member.cpp (+8) 


``````````diff
diff --git a/clang/include/clang/Basic/ABIVersions.def b/clang/include/clang/Basic/ABIVersions.def
index a9f72481ee0ea..967a409ca22bf 100644
--- a/clang/include/clang/Basic/ABIVersions.def
+++ b/clang/include/clang/Basic/ABIVersions.def
@@ -136,8 +136,13 @@ ABI_VER_MAJOR(20)
 ABI_VER_MAJOR(21)
 
 /// Attempt to be ABI-compatible with code generated by Clang 22.0.x.
-/// This causes clang to mangle lambdas in default member initializers of
-/// local classes as nested-name entities instead of local-name entities.
+/// This causes clang to:
+///   - Mangle lambdas in default member initializers of local classes as
+///     nested-name entities instead of local-name entities.
+///   - On x86_64 Windows, apply the System V psABI "large array" alignment
+///     rule instead of matching MSVC.
+///   - On x86_64 Windows, omit MSVC's size-based minimum alignment of global
+///     variables.
 ABI_VER_MAJOR(22)
 
 /// Conform to the underlying platform's C and C++ ABIs as closely as we can.
diff --git a/clang/lib/Basic/Targets/X86.cpp b/clang/lib/Basic/Targets/X86.cpp
index 7d8a74d62be74..8ab39b750dc99 100644
--- a/clang/lib/Basic/Targets/X86.cpp
+++ b/clang/lib/Basic/Targets/X86.cpp
@@ -1882,5 +1882,24 @@ MicrosoftX86_64TargetInfo::getMinGlobalAlign(uint64_t TypeSize,
   unsigned Align =
       WindowsX86_64TargetInfo::getMinGlobalAlign(TypeSize, HasNonWeakDef);
 
+  // Skip the MSVC size-based global-alignment increase under
+  // -fclang-abi-compat<=22.
+  if (!UseMSVCCompatGlobalAlign)
+    return Align;
+
   return std::max(Align, Microsoft64BitMinGlobalAlign(TypeSize));
 }
+
+void MicrosoftX86_64TargetInfo::adjust(DiagnosticsEngine &Diags,
+                                       LangOptions &Opts,
+                                       const TargetInfo *Aux) {
+  WindowsX86_64TargetInfo::adjust(Diags, Opts, Aux);
+  // Under -fclang-abi-compat<=22, restore the prior x86_64-windows-msvc
+  // behavior: apply the Sys V "large array" alignment increase and skip the
+  // MSVC size-based global-alignment increase.
+  if (Opts.isCompatibleWith(LangOptions::ClangABI::Ver22)) {
+    UseMSVCCompatGlobalAlign = false;
+    LargeArrayMinWidth = 128;
+    LargeArrayAlign = 128;
+  }
+}
diff --git a/clang/lib/Basic/Targets/X86.h b/clang/lib/Basic/Targets/X86.h
index e305d9017d897..27f33c9d03672 100644
--- a/clang/lib/Basic/Targets/X86.h
+++ b/clang/lib/Basic/Targets/X86.h
@@ -982,6 +982,14 @@ class LLVM_LIBRARY_VISIBILITY MicrosoftX86_64TargetInfo
 
   unsigned getMinGlobalAlign(uint64_t TypeSize,
                              bool HasNonWeakDef) const override;
+
+  void adjust(DiagnosticsEngine &Diags, LangOptions &Opts,
+              const TargetInfo *Aux) override;
+
+private:
+  // Whether to apply the MSVC size-based global-alignment scheme. Disabled by
+  // -fclang-abi-compat<=22 to restore prior x86_64-windows-msvc behavior.
+  bool UseMSVCCompatGlobalAlign = true;
 };
 
 // x86-64 MinGW target
diff --git a/clang/test/CodeGen/align-x68_64.c b/clang/test/CodeGen/align-x68_64.c
index 91f5fac199136..af20ffa3a4d1e 100644
--- a/clang/test/CodeGen/align-x68_64.c
+++ b/clang/test/CodeGen/align-x68_64.c
@@ -1,6 +1,8 @@
 // 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
+// RUN: %clang_cc1 -triple x86_64-windows-msvc -fclang-abi-compat=22 \
+// RUN:   -emit-llvm %s -o - | FileCheck %s
 // PR5599
 
 char arr[16];
diff --git a/clang/test/CodeGenCXX/ms-constexpr-static-data-member.cpp b/clang/test/CodeGenCXX/ms-constexpr-static-data-member.cpp
index 4b191fd472c20..623eb312ccf3c 100644
--- a/clang/test/CodeGenCXX/ms-constexpr-static-data-member.cpp
+++ b/clang/test/CodeGenCXX/ms-constexpr-static-data-member.cpp
@@ -1,4 +1,6 @@
 // RUN: %clang_cc1 -emit-llvm -triple=x86_64-windows-msvc %s -o - | FileCheck %s
+// RUN: %clang_cc1 -emit-llvm -triple=x86_64-windows-msvc \
+// RUN:   -fclang-abi-compat=22 %s -o - | FileCheck %s --check-prefix=COMPAT
 
 struct Foo { int x, y; };
 
@@ -24,3 +26,9 @@ void usethem() {
 // 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 8
+
+// COMPAT-DAG: @"?sdm_char_array at S@@2QBDB" = linkonce_odr dso_local constant [5 x i8] c"asdf\00", comdat, align 1
+
+// COMPAT-DAG: @"?sdm_char_ptr at S@@2QEBDEB" = linkonce_odr dso_local constant ptr @"??_C at _04JIHMPGLA@asdf?$AA@", comdat, align 8
+
+// COMPAT-DAG: @"?sdm_udt at S@@2UFoo@@B" = linkonce_odr dso_local constant %struct.Foo { i32 1, i32 2 }, comdat, align 4

``````````

</details>


https://github.com/llvm/llvm-project/pull/212122