Add option to use older clang ABI behavior when passing certain union types as function arguments

Recently commit D78699 (commit 26cfb6e562), fixed clang's behavior with respect
to passing a union type through a register to correctly follow the ABI. However,
this is an ABI breaking change with earlier versions of the clang compiler, so we
should add an -fclang-abi-compat option to address this. Additionally, the PS4 ABI
requires the older behavior, so that is added as well.

This change adds a Ver11 value to the ClangABI enum that when it is set (or the
target is the PS4 triple), we skip the ABI fix introduced in D78699.

Differential Revision: https://reviews.llvm.org/D89747

clang/include/clang/Basic/LangOptions.h

@@ -153,6 +153,11 @@ public:
     /// NetBSD.
     Ver9,
 
+    /// Attempt to be ABI-compatible with code generated by Clang 11.0.x
+    /// (git  2e10b7a39b93). This causes clang to pass unions with a 256-bit
+    /// vector member on the stack instead of using registers.
+    Ver11,
+
     /// Conform to the underlying platform's C and C++ ABIs as closely
     /// as we can.
     Latest

clang/lib/CodeGen/TargetInfo.cpp

@@ -3061,7 +3061,11 @@ void X86_64ABIInfo::classify(QualType Ty, uint64_t OffsetBase,
 
     // Classify the fields one at a time, merging the results.
     unsigned idx = 0;
-    bool IsUnion = RT->isUnionType();
+    bool UseClang11Compat = getContext().getLangOpts().getClangABICompat() <=
+                                LangOptions::ClangABI::Ver11 ||
+                            getContext().getTargetInfo().getTriple().isPS4();
+    bool IsUnion = RT->isUnionType() && !UseClang11Compat;
+
     for (RecordDecl::field_iterator i = RD->field_begin(), e = RD->field_end();
            i != e; ++i, ++idx) {
       uint64_t Offset = OffsetBase + Layout.getFieldOffset(idx);

clang/lib/Frontend/CompilerInvocation.cpp

@@ -3460,6 +3460,8 @@ static void ParseLangArgs(LangOptions &Opts, ArgList &Args, InputKind IK,
         Opts.setClangABICompat(LangOptions::ClangABI::Ver7);
       else if (Major <= 9)
         Opts.setClangABICompat(LangOptions::ClangABI::Ver9);
+      else if (Major <= 11)
+        Opts.setClangABICompat(LangOptions::ClangABI::Ver11);
     } else if (Ver != "latest") {
       Diags.Report(diag::err_drv_invalid_value)
           << A->getAsString(Args) << A->getValue();

clang/test/CodeGen/X86/avx-union.c

@@ -1,5 +1,11 @@
 // RUN: %clang_cc1 -w -ffreestanding -triple x86_64-linux-gnu -target-feature +avx -emit-llvm -o - %s | FileCheck %s --check-prefix=CHECK --check-prefix=AVX
 // RUN: %clang_cc1 -w -ffreestanding -triple x86_64-linux-gnu -target-feature +avx512f -emit-llvm -o - %s | FileCheck %s --check-prefix=CHECK --check-prefix=AVX512
+
+// Test Clang 11 and earlier behavior
+// RUN: %clang_cc1 -w -ffreestanding -triple x86_64-linux-gnu -target-feature +avx -fclang-abi-compat=10.0 -emit-llvm -o - %s | FileCheck %s --check-prefix=AVX --check-prefix=CHECK-LEGACY
+// RUN: %clang_cc1 -w -ffreestanding -triple x86_64-linux-gnu -target-feature +avx512f -fclang-abi-compat=11.0 -emit-llvm -o - %s | FileCheck %s --check-prefix=CHECK-LEGACY --check-prefix=AVX512-LEGACY
+// RUN: %clang_cc1 -w -ffreestanding -triple x86_64-scei-ps4 -target-feature +avx -emit-llvm -o - %s | FileCheck %s --check-prefix=CHECK-LEGACY --check-prefix=AVX-LEGACY
+
 // This tests verifies that a union parameter should pass by a vector regitster whose first eightbyte is SSE and the other eightbytes are SSEUP.
 
 typedef int __m256 __attribute__ ((__vector_size__ (32)));
@@ -21,8 +27,10 @@ union M256 m1;
 union M512 m2;
 // CHECK-LABEL:   define void @test()
 // CHECK:         call void @foo1(<4 x double>
+// CHECK-LEGACY:  call void @foo1(%union.M256* byval(%union.M256) align 32
 // AVX:           call void @foo2(%union.M512* byval(%union.M512) align 64
 // AVX512:        call void @foo2(<8 x double>
+// AVX512-LEGACY: call void @foo2(%union.M512* byval(%union.M512) align 64
 void test() {
   foo1(m1);
   foo2(m2);