[X86] Fix i386 struct and union parameter alignment

According to i386 System V ABI 2.1: Structures and unions assume the
alignment of their most strictly aligned component. But current
implementation always takes them as 4-byte aligned which will result
in incorrect code, e.g:

 1 #include <immintrin.h>
 2 typedef union {
 3         int d[4];
 4         __m128 m;
 5 } M128;
 6 extern void foo(int, ...);
 7 void test(void)
 8 {
 9   M128 a;
10   foo(1, a);
11   foo(1, a.m);
12 }

The first call (line 10) takes the second arg as 4-byte aligned while
the second call (line 11) takes the second arg as 16-byte aligned.
There is oxymoron for the alignment of the 2 calls because they should
be the same.

This patch fixes the bug by following i386 System V ABI and apply it to
Linux only since other System V OS (e.g Darwin, PS4 and FreeBSD) don't
want to spend any effort dealing with the ramifications of ABI breaks
at present.

Patch by Wei Xiao (wxiao3)

Differential Revision: https://reviews.llvm.org/D60748

llvm-svn: 361934

1 files changed, 11 insertions, 2 deletions

diff --git a/clang/lib/CodeGen/TargetInfo.cpp b/clang/lib/CodeGen/TargetInfo.cpp
index 24b7b9f97f9..4b96aa13d00 100644
--- a/clang/lib/CodeGen/TargetInfo.cpp
+++ b/clang/lib/CodeGen/TargetInfo.cpp
@@ -1010,6 +1010,7 @@ class X86_32ABIInfo : public SwiftABIInfo {
   bool IsWin32StructABI;
   bool IsSoftFloatABI;
   bool IsMCUABI;
+  bool IsLinuxABI;
   unsigned DefaultNumRegisterParameters;
 
   static bool isRegisterSize(unsigned Size) {
@@ -1076,6 +1077,7 @@ public:
       IsWin32StructABI(Win32StructABI),
       IsSoftFloatABI(SoftFloatABI),
       IsMCUABI(CGT.getTarget().getTriple().isOSIAMCU()),
+      IsLinuxABI(CGT.getTarget().getTriple().isOSLinux()),
       DefaultNumRegisterParameters(NumRegisterParameters) {}
 
   bool shouldPassIndirectlyForSwift(ArrayRef<llvm::Type*> scalars,
@@ -1492,8 +1494,15 @@ unsigned X86_32ABIInfo::getTypeStackAlignInBytes(QualType Ty,
   if (Align <= MinABIStackAlignInBytes)
     return 0; // Use default alignment.
 
-  // On non-Darwin, the stack type alignment is always 4.
-  if (!IsDarwinVectorABI) {
+  if (IsLinuxABI) {
+    // i386 System V ABI 2.1: Structures and unions assume the alignment of their
+    // most strictly aligned component.
+    //
+    // Exclude other System V OS (e.g Darwin, PS4 and FreeBSD) since we don't
+    // want to spend any effort dealing with the ramifications of ABI breaks.
+    return Align;
+  } else if (!IsDarwinVectorABI) {
+    // On non-Darwin and non-Linux, the stack type alignment is always 4.
     // Set explicit alignment, since we may need to realign the top.
     return MinABIStackAlignInBytes;
   }