[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, 25 insertions, 0 deletions

diff --git a/clang/test/CodeGen/x86_32-align-linux.c b/clang/test/CodeGen/x86_32-align-linux.c
new file mode 100644
index 00000000000..5fce3f5f295
--- /dev/null
+++ b/clang/test/CodeGen/x86_32-align-linux.c
@@ -0,0 +1,25 @@
+// RUN: %clang_cc1 -w -fblocks -ffreestanding -triple i386-pc-linux-gnu -emit-llvm -o %t %s
+// RUN: FileCheck < %t %s
+
+#include <immintrin.h>
+
+typedef union {
+        int d[4];
+        __m128 m;
+} M128;
+
+extern void foo(int, ...);
+
+M128 a;
+
+// CHECK-LABEL: define void @test
+// CHECK: entry:
+// CHECK: call void (i32, ...) @foo(i32 1, %union.M128* byval align 16
+// CHECK: call void (i32, ...) @foo(i32 1, <4 x float>
+
+void test(void)
+{
+  foo(1, a);
+  foo(1, a.m);
+}
+