[PATCH] D60748: Fix i386 struct and union parameter alignment

Wei Xiao via Phabricator via cfe-commits cfe-commits at lists.llvm.org
Mon Apr 15 19:35:45 PDT 2019

wxiao3 created this revision.
wxiao3 added reviewers: annita.zhang, LuoYuanke, smaslov, hjl.tools, RKSimon, rnk, andreadb.
wxiao3 added a project: clang.
Herald added a subscriber: cfe-commits.

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.

  rC Clang



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