stack realignment

[Marcus Meissner]

Hi,

(as small heads up)
As we have this problem with the Win64 code calling us with non-16 byte aligned stacks,
the gcc folks have now commited code to trunk gcc that allows -mincoming-stack-boundary=3
on October 7th.

Uros also plans a backport for gcc 5.

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66697

$ cat xx.c
#include <stdio.h>
int f(int a,int b) {
        printf("a=%d, b=%d\n", a, b);
}


-O2 build:
0000000000000000 <f>:
   0:   89 f2                   mov    %esi,%edx
   2:   31 c0                   xor    %eax,%eax
   4:   89 fe                   mov    %edi,%esi
   6:   bf 00 00 00 00          mov    $0x0,%edi
   b:   e9 00 00 00 00          jmpq   10 <f+0x10>


-O2 -mincoming-stack-boundary=3
0000000000000000 <f>:
   0:   4c 8d 54 24 08          lea    0x8(%rsp),%r10
   5:   48 83 e4 f0             and    $0xfffffffffffffff0,%rsp
   9:   89 f2                   mov    %esi,%edx
   b:   31 c0                   xor    %eax,%eax
   d:   89 fe                   mov    %edi,%esi
   f:   bf 00 00 00 00          mov    $0x0,%edi
  14:   41 ff 72 f8             pushq  -0x8(%r10)
  18:   55                      push   %rbp
  19:   48 89 e5                mov    %rsp,%rbp
  1c:   41 52                   push   %r10
  1e:   48 83 ec 08             sub    $0x8,%rsp
  22:   e8 00 00 00 00          callq  27 <f+0x27>
  27:   48 83 c4 08             add    $0x8,%rsp
  2b:   41 5a                   pop    %r10
  2d:   5d                      pop    %rbp
  2e:   49 8d 62 f8             lea    -0x8(%r10),%rsp
  32:   c3                      retq   

It is more code emitted, but we can at least fix it without generating our own thunks.

Ciao, Marcus