Re: generated movaps with unaligned memory

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Mon, Feb 23, 2009 at 10:05 AM, Jiri Olsa <olsajiri@gmail.com> wrote:
> Hi,
>
> my shared library crashes with movaps instruction using not aligned memory.
>
> Since the shared library function is being called from dynamic linker, which
> basically prepares the memory location, I'm not sure whoose side issues this is.
>
> I have following function in C:
>
> typedef float La_x86_64_xmm __attribute__ ((__vector_size__ (16)));
>
> typedef struct La_x86_64_retval
> {
>  uint64_t lrv_rax;
>  uint64_t lrv_rdx;
>  La_x86_64_xmm lrv_xmm0;
>  La_x86_64_xmm lrv_xmm1;
>  long double lrv_st0;
>  long double lrv_st1;
> } La_x86_64_retval;
>
> unsigned int la_x86_64_gnu_pltexit (Elf64_Sym *__sym,
>               unsigned int __ndx, uintptr_t *__refcook, uintptr_t *__defcook,
>               const La_x86_64_regs *__inregs, La_x86_64_retval
> *__outregs, const char *symname)
> {
>       La_x86_64_xmm b __attribute__ ((aligned(16)));
>       b = __outregs->lrv_xmm0;
>       return 0;
> }
>
> this will endup in following assembly:
>
> 00000000000007d7 <la_x86_64_gnu_pltexit>:
>  7d7:   55                      push   %rbp
>  7d8:   48 89 e5                mov    %rsp,%rbp
>  7db:   48 89 7d e8             mov    %rdi,-0x18(%rbp)
>  7df:   89 75 e4                mov    %esi,-0x1c(%rbp)
>  7e2:   48 89 55 d8             mov    %rdx,-0x28(%rbp)
>  7e6:   48 89 4d d0             mov    %rcx,-0x30(%rbp)
>  7ea:   4c 89 45 c8             mov    %r8,-0x38(%rbp)
>  7ee:   4c 89 4d c0             mov    %r9,-0x40(%rbp)
>  7f2:   48 8b 45 c0             mov    -0x40(%rbp),%rax
>  7f6:   0f 28 40 10             movaps 0x10(%rax),%xmm0
>  7fa:   0f 29 45 f0             movaps %xmm0,-0x10(%rbp)
>  7fe:   b8 00 00 00 00          mov    $0x0,%eax
>  803:   c9                      leaveq
>  804:   c3                      retq
>
>
> Looks like xmm0 register is being used to transfer the data. However
> the structure's alignment is not 16, so it will crash.
>

Where exactly is it crashed? Which the structure isn't aligned at 16byte?



-- 
H.J.