Date: Sun, 6 Sep 2015 17:17:03 +0300 From: Solar Designer <solar@...nwall.com> To: john-dev@...ts.openwall.com Subject: Re: Large stack alignment On Sun, Sep 06, 2015 at 03:15:11PM +0200, magnum wrote: > On 2015-09-06 13:20, magnum wrote: > >So Lei reminded me of this: > >http://www.openwall.com/lists/john-dev/2015/08/24/8 > > > >We have an issue for changing all stack allocs using MEM_ALIGN_SIMD to > >align ourselves. > > This is fixed in 1bd8d9d. I think we (including me) still don't have adequate understanding of the problem. This commit fixes the instances where we were using gcc's alignment attributes, but it does not touch explicit vector variables on the stack such as SIMDmd5body()'s: vtype w[16*SIMD_PARA_MD5]; vtype a[SIMD_PARA_MD5]; vtype b[SIMD_PARA_MD5]; vtype c[SIMD_PARA_MD5]; vtype d[SIMD_PARA_MD5]; vtype tmp[SIMD_PARA_MD5]; vtype tmp2[SIMD_PARA_MD5]; vtype mask; We're hoping these will be in registers, but when not will they be properly aligned for AVX2? I guess this is just as dependent on stack pointer alignment and gcc's capabilities as uses of gcc's alignment attribute were. (And if so, the 1bd8d9d commit makes little sense.) Also, when gcc spills AVX2 registers to stack, does it ensure proper alignment? Or does it use unaligned-capable instructions? Or neither? We need to figure all of this out. A related issue are library callbacks when our code is called by a library that was compiled with a smaller -mpreferred-stack-boundary or with non-gcc (and for an ABI permitting a smaller stack alignment than we need). This may be the cause of some of the problems, such as what Lei and Jim saw with OpenMP (the threads are started via OpenMP runtime library), even when the program initially started with proper stack alignment (which is also dependent not only on gcc, but on dynamic linker and libc startup code). We might want to consider reading up on and using -mstackrealign or/and -mincoming-stack-boundary. And maybe we'd be able to revert the explicit alignment commits then, which as I suggested above are likely not a complete solution anyway. I expect that forcing gcc to realign the stack would have performance impact, though. And then there are non-gcc compilers. It's a mess. Alexander
Powered by blists - more mailing lists
Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.