Date: Sun, 12 Jul 2015 17:30:27 +0300 From: Solar Designer <solar@...nwall.com> To: john-dev@...ts.openwall.com Subject: Re: extend SIMD intrinsics On Fri, Jul 10, 2015 at 05:47:37PM +0800, Lei Zhang wrote: > On Jul 7, 2015, at 9:16 PM, Solar Designer <solar@...nwall.com> wrote: > > On Mon, Jul 06, 2015 at 11:15:41PM -0400, Alain Espinosa wrote: > >> In Visual C the difference of a simple assignment and a vload is that for the assignment the compiler generate an unaligned SIMD load instruction, and for vload it generates an aligned SIMD load with the usual restriction: if this memory access isn't aligned the required byte amount an exception is raised. In general the performance difference is negligible, if any. > > > > When I corrected my code (bitslice DES code in JtR) to > > make the alignment apparent to gcc, it stopped generating the unaligned > > load instructions, generating the aligned ones instead. > > Could you exemplify how you did it with some code? It's commit fd875ac13ba0b57b5c84157a440dd03fac714eb1: Added CC_CACHE_ALIGN to the declaration of DES_bs_all in the non-OpenMP case, per advice from Richard Biener in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51017#c11 and @sevenps tweeting "Different alignments in definition and declaration is undefined behavior (C11, alignas)" So it was a bug fix, and it only applied to non-OpenMP builds (where the struct was at a fixed address, and gcc's attribute was used to provide and then to declare alignment). I now think Alain's message suggests we may in fact need to use the load/store intrinsics. His code is probably multi-threaded, so he must have been accessing the vectors via pointers. gcc in this case uses aligned loads; per Alain, apparently Visual C uses unaligned loads. I don't know if we care about Visual C in particular, but it's a bad sign. And JimF has in fact been building jumbo with Visual C. 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.