On 24.10.2016 16:14, Ronald S. Bultje wrote: > On Mon, Oct 24, 2016 at 8:47 AM, wm4 <nfx...@googlemail.com> wrote: >> On Mon, 24 Oct 2016 07:54:47 -0400 >> "Ronald S. Bultje" <rsbul...@gmail.com> wrote: >>> On Mon, Oct 24, 2016 at 3:36 AM, wm4 <nfx...@googlemail.com> wrote: >>>> I was under the impression that it is UB to have the FPU in MMX state >>>> at any time while in C, not just while e.g. calling the stdlib. Maybe I >>>> got that wrong (how would MMX intrinsics even work?) - can anyone shed >>>> light on the exact requirements? (Possibly again, sorry.) >>> >>> I'm under the impression that it's part of the calling convention. That >> is, >>> any code anywhere (including mmx intrinsics, indeed) can - when called - >>> expect the state to be cleared by the caller, just like you'd expect >>> eax/edx to be caller-save (whereas esi/edi are callee-save). >>> >>> However, if you never call external code (including intrinsics), you can >>> ignore this, just as you can ignore / create your own calling >>> convention (remember fastcall etc.?). However, when calling any external >>> code, this could (in theory) crash; it's just that right now it only >>> crashes with musl when calling malloc/free. So basically, ffmpeg has its >>> own calling convention, and manually calling emms_c() fixes "ffmpeg" >>> calling convention to be compatible with "standard" calling >> convention...? >> >> It can't really be a calling convention unless the compiler is aware of >> it?
It is defined as part of the System V Application Binary Interface [1]: "The CPU shall be in x87 mode upon entry to a function. Therefore, every function that uses the MMX registers is required to issue an emms or femms instruction after using MMX registers, before returning or calling another function." >> We're doing things behind the compiler's back here. The safest >> assumption would be that leaving the FPU state invalid while in C is >> not allowed, period. That's what the standard says. >> The next safest assumption is that it's fine as long as we explicitly >> don't use floating point or call external functions that aren't >> MMX-aware. This would mean calling any libc functions or user callbacks >> (including indirectly through e.g. av_log) is not fine. This is probably OK in practice and likely has a significant performance benefit. Best regards, Andreas 1: https://software.intel.com/sites/default/files/article/402129/mpx-linux64-abi.pdf _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
