On Fri, May 08, 2015 at 10:50:49AM -0700, Nick Lewycky wrote: > On 6 May 2015 at 18:03, Michael Niedermayer <michae...@gmx.at> wrote: > > > On Wed, May 06, 2015 at 04:08:09PM -0700, Nick Lewycky wrote: > > > On 6 May 2015 at 15:06, Michael Niedermayer <michae...@gmx.at> wrote: > > > > > > > Hi > > > > > > > > On Wed, May 06, 2015 at 11:52:59AM -0700, Nick Lewycky wrote: > > > > > Newer versions of clang will allocate %rbx as one of the inline asm > > > > inputs, > > > > > > > > Thats great > > > > > > > > > > > > > even in PIC. This occurs when building ffmpeg with clang > > > > -fsanitize=address > > > > > -O1 -fPIE. Because the asm does clobber %bx whether PIC is on or off, > > > > just > > > > > include %bx in the clobber list regardless of whether PIC is on or > > off. > > > > > > > > you cant include *bx in the clobber list with PIC > > > > it breaks compilers that are less great, like gcc > > > > > > > > > > Doh!! I did check for this, but only tried x86-64, not x86-32. Sorry! > > > > > > gcc (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3 > > > > ../libswscale/x86/hscale_fast_bilinear_simd.c: In function > > > > ‘ff_hyscale_fast_mmxext’: > > > > ../libswscale/x86/hscale_fast_bilinear_simd.c:205:5: error: PIC > > register > > > > clobbered by ‘%ebx’ in ‘asm’ > > > > ../libswscale/x86/hscale_fast_bilinear_simd.c: In function > > > > ‘ff_hcscale_fast_mmxext’: > > > > ../libswscale/x86/hscale_fast_bilinear_simd.c:276:5: error: PIC > > register > > > > clobbered by ‘%ebx’ in ‘asm’ > > > > > > > > > > > > also what exactly are you trying to fix ? > > > > or rather what exactly goes how exactly wrong with the code as it is > > > > if rbx is used ? > > > > > > > > > > Ok, let's look at ff_hcscale_fast_mmext. Preprocessor directives > > evaluated > > > in PIC x86-64, the inline constraints work out to: > > > > > > :: "m" (src1), "m" (dst1), "m" (filter), "m" (filterPos), > > > "m" (mmxextFilterCode), "m" (src2), "m"(dst2) > > > ,"m" (ebxsave) > > > ,"m"(retsave) > > > : "%"REG_a, "%"REG_c, "%"REG_d, "%"REG_S, "%"REG_D > > > > > > so clang looks at that and decides that it can pick src1 = (%r10), dst1 = > > > (%r8), filter = (%r11), filterPos = (%r12), mmxextFilterCode = (%r15), > > src2 > > > = (%rbx), dst2 = (%r14), ebxsave = (%r13), retsave = (%r9). The problem > > > there is src2 being (%rbx). > > > > > > Now let's look at how we use them: > > > > > > "mov %0, %%"REG_c" \n\t" > > > "mov %1, %%"REG_D" \n\t" > > > "mov %2, %%"REG_d" \n\t" > > > "mov %3, %%"REG_b" \n\t" // Clobbers %rbx / > > src2 > > > / %5 here > > > "xor %%"REG_a", %%"REG_a" \n\t" > > > PREFETCH" (%%"REG_c") \n\t" > > > PREFETCH" 32(%%"REG_c") \n\t" > > > PREFETCH" 64(%%"REG_c") \n\t" > > > > > > CALL_MMXEXT_FILTER_CODE > > > CALL_MMXEXT_FILTER_CODE > > > CALL_MMXEXT_FILTER_CODE > > > CALL_MMXEXT_FILTER_CODE > > > "xor %%"REG_a", %%"REG_a" \n\t" > > > "mov %5, %%"REG_c" \n\t" // %5 is read too > > late, > > > we get %3 / filterPos instead > > > "mov %6, %%"REG_D" \n\t" > > > PREFETCH" (%%"REG_c") \n\t" > > > PREFETCH" 32(%%"REG_c") \n\t" > > > PREFETCH" 64(%%"REG_c") \n\t" > > > > > > CALL_MMXEXT_FILTER_CODE // Crash... > > > > > > The two marked instructions are intended to refer to different contents. > > > CALL_MMXEXT_FILTER_CODE moves RBX to ESI and calls mmxextFilterCode: > > > > > > "movl (%%"REG_b"), %%esi \n\t"\ > > > "call *%4 \n\t"\ // Crash... > > > > > > That callee ultimately segfaults: > > > > > > => 0x7fffefa45000: movq (%rdx,%rax,1),%mm3 > > > => 0x7fffefa45004: movd (%rcx,%rsi,1),%mm0 > > > Program received signal SIGSEGV, Segmentation fault. > > > 0x00007fffefa45004 in ?? () > > > => 0x7fffefa45004: movd (%rcx,%rsi,1),%mm0 > > > > > > I'm trying to fix this segfault. > > > > > > also ideally changes to this code should be tested against gcc/clang/icc > > > > of various versions with and without PIC, 32 and 64 bit > > > > this code is more tricky than than the average so theres a good > > > > change changes to it will break some compiler > > > > > > > > > > I understand that, but I may not be able to test as many environments as > > > you like. I'm going to give testing it my best effort, but I can tell you > > > up front that I'm only going to test gcc and clang, on an x86 Ubuntu > > > machine. I don't have ICC, so I can't test that. > > > > iam not sure it would work but, configure could easily test if > > ebx can be put on the clobber list and that then could be used > > in place of PIC in the checks > > > > I checked gcc 4.4, 4.6, 4.7, 4.8, and clang 3.3, 3.4, 3.5 in 32 and 64-bit > builds, and PIC vs. not, cross product thereof. The only one that emits > this error is all versions of gcc in 32-bit mode with PIC enabled. On > 64-bit, gcc and clang do the same thing, they'll spill BX if they want to > preserve it. > > Based on that, I decided we could sink the manual saving and restoring of > ebx down to 32-bit+PIC mode, and just list it as a clobber in 64-bit mode > regardless of the PIC setting. That works in every compiler and mode I > tried. > > Tested with make + make check + make fate but not "make fake SAMPLES=...". > Updated patch attached! Let me know what you think. > > and thanks for the detailed informaion > > > > No problem, thanks for the review!
please provide a git patch with commit message [...] > int i; > +#if ARCH_X86_64 > + uint64_t retsave; > +#else > #if defined(PIC) > uint64_t ebxsave; > #endif > -#if ARCH_X86_64 > - uint64_t retsave; > #endif > > __asm__ volatile( > -#if defined(PIC) > - "mov %%"REG_b", %5 \n\t" > #if ARCH_X86_64 > "mov -8(%%rsp), %%"REG_a" \n\t" > - "mov %%"REG_a", %6 \n\t" > -#endif > + "mov %%"REG_a", %5 \n\t" // retsave > #else > -#if ARCH_X86_64 > - "mov -8(%%rsp), %%"REG_a" \n\t" > - "mov %%"REG_a", %5 \n\t" > +#if defined(PIC) > + "mov %%"REG_b", %5 \n\t" // ebxsave > #endif > #endif > "pxor %%mm7, %%mm7 \n\t" > @@ -256,28 +253,25 @@ void ff_hyscale_fast_mmxext(SwsContext *c, int16_t *dst, > CALL_MMXEXT_FILTER_CODE > CALL_MMXEXT_FILTER_CODE > > -#if defined(PIC) > - "mov %5, %%"REG_b" \n\t" > -#if ARCH_X86_64 > - "mov %6, %%"REG_a" \n\t" > - "mov %%"REG_a", -8(%%rsp) \n\t" > -#endif > -#else > #if ARCH_X86_64 > "mov %5, %%"REG_a" \n\t" > "mov %%"REG_a", -8(%%rsp) \n\t" > +#else > +#if defined(PIC) > + "mov %5, %%"REG_b" \n\t" > #endif > #endif > :: "m" (src), "m" (dst), "m" (filter), "m" (filterPos), > "m" (mmxextFilterCode) > +#if ARCH_X86_64 > + ,"m"(retsave) > +#else > #if defined(PIC) > ,"m" (ebxsave) > #endif > -#if ARCH_X86_64 > - ,"m"(retsave) > #endif > : "%"REG_a, "%"REG_c, "%"REG_d, "%"REG_S, "%"REG_D > -#if !defined(PIC) > +#if ARCH_X86_64 || !defined(__PIC__) this mixes __PIC__ and PIC [...] -- Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB Into a blind darkness they enter who follow after the Ignorance, they as if into a greater darkness enter who devote themselves to the Knowledge alone. -- Isha Upanishad
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