Le sunnuntaina 16. heinäkuuta 2023, 19.00.35 EEST Andreas Rheinhardt a écrit : > > @@ -113,19 +121,31 @@ __asm__ volatile(\ > > > > // avoid +32 for shift optimization (gcc should do that ...) > > #define NEG_SSR32 NEG_SSR32 > > static inline int32_t NEG_SSR32( int32_t a, int8_t s){ > > > > + if (__builtin_constant_p(s)) > > > > __asm__ ("sarl %1, %0\n\t" > > > > : "+r" (a) > > > > - : "ic" ((uint8_t)(-s)) > > + : "i" (-s & 0x1F) > > > > ); > > > > + else > > + __asm__ ("sarl %1, %0\n\t" > > + : "+r" (a) > > + : "c" ((uint8_t)(-s)) > > + ); > > > > return a; > > > > } > > > > #define NEG_USR32 NEG_USR32 > > static inline uint32_t NEG_USR32(uint32_t a, int8_t s){ > > > > + if (__builtin_constant_p(s)) > > > > __asm__ ("shrl %1, %0\n\t" > > > > : "+r" (a) > > > > - : "ic" ((uint8_t)(-s)) > > + : "i" (-s & 0x1F) > > > > ); > > > > + else > > + __asm__ ("shrl %1, %0\n\t" > > + : "+r" (a) > > + : "c" ((uint8_t)(-s)) > > + ); > > > > return a; > > > > } > > Does this have a performance or codesize impact?
It performs the masking at compilation time rather than run-time, thus saving one masking instruction and sparing an implicit clobber on ECX. However, since we are dealing with constants, we the shift amount could *presumably* just as well be fixed in the calling code. Not that I'd know. > And is the inline ASM actually any good? To be honest, even if it's good, it inhibits instruction scheduling by the compiler. So IMO the threshold should be for assembler to be strictly *better* than the C code. (And I don't know the answer to that question.) -- Rémi Denis-Courmont http://www.remlab.net/ _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org https://ffmpeg.org/mailman/listinfo/ffmpeg-devel To unsubscribe, visit link above, or email ffmpeg-devel-requ...@ffmpeg.org with subject "unsubscribe".