Hi! On Wed, Aug 05, 2020 at 04:40:16PM +0000, Christophe Leroy wrote: > >It cannot optimise it because it does not know shift < 32. The code > >below is incorrect for shift equal to 32, fwiw. > > Is there a way to tell it ? Sure, for example the &31 should work (but it doesn't, with the GCC version you used -- which version is that?) > >What does the compiler do for just > > > >static __always_inline u64 vdso_shift_ns(u64 ns, unsigned long shift) > > return ns >> (shift & 31); > >} > > > > Worse: I cannot make heads or tails of all that branch spaghetti, sorry. > 73c: 55 8c 06 fe clrlwi r12,r12,27 > 740: 7f c8 f0 14 addc r30,r8,r30 > 744: 7c c6 4a 14 add r6,r6,r9 > 748: 7c c6 e1 14 adde r6,r6,r28 > 74c: 34 6c ff e0 addic. r3,r12,-32 > 750: 41 80 00 70 blt 7c0 <__c_kernel_clock_gettime+0x114> This branch is always true. Hrm. Segher
