Hi!

On Wed, Aug 05, 2020 at 04:40:16PM +0000, Christophe Leroy wrote:

quoted

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?)

quoted

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