https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97445
--- Comment #43 from Christophe Leroy <christophe.leroy at csgroup dot eu> ---
(In reply to Christophe Leroy from comment #42)
> (In reply to Jakub Jelinek from comment #41)
> > It is documented to be undefined:
> > -- Built-in Function: int __builtin_clz (unsigned int x)
> > Returns the number of leading 0-bits in X, starting at the most
> > significant bit position. If X is 0, the result is undefined.
> > Especially GCC 11 (but e.g. clang too) will e.g. during value range
> > propagation assume that e.g. the builtin return value will be only 0 to 31,
> > not to 32, etc.
> > The portable way how to write this is x ? __builtin_clz (x) :
> > whatever_value_you_want_for_clz_0;
> > and the compiler should recognize that and if the instruction is well
> > defined for 0 and matches your choice, use optimal sequence.
>
> int f(int x)
> {
> return x ? __builtin_clz(x) : 32;
> }
>
> Is compiled into (with -O2):
>
> 00000000 <f>:
> 0: 2c 03 00 00 cmpwi r3,0
> 4: 40 82 00 0c bne 10 <f+0x10>
> 8: 38 60 00 20 li r3,32
> c: 4e 80 00 20 blr
> 10: 7c 63 00 34 cntlzw r3,r3
> 14: 4e 80 00 20 blr
>
>
>
> Allthough
>
> int g(void)
> {
int g(int x)
{
return __builtin_clz(0);
}
Gives
00000018 <g>:
18: 38 60 00 20 li r3,32
1c: 4e 80 00 20 blr
