https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97445
--- Comment #42 from Christophe Leroy <christophe.leroy at csgroup dot eu> ---
(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)
{
