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From: "christophe.leroy at csgroup dot eu" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/97445] Some fonctions marked static inline in Linux kernel
 are not inlined
Date: Tue, 20 Oct 2020 14:24:55 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97445

--- 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.
>=20
> int f(int x)
> {
> 	return x ? __builtin_clz(x) : 32;
> }
>=20
> Is compiled into (with -O2):
>=20
> 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
>=20
>=20
>=20
> Allthough
>=20
> 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=