On Thu, Feb 11, 2021 at 07:41:52AM +0000, Christophe Leroy wrote:
> powerpc BUG_ON() is based on using twnei or tdnei instruction,
> which obliges gcc to format the condition into a 0 or 1 value
> in a register.
Huh? Why is that?
Will it work better if this used __builtin_trap? Or does the kernel only
detect very specific forms of trap instructions?
> By using a generic implementation, gcc will generate a branch
> to the unconditional trap generated by BUG().
That is many more instructions than ideal.
> As modern powerpc implement branch folding, that's even more efficient.
What PowerPC cpus implement branch folding? I know none.
Some example code generated via __builtin_trap:
void trap(void) { __builtin_trap(); }
void trap_if_0(int x) { if (x == 0) __builtin_trap(); }
void trap_if_not_0(int x) { if (x != 0) __builtin_trap(); }
-m64:
trap:
trap
trap_if_0:
tdeqi 3,0
blr
trap_if_not_0:
tdnei 3,0
blr
-m32:
trap:
trap
trap_if_0:
tweqi 3,0
blr
trap_if_not_0:
twnei 3,0
blr
Segher
