Messages in this thread | | | Date | Thu, 11 Feb 2021 05:49:10 -0600 | From | Segher Boessenkool <> | Subject | Re: [PATCH] powerpc/bug: Remove specific powerpc BUG_ON() |
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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
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