Re: [PATCH] powerpc/bug: Remove specific powerpc BUG_ON()

[Segher Boessenkool <segher@kernel.crashing.org>]

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