On Mon, Nov 15, 2021 at 11:20:07AM -0800, H. Peter Anvin wrote:
> [Cc: Peter Z.]
> 
> This seems totally bizarre... that is an *enormous* change, and if I'm
> reading it right it seems like this somehow related to the performance
> monitoring framework itself?
> 
> The lower-performance init code is all pushed into the pre-boot path, unless
> for some strange reason not all code gets patched e.g. at module loading
> time.
> 
> A quick peek around made me notice a few minor possibilities, but none of
> them look particularly sane:
> 
> 1. We don't use "asm inline" in asm_volatile_goto, and we probably
>    should; otherwise gcc might get the idea this is a more heavyweight
>    operation than it actually is.
> 2. There is a workaround in asm_volatile_goto for a bug which apparently
>    was fixed in gcc 4.8.x that might mislead gcc's code generator into
>    generating worse code.
> 
> Did you see any functions for which the code got *bigger*?

Urgh, that code uses _4_ static_cpu_has(X86_FEATURE_ARCH_LBR) which,
IIRC, GCC can't CSE. I've been asking for CSE on jump-labels for a
while, but that's not actually got me anywhere.

https://lore.kernel.org/all/YG80wg/2iZjXfCDJ(a)hirez.programming.kicks-ass.net/?q=static_branch%2Fjump_label+vs+branch+merging

Let me see if I can't re-arrange that code differently.