Re: static_branch/jump_label vs branch merging

[Peter Zijlstra]

On Fri, Apr 09, 2021 at 12:55:18PM +0200, Florian Weimer wrote:
> * Ard Biesheuvel:
> 
> > Wouldn't that require the compiler to interpret the contents of the
> > asm() block?
> 
> Yes and no.  It would require proper toolchain support, so in this case
> a new ELF relocation type, with compiler, assembler, and linker support
> to generate those relocations and process them.  As far as I understand
> it, the kernel doesn't do things this way.

I don't think that all is needed. All we need is for the compiler to
recognise that:

	if (cond) {
		stmt-A;
	}
	if (cond) {
		stmt-B;
	}

both cond are equivalent and hence can merge the blocks like:

	if (cond) {
		stmt-A;
		stmt-B;
	}

But because @cond is some super opaque asm crap, the compiler throws up
it's imaginry hands and says it cannot possibly tell and leaves them as
is.