linux-kernel - Re: static_branch/jump

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Message-ID: <YHA3iGyT5dMq7/06@hirez.programming.kicks-ass.net>
Date:   Fri, 9 Apr 2021 13:16:24 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Florian Weimer <fweimer@...hat.com>
Cc:     Ard Biesheuvel <ardb@...nel.org>, linux-toolchains@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jason Baron <jbaron@...mai.com>,
        "Steven Rostedt (VMware)" <rostedt@...dmis.org>
Subject: Re: static_branch/jump_label vs branch merging

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.