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Date:   Wed, 10 Nov 2021 06:14:28 -0600
From:   Segher Boessenkool <segher@kernel.crashing.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        Bill Wendling <morbo@google.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, mark.rutland@arm.com,
        dvyukov@google.com, seanjc@google.com, pbonzini@redhat.com,
        mbenes@suse.cz, llvm@lists.linux.dev,
        linux-toolchains@vger.kernel.org
Subject: Re: [PATCH 20/22] x86,word-at-a-time: Remove .fixup usage
Message-ID: <20211110121428.GD614@gate.crashing.org>
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Hi!

On Tue, Nov 09, 2021 at 10:07:36PM +0100, Peter Zijlstra wrote:
> On Tue, Nov 09, 2021 at 11:22:44AM -0800, Nick Desaulniers wrote:
> > I think the use of this feature (label-attributes) here isn't
> > necessary though; because of the use of outputs, the "fallthrough"
> > basic block needs to be placed immediately after the basic block
> > terminated by the asm goto, at least in LLVM.  Was different ordering
> > of basic blocks observed with GCC without this label attribute?
> 
> GCC does the same, but I wanted to have the exception stuff be in
> .text.cold, but alas it doesn't do that. I left the attribute because of
> it's descriptive value.
> 
> >  Unless the cold attribute is helping move
> > ("shrink-wrap"?)

Shrink-wrapping is something else entirely.

>>  the basic block to a whole other section
> > (.text.cold.)?
> 
> I was hoping it would do that, but it doesn't on gcc-11.

A cold basic block can never dominate a non-cold basic block.  GCC will
fix things up when it notices this property is violated, so marking
random blocks as "cold" will not be very effective.


Segher