Re: [Qemu-devel] [PATCH] tcg: Fix complilation with TCG

From: Paolo Bonzini <pbonzini@redhat.com>
To: Peter Maydell <peter.maydell@linaro.org>,
	Richard Henderson <richard.henderson@linaro.org>
Cc: Laurent Vivier <lvivier@redhat.com>, Peter Xu <peterx@redhat.com>,
	QEMU Developers <qemu-devel@nongnu.org>,
	"Dr. David Alan Gilbert" <dgilbert@redhat.com>,
	Juan Quintela <quintela@redhat.com>
Subject: Re: [Qemu-devel] [PATCH] tcg: Fix complilation with TCG
Date: Thu, 23 Nov 2017 23:08:27 +0100	[thread overview]
Message-ID: <d25e4d2f-2ca2-55fb-f314-8ed17da22b9b@redhat.com> (raw)
In-Reply-To: <CAFEAcA9zwnY5+=Em2gta7HVoKGv2EBS3UVH+zbwCzOCovyUVdg@mail.gmail.com>

On 22/11/2017 13:14, Peter Maydell wrote:
>> We do this all of the time for more regular tests that are obviously
>> compile-time.  I am a really big fan of this, because it makes sure that the
>> (usually 32-bit) else branch continues to compile.
> I'm happy with code that is "we assume the compiler is going to
> be sensible here so we don't have to use #ifdefs in the name
> of performance". I just don't think we should write code that
> can't compile at all if the compiler happens to not identify
> the codepath as dead. There's no guarantee by the compiler
> that it's going to do that.

There's plenty of cases where "if (kvm_enabled())" code would fail to
link if the compiler didn't do that optimization.

(In fact, it's one of the very few optimizations that the compiler
performs at -O0, because it's extremely effective).

Paolo