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Date: Fri, 4 Dec 2020 12:33:17 -0600
From: Segher Boessenkool <segher@kernel.crashing.org>
To: Andrea Corallo <andrea.corallo@arm.com>
Cc: stefan@franke.ms, gcc-help@gcc.gnu.org
Subject: Re: Correct way to express to the compiler "this does not get
 clobbered"?
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On Fri, Dec 04, 2020 at 07:16:45PM +0100, Andrea Corallo via Gcc-help wrote:
> Segher Boessenkool <segher@kernel.crashing.org> writes:
> 
> > On Fri, Dec 04, 2020 at 10:52:17AM +0100, Andrea Corallo via Gcc-help wrote:
> >> stefan@franke.ms writes:
> >> I might open a bug but my understanding is that const is generally
> >> not used for optimizations.  Am I wrong?
> >
> > extern const int x = 42;
> > int f(void) { return x; }
> >
> > The code generated for f does not load the value for x from memory: it
> > returns 42 always.

> Are you suggesting we should treat this as a bug?

Huh?  No, I am just saying that const *is* used for optimisation, with a
dumb simple example.  Remove const from this code and you get different
generated machine code (that does load x from memory always).

If you think you have found a missing optimisation, please make a
self-contained demonstrator for that, and a file a PR?


Segher