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Re: Do not produce empty try-finally statements
- From: Jakub Jelinek <jakub at redhat dot com>
- To: Jan Hubicka <hubicka at ucw dot cz>
- Cc: gcc-patches at gcc dot gnu dot org, jason at redhat dot com
- Date: Mon, 20 Jan 2014 08:07:35 +0100
- Subject: Re: Do not produce empty try-finally statements
- Authentication-results: sourceware.org; auth=none
- References: <20140119224609 dot GB22057 at kam dot mff dot cuni dot cz>
- Reply-to: Jakub Jelinek <jakub at redhat dot com>
On Sun, Jan 19, 2014 at 11:46:09PM +0100, Jan Hubicka wrote:
> Bootstrapped/regtested x86_64-linux, OK?
This looks very wrong. By ignoring the clobbers in the cleanups you
ensure clobbers are hardly ever emitted, but the TRY/FINALLY is the way how
to make sure the clobbers appear in all the places where they are supposed
to be, so that DSE, expansion etc. can take advantage of them.
Just look at say:
struct A { char buf[64]; };
void foo (char *);
void test ()
{
{ A a; foo (a.buf); a.buf[6] = 1; a.buf[7] = 8; }
{ A a; foo (a.buf); a.buf[6] = 1; a.buf[7] = 8; }
{ A a; foo (a.buf); a.buf[6] = 1; a.buf[7] = 8; }
{ A a; foo (a.buf); a.buf[6] = 1; a.buf[7] = 8; }
{ A a; foo (a.buf); a.buf[6] = 1; a.buf[7] = 8; }
{ A a; foo (a.buf); a.buf[6] = 1; a.buf[7] = 8; }
{ A a; foo (a.buf); a.buf[6] = 1; a.buf[7] = 8; }
}
where with your patch we don't DSE the a.buf[6] and a.buf[7] stores anymore.
If optimize_clobbers isn't performed at -O0, perhaps we should consider
performing at at -O0 (just that and not the other EH optimizations)?
Jakub