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Re: changing "configure" to default to "gcc -g -O2 -fwrapv ..."

From: Richard Guenther
Subject: Re: changing "configure" to default to "gcc -g -O2 -fwrapv ..."
Date: Tue, 2 Jan 2007 11:04:22 +0100

On 1/1/07, Geert Bosch <address@hidden> wrote:

On Dec 31, 2006, at 19:13, Daniel Berlin wrote:
> Note the distinct drop in performance across almost all the benchmarks
> on Dec 30, including popular programs like bzip2 and gzip.
Not so.

To my eyes, the specint 2000 mean went UP by about 1% for the
base -O3 compilation. The peak enabled more unrolling, which
is helped by additional range information provided by absence
of -frwapv.

So, I'd say this run would suggest enabling -fwrapv for
at least -O1 and -O2. Also, note that we never have
focussed on performance with -fwrapv, and it is quite
likely there is quite some improvement possible.

I'd really like using -fwrapv by default for -O, -O[s12].
The benefit of many programs moving from "undefined semantics"
to "implementation-defined semantics, overflow wraps like in
old compilers" far outweighs even an average performance loss
of 2% as seen in specfp.

I would support the proposal to enable -fwrapv for -O[01], but
not for -O2 as that is supposed to be "optimize for speed" and
as -O3 is not widely used to optimize for speed (in fact it may
make code slower).  I'm undecided for -Os but care less about it.


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