This is the mail archive of the
gcc@gcc.gnu.org
mailing list for the GCC project.
Re: changing "configure" to default to "gcc -g -O2 -fwrapv ..."
- From: Ian Lance Taylor <iant at google dot com>
- To: kenner at vlsi1 dot ultra dot nyu dot edu (Richard Kenner)
- Cc: autoconf-patches at gnu dot org, bug-gnulib at gnu dot org, gcc at gcc dot gnu dot org
- Date: 31 Dec 2006 17:34:08 -0800
- Subject: Re: changing "configure" to default to "gcc -g -O2 -fwrapv ..."
- References: <200612300047.kBU0lFwk014817@localhost.localdomain> <45970416.80407@adacore.com> <8764bssikm.fsf@penguin.cs.ucla.edu> <571f6b510612310047r3b7ef9acl4a0ab082e88e17a3@mail.gmail.com> <87bqlkmm3r.fsf@penguin.cs.ucla.edu> <4aca3dc20612310641n78e8898euf1c003444e2303eb@mail.gmail.com> <87fyavhphc.fsf@penguin.cs.ucla.edu> <45980B43.7080301@adacore.com> <84fc9c000612311121x2ed1772fya44e59d373f0b57b@mail.gmail.com> <10612311945.AA05131@vlsi1.ultra.nyu.edu> <84fc9c000612311200p72f20f4g62beaa28cf2f920c@mail.gmail.com> <10612312045.AA06726@vlsi1.ultra.nyu.edu>
kenner@vlsi1.ultra.nyu.edu (Richard Kenner) writes:
> > Note that -fwrapv also _enables_ some transformations on signed
> > integers that are disabled otherwise. We for example constant fold
> > -CST for -fwrapv while we do not if signed overflow is undefined.
> > Would you change those?
>
> I don't understand the rationale for not wrapping constant folding when
> signed overflow is undefined: what's the harm in "defining" it as wrapping
> for that purpose? If it's undefined, then why does it matter what we
> fold it to? So we might as well fold it to what traditional code expects.
If flag_wrapv is false, we can't do any optimization which may
introduce signed overflow when it did not already exist. If we do,
future optimization passes may take advantage of the undefined signed
overflow to break the code unjustifiably. The problem is that we
don't have a way to annotate particular tree or RTL nodes as using
wrapping arithmetic. (This is something we will have to change for
LTO.)
Ian