Re: [PATCH][4.3] Deprecate -ftrapv

[Richard Guenther <rguenther at suse dot de>]

On Fri, 29 Feb 2008, Mark Mitchell wrote:

> Richard Guenther wrote:
> > This is a request to depcrecate -ftrapv which has numerous problems
> > making it not work as expected since a long time.  As nobody seems
> > to have the energy to ensure it is working I propose to deprecate it
> > for GCC 4.3 to make it possible to remove it during 4.4 (and not
> > block removing libcall notes by it).
> 
> Why doesn't it work?  Can it be made to work relatively easily?  Do we need
> functionality like this for Ada or Java?

The tree optimizers do not recognize +-* of singed integers as
possibly trapping which causes functions to become const/pure that
should not and causes 'dead' code to be removed that should not.

We do expand +-* allowing LIBCALL_WIDEN which will for example
use DImode on x86_64 where the libgcc implementation then of course
traps on DImode overflow rather than whatever smaller mode we
were interested in.

We wrap the libcalls inside libcall notes using REG_EQUAL notes
which indicate the libcalls compute non-trapping +-* (there's no
RTX code for the trappingness), so we combine and simplify the
operations making the libcall possibly dead and remove it again.

Just to mention the few problems I stumbled over while looking
at PR19020 some time ago.

Richard.