[Patch] PR 51938: extend ifcombine

[Marc Glisse]

On Wed, 20 Jun 2012, Marc Glisse wrote:

> On Wed, 20 Jun 2012, Richard Guenther wrote:
>
>> On Sun, Jun 10, 2012 at 4:16 PM, Marc Glisse <marc.glisse@inria.fr> wrote:
>>> Hello,
>>> 
>>> currently, tree-ssa-ifcombine handles pairs of imbricated "if"s that share
>>> the same then branch, or the same else branch. There is no particular 
>>> reason
>>> why it couldn't also handle the case where the then branch of one is the
>>> else branch of the other, which is what I do here.
>>> 
>>> Any comments?
>> 
>> The general idea looks good, but I think the patch is too invasive.  As far
>> as I can see the only callers with a non-zero 'inv' argument come from
>> ifcombine_ifnotorif and ifcombine_ifnotandif (and both with inv == 2).
>> I would rather see a more localized patch that makes use of
>> invert_tree_comparison to perform the inversion on the call arguments
>> of maybe_fold_and/or_comparisons.
>> 
>> Is there any reason that would not work?
>
> invert_tree_comparison is useless for floating point (the case I am most 
> interested in) unless we specify -fno-trapping-math (writing this patch 
> taught me to add this flag to my default flags, but I can't expect everyone 
> to do the same). An issue is that gcc mixes the behaviors of qnan and snan 
> (it is not really an issue, it just means that !(comparison) can't be 
> represented as comparison2).

Actually, what would you think of 
s/flag_trapping_math/flag_signaling_nans/
in invert_tree_comparison? Are there other possible ways a<b can trap than 
having a sNaN for a or b?

-- 
Marc Glisse