[PATCH] Transform C - zext(bool) -> bool ? C - 1 : C

[David Majnemer]

On Fri, Jul 19, 2013 at 3:50 PM, Evan Cheng <evan.cheng at apple.com> wrote:

> I don't have a problem with the patch. But is there a follow up patch to
> fix the lowering problem?
>
> Looking back at the history. It mentioned this impacts a couple of small
> test cases:
>
> int
> blah(_Bool x, _Bool y) {
>   return (x - y) + 1;
> }
>
> int
> blah2(bool x) {
>   return x ? 0 : -1;
> }
>

I'll take a look to see what is going on with those cases this weekend.


>
> BTW, David filed the bug report that motivated the patch.
>

Heh, funny that.


>
>

> Evan
>
> On Jul 19, 2013, at 2:10 PM, Nick Lewycky <nlewycky at google.com> wrote:
>
> Hi Evan, this is about your r159230,
> http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20120625/145470.html
>  . The canonicalization was added, then removed by you. In post-commit
> review, Eli asked you to pick one of the two forms to be canonical.
>
> I have no problem with David landing this patch as is -- it looks like
> goodness to me, particularly the "C - zext(X)" transform -- but I don't
> want to mess you up. Could you please chime in?
>
> Nick
>
>
> On 12 July 2013 16:03, David Majnemer <david.majnemer at gmail.com> wrote:
>
>> The attached patch essentially reverts r159230.
>>
>> The reasons why are because:
>> 1. It creates an asymmetry in the IR by hurting our ability to
>> canonicalize things like C - zext(X) and C + zext(!X) into the same thing.
>> 2. Adding it back does not break any tests and without it we still
>> generate the inferior code mentioned in the commit log.
>>
>> It seems like we should keep the canonicalization and fix (the apparently
>> still broken problem) in the lowering stage.
>> The commit mentioned that some targets don't have a sext i1. This
>> shouldn't be a concern at the IR level, should it?
>>
>> --
>> David Majnemer
>>
>> _______________________________________________
>> llvm-commits mailing list
>> llvm-commits at cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>
>
>
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