[llvm] r216523 - InstCombine: Optimize GEP's involving ptrtointbetter

[David Majnemer]

On Thu, Aug 28, 2014 at 2:44 PM, Nuno Lopes <nunoplopes at sapo.pt> wrote:

> On 27 August 2014 06:16, David Majnemer <david.majnemer at gmail.com> wrote:
>>
>>> Author: majnemer
>>> Date: Wed Aug 27 00:16:04 2014
>>> New Revision: 216523
>>>
>>> URL: http://llvm.org/viewvc/llvm-project?rev=216523&view=rev
>>> Log:
>>> InstCombine: Optimize GEP's involving ptrtoint better
>>>
>>
>>
>> Hi David,
>>
>> I've done a bisect and this commit broke out test-suite bot:
>>
>> http://lab.llvm.org:8011/builders/clang-native-arm-lnt/builds/8059
>>
>> The big problem is that the test "result" is just an MD5. :(
>>
>> I'm investigating.
>>
>> cheers,
>> --renato
>>
>
>
> I tried to specify this transformation in Alive.  The transformation
> supports more complex cases, but I get a failure even for the simplest case:
>
> $ ./alive.py < gep.opt
> Precondition: true
> %0 = ptrtoint i1* %Y
> %1 = sub i64 0, %0
> %r = getelementptr i1* %X, %1
>  =>
> %0 = ptrtoint i1* %Y
> %2 = ptrtoint * %X to i64
> %3 = sub %2, %0
> %r = inttoptr %3 to *
>
>
> ERROR: Mismatch in values of i1* %r
>
> Example:
> %Y i1* = 18446744073709551615 (0xffffffffffffffffL)
> %0 i64 = 18446744073709551615 (0xffffffffffffffffL)
> %1 i64 = 1 (0x1)
> %X i1* = 0 (0x0)
> %2 i64 = 0 (0x0)
> %3 i64 = 1 (0x1)
> Source value: 64 (0x40)
> Target value: 1 (0x1)
>
> Note that the counterexample is assuming pointer-size=8 bits.
> Let me know if I can help with debugging the transformation. (I may be
> missing some precondition; I just quickly glanced through your patch)



Hi Nuno,

I believe Alive isn't ready for getelementptr:

----------------------------------------
Optimization: 1
Precondition: true
%gep = getelementptr i8* %A, i64 1
%ret = ptrtoint i8* %gep to i64
  =>
%ptrtoint = ptrtoint i8* %A to i64
%ret = add i64 %ptrtoint, 1


ERROR: Mismatch in values of i64 %ret

Example:
%A i8* = 0 (0x0)
%gep i8* = 64 (0x40)
%ptrtoint i64 = 0 (0x0)
Source value: 64 (0x40)
Target value: 1 (0x1)

%gep should not be 64.



>
>
> Nuno
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20140828/c8d5989b/attachment.html>