[PATCH] PR17473: Disallow LSR to perform non-invertible normalization

[Michael Zolotukhin]

Hi Andy,

I’ve run specs+test-suite with the same compiler (which aborts when LSR wants to discard a user), and got the following list of fails:

New Failures - Compile Time ▾
External/SPEC/CINT2000/254_gap/254_gap
External/SPEC/CINT2000/255_vortex/255_vortex
External/SPEC/CINT2006/400_perlbench/400_perlbench
External/SPEC/CINT95/147_vortex/147_vortex
MultiSource/Applications/ClamAV/clamscan
MultiSource/Benchmarks/7zip/7zip-benchmark
MultiSource/Benchmarks/mediabench/mpeg2/mpeg2dec/mpeg2decode

Then I tried a possible fix, that we discussed yesterday: I added normalization of the stride before subtracting/adding it during NormalizeAutodetect/Denormalize. However, that fixed only one fail, the one from ‘make check’, described in the previous mail. The fails in specs and tests from test-suite persisted.

I again looked at a couple of expressions, which we are failing to denormalize to the original version, and they were like this one:
   ORIGINAL ISE: 	{(trunc i64 (1 + %i.0.ph) to i32),+,1}<%while.cond>
   NORMALIZED ISE: 	{(trunc i64      %i.0.ph  to i32),+,1}<%while.cond>
   DENORMALIZED ISE: 	{(1 + (trunc i64 %i.0.ph to i32)),+,1}<%while.cond>

I.e. we didn’t move constants to trunc. I suppose this behavior is correct.

Thus, I think we could commit the original patch, and then I’ll send a patch, which adds step normalization. What do you think?

Thanks,
Michael

On Mar 10, 2014, at 5:55 PM, Andrew Trick <atrick at apple.com> wrote:

> 
> On Mar 10, 2014, at 5:12 PM, Michael Zolotukhin <mzolotukhin at apple.com> wrote:
> 
>> I’ve run 'make check-all’ with llvm_unreachable in the branch where we discard a user, and actually got some interesting result. There were two failing tests, one of which is obviously the just added test.
>> 
>> The second one is CodeGen/X86/lsr-normalization.ll, and here is what I got there:
>> 
>>    ORIGINAL ISE: 				{(100 /u {1,+,1}<%bb16>),+,(100 /u {1,+,1}<%bb16>)}<%bb25>
>>    NORMALIZED ISE: 			{((-1 * (100 /u {1,+,1}<%bb16>)) + (100 /u {0,+,1}<%bb16>)),+,(100 /u {0,+,1}<%bb16>)}<%bb25>
>>    DENORMALIZED BACK ISE: 	{((2 * (100 /u {1,+,1}<%bb16>)) + (-1 * (100 /u {2,+,1}<%bb16>))),+,(100 /u {1,+,1}<%bb16>)}<%bb25>
>> 
>> This could give an idea of what opportunities we could lose due to this change. Maybe we need to act so conservatively only in danger of overflow, i.e. when the original expression has sign-extensions/wrap-flags?
> 
> Your current fix is conservatively correct. However, it also caught a more pervasive bug (the original bug only had to do with overflow at the induction variable’s boundary). As a follow-up, I agree with your suggestion to fix normalization so that it recursively normalizes the stride before subtracting it from the initial value. Your normalize-denormalize round trip check should also be sufficient to test that fix (you should only be eliminating cases in which the check fails, not adding any new ones).
> 
> -Andy
> 
>> Thanks,
>> Michael
>> 
>> On Mar 10, 2014, at 5:01 PM, Andrew Trick <atrick at apple.com> wrote:
>> 
>>> 
>>> On Mar 10, 2014, at 5:00 PM, Eric Christopher <echristo at gmail.com> wrote:
>>> 
>>>>> As we discussed, once you have finished running the test-suite with a check
>>>>> to find out if any important cases are no longer normalized as expected,
>>>>> then I think you can commit. I know you've already verified performance, so
>>>>> this is just an extra sanity check.
>>>>> 
>>>> 
>>>> Any interesting results?
>>> 
>>> No performance changes, just fixing PR17473.
>>> 
>>> -Andy
>>> 
>>>> 
>>>> -eric
>>>> 
>>>>> Thanks.
>>>>> 
>>>>> -Andy
>>>>> 
>>>>> 
>>>>> Thanks,
>>>>> Michael
>>>>> 
>>>>> Thanks,
>>>>> Arnold
>>>>> 
>>>>> On Mar 10, 2014, at 2:56 PM, Michael Zolotukhin <mzolotukhin at apple.com>
>>>>> wrote:
>>>>> 
>>>>> Hi,
>>>>> 
>>>>> This is a fix for PR17473. The issue is that LSR performs normalization of
>>>>> detected IV users and sometimes wants to denormalize it back hoping to get
>>>>> the original expression. But normalization isn't always invertible, and the
>>>>> new expression might be not equivalent to the original one. In the test
>>>>> case, we were losing sign-extension in such transformation. The patch simply
>>>>> adds a check, if the transformation is invertible.
>>>>> 
>>>>> <pr17473.patch>
>>>>> 
>>>>> Ok for trunk?
>>>>> 
>>>>> Thanks,
>>>>> Michael
>>>>> 
>>>>> 
>>>>> 
>>>>> _______________________________________________
>>>>> llvm-commits mailing list
>>>>> llvm-commits at cs.uiuc.edu
>>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>>> 
>> 
> 

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