[PATCH] D55870: [X86] Don't match TESTrr from (cmp (and X, Y), 0) during isel. Defer to post processing

[Andrea Di Biagio via Phabricator via llvm-commits]

andreadb added inline comments.


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Comment at: test/CodeGen/X86/bmi.ll:531-539
 ; X64-NEXT:    movl %esi, %eax
-; X64-NEXT:    movl %edi, %ecx
-; X64-NEXT:    negl %ecx
-; X64-NEXT:    testl %edi, %ecx
+; X64-NEXT:    blsil %edi, %ecx
 ; X64-NEXT:    cmovnel %edx, %eax
 ; X64-NEXT:    retq
   %t0 = sub i32 0, %a
   %t1 = and i32 %t0, %a
   %t2 = icmp eq i32 %t1, 0
----------------
craig.topper wrote:
> andreadb wrote:
> > This is a strange/interesting test.
> > 
> > If %a is zero, then %t1 is also zero.
> > If %a is not zero, then %t1 has exactly one bit set.
> > 
> > -->
> > 
> > Testing if %t1 is equal to 0, is equivalent to testing if %a is 0.
> > 
> > The only case where %t2 is TRUE, is if %a is 0.
> > This whole logic could be folded into a icmp + select. So we don't even need to select a BLSI.
> > 
> > This sequence should be optimized at IR level. I didn't test if it is what happens.
> > 
> > That being said. I take that the the purpose of this test was different. Probably, this test should be rewritten in a way that doesn't expose that simplification?
> > 
> The tests were intended to test use the Z flag from the BMI instructions.
I see.

In that case, then don't worry about changing those tests.

I think it is still worthy to raise a bug about teaching how to fold that computation at IR level. We keep these tests to verify that the Z flag is actually used.


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  https://reviews.llvm.org/D55870/new/

https://reviews.llvm.org/D55870