[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 accepted this revision.
andreadb added a comment.
This revision is now accepted and ready to land.

The change LGTM.

However, tests look a bit too fragile. If the goal is to verify the selection of BMI/TBM instructions, then tests should be made a bit more robust.
Future improvemenets may break some of those patterns; the code from some of those tests can be aggressively simplified...
I suggest to improve the tests first, and then commit this patch.
I think that we should probably raise a couple of bugs for missing simplifications. Most of those missing simplifications can be probably catched at IR level.



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Comment at: test/CodeGen/X86/bmi.ll:624-635
 ; X64-LABEL: blsi64_z2:
 ; X64:       # %bb.0:
 ; X64-NEXT:    movq %rsi, %rax
-; X64-NEXT:    movq %rdi, %rcx
-; X64-NEXT:    negq %rcx
-; X64-NEXT:    testq %rdi, %rcx
+; X64-NEXT:    blsiq %rdi, %rcx
 ; X64-NEXT:    cmovneq %rdx, %rax
 ; X64-NEXT:    retq
   %t0 = sub i64 0, %a
----------------
Again. Here we may prefer POPCNT to BLSI. It tends to have better latency/throughput overall. I think it is worthy to raise a bug for this.

Speaking about these tests in general:
I think that we should make these more robust (maybe in a separate patch). 

We can probably make this test more robust by changing how we check the result. For example, rather than comparing against zero, we can compare against a specific power-of-2. That would force the selection of BLSI, since we would need to know the position of that bit.

We can probably do something similar to improve the other test.


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Comment at: test/CodeGen/X86/bmi.ll:880-882
 ; X64-NEXT:    movl %esi, %eax
-; X64-NEXT:    # kill: def $edi killed $edi def $rdi
-; X64-NEXT:    leal -1(%rdi), %ecx
-; X64-NEXT:    testl %edi, %ecx
+; X64-NEXT:    blsrl %edi, %ecx
 ; X64-NEXT:    cmovnel %edx, %eax
----------------
Same.
Could be a simple `test+cmov`. But - again - I take that the purpose of this test is not to check if we are smart enough to fold away that sequence...


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https://reviews.llvm.org/D55870