[PATCH] D66232: [InstCombine] Try to reuse constant from select in leading comparison

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri added inline comments.


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Comment at: llvm/test/Transforms/InstCombine/unrecognized_three-way-comparison.ll:92-106
 define i32 @compare_against_two(i32 %x) {
 ; CHECK-LABEL: @compare_against_two(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[TMP0:%.*]] = icmp sgt i32 [[X:%.*]], 2
-; CHECK-NEXT:    br i1 [[TMP0]], label [[CALLFOO:%.*]], label [[EXIT:%.*]]
+; CHECK-NEXT:    [[CMP1:%.*]] = icmp eq i32 [[X:%.*]], 2
+; CHECK-NEXT:    [[CMP2_INV:%.*]] = icmp sgt i32 [[X]], 1
+; CHECK-NEXT:    [[SELECT1:%.*]] = select i1 [[CMP2_INV]], i32 1, i32 -1
+; CHECK-NEXT:    [[SELECT2:%.*]] = select i1 [[CMP1]], i32 0, i32 [[SELECT1]]
----------------
xbolva00 wrote:
> lebedev.ri wrote:
> > This is clearly a regression.
> > I suppose it means this fold happens before some other fold that used to happen,
> > and that fold does not know how to deal with the new pattern.
> > I'd prefer to look into this afterwards.
> No status update here? Or did you find what is broken?
> 
> Patch looks fine I think, but this worries me -   we may realize later that the root cause of regression is possibly nontrivial thing which could lead us to rever this one. (and wasted time).
Added some tests in rL369667.
Whatever handles this is "simply" ignorant about commutativity.


Repository:
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  https://reviews.llvm.org/D66232/new/

https://reviews.llvm.org/D66232