[PATCH] D129726: [InstCombine] add a constraint for x <u minSigned --> x >s -1

[Chenbing.Zheng via Phabricator via llvm-commits]

Chenbing.Zheng created this revision.
Chenbing.Zheng added reviewers: spatel, RKSimon, benshi001.
Chenbing.Zheng added a project: LLVM.
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Chenbing.Zheng requested review of this revision.
Herald added subscribers: llvm-commits, jacquesguan.

Fold  x <u minSigned --> x >s -1 will break some combine, 
so we add a constraint for it.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D129726

Files:
  llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
  llvm/test/Transforms/InstCombine/icmp-shr.ll


Index: llvm/test/Transforms/InstCombine/icmp-shr.ll
===================================================================
--- llvm/test/Transforms/InstCombine/icmp-shr.ll
+++ llvm/test/Transforms/InstCombine/icmp-shr.ll
@@ -1146,12 +1146,9 @@
   ret i1 %r
 }
 
-; TODO: This should reduce to X != 0.
-
 define i1 @lshr_pow2_ult_smin(i8 %x) {
 ; CHECK-LABEL: @lshr_pow2_ult_smin(
-; CHECK-NEXT:    [[S:%.*]] = lshr i8 -128, [[X:%.*]]
-; CHECK-NEXT:    [[R:%.*]] = icmp sgt i8 [[S]], -1
+; CHECK-NEXT:    [[R:%.*]] = icmp ne i8 [[X:%.*]], 0
 ; CHECK-NEXT:    ret i1 [[R]]
 ;
   %s = lshr i8 128, %x
Index: llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -6169,8 +6169,10 @@
       return new ICmpInst(ICmpInst::ICMP_SLT, Op0, Zero);
     }
 
-    // For i32: x <u 2147483648 -> x >s -1  -> true if sign bit clear
-    if (Pred == ICmpInst::ICMP_ULT && C->isMinSignedValue()) {
+    // For i32: x <u 2147483648 -> x >s -1  -> true if sign bit clear,
+    // and exclude a case that can be matched by other patterns.
+    if (Pred == ICmpInst::ICMP_ULT && C->isMinSignedValue() &&
+        !match(Op0, m_LShr(m_SpecificInt(*C), m_Value()))) {
       Constant *AllOnes = Constant::getAllOnesValue(Op0->getType());
       return new ICmpInst(ICmpInst::ICMP_SGT, Op0, AllOnes);
     }


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