[llvm] [InstCombine] Simplify compare abs(X) and X. (PR #76385)

[Yingwei Zheng via llvm-commits]

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@@ -6860,6 +6860,54 @@ Instruction *InstCombinerImpl::foldICmpCommutative(ICmpInst::Predicate Pred,
       return foldICmpAddOpConst(X, *C, Pred);
   }
 
+  // abs(X) >=  X --> true
+  // abs(X) u<= X --> true
+  // abs(X) <   X --> false
+  // abs(X) u>  X --> false
+  // abs(X) u>= X --> IsIntMinPosion ? `X > -1`: `X u<= INTMIN`
+  // abs(X) <=  X --> IsIntMinPosion ? `X > -1`: `X u<= INTMIN`
+  // abs(X) ==  X --> IsIntMinPosion ? `X > -1`: `X u<= INTMIN`
+  // abs(X) u<  X --> IsIntMinPosion ? `X < 0` : `X >   INTMIN`
+  // abs(X) >   X --> IsIntMinPosion ? `X < 0` : `X >   INTMIN`
+  // abs(X) !=  X --> IsIntMinPosion ? `X < 0` : `X >   INTMIN`
+  {
+    Value *X;
+    Constant *C;
+    if ((match(Op0, m_Intrinsic<Intrinsic::abs>(m_Value(X), m_Constant(C))) &&
+         match(Op1, m_Specific(X)))) {
+      Value *NullValue = Constant::getNullValue(X->getType());
+      Value *AllOnesValue = Constant::getAllOnesValue(X->getType());
+      Value *MinVal = ConstantInt::get(
+          X->getType(),
+          APInt::getSignedMinValue(X->getType()->getScalarSizeInBits()));
+      bool IsIntMinPosion = C->isAllOnesValue();
+      switch (Pred) {
+      case CmpInst::ICMP_ULE:
+      case CmpInst::ICMP_SGE:
+        return replaceInstUsesWith(CxtI, ConstantInt::getTrue(CxtI.getType()));
+      case CmpInst::ICMP_UGT:
+      case CmpInst::ICMP_SLT:
+        return replaceInstUsesWith(CxtI, ConstantInt::getFalse(CxtI.getType()));
+      case CmpInst::ICMP_UGE:
+      case CmpInst::ICMP_SLE:
+      case CmpInst::ICMP_EQ: {
+        return replaceInstUsesWith(
+            CxtI, IsIntMinPosion ? Builder.CreateICmpSGT(X, AllOnesValue)
+                                 : Builder.CreateICmpULE(X, MinVal));
----------------
dtcxzyw wrote:

`icmp ule X, C` will be canonicalized into `icmp ult X, C+1`.


https://github.com/llvm/llvm-project/pull/76385