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

[Nikita Popov via llvm-commits]

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@@ -7109,6 +7109,56 @@ Instruction *InstCombinerImpl::visitICmpInst(ICmpInst &I) {
     }
   }
 
+  // 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))) ||
+        (match(Op0, m_Value(X)) &&
+         match(Op1,
+               m_Intrinsic<Intrinsic::abs>(m_Specific(X), m_Constant(C))))) {
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nikic wrote:

You are matching the operands in swapped order here, but keep the old predicate. This will result in a miscompile (as your last test shows).

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