[llvm] [InstCombine] Decompose more icmps into masks (PR #110836)

[via llvm-commits]

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@@ -100,29 +100,65 @@ llvm::decomposeBitTestICmp(Value *LHS, Value *RHS, CmpInst::Predicate Pred,
   switch (Pred) {
   default:
     llvm_unreachable("Unexpected predicate");
-  case ICmpInst::ICMP_SLT:
+  case ICmpInst::ICMP_SLT: {
     // X < 0 is equivalent to (X & SignMask) != 0.
-    if (!C.isZero())
-      return std::nullopt;
-    Result.Mask = APInt::getSignMask(C.getBitWidth());
-    Result.Pred = ICmpInst::ICMP_NE;
-    break;
+    if (C.isZero()) {
+      Result.Mask = APInt::getSignMask(C.getBitWidth());
+      Result.Cmp = APInt::getZero(C.getBitWidth());
+      Result.Pred = ICmpInst::ICMP_NE;
+      break;
+    }
+
+    APInt FlippedSign = C ^ APInt::getSignMask(C.getBitWidth());
+    if (FlippedSign.isPowerOf2()) {
+      // X s< 10000100 is equivalent to (X & 11111100 == 10000000)
+      Result.Mask = -FlippedSign;
+      Result.Cmp = APInt::getSignMask(C.getBitWidth());
+      Result.Pred = ICmpInst::ICMP_EQ;
+      break;
+    }
+
+    if (FlippedSign.isNegatedPowerOf2()) {
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goldsteinn wrote:

Missing test for this case?

https://github.com/llvm/llvm-project/pull/110836