[llvm] [SelectionDAG] Detect impossible conditions using known bits analysis (PR #150715)

[Craig Topper via llvm-commits]

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@@ -13529,6 +13529,58 @@ SDValue DAGCombiner::visitSETCC(SDNode *N) {
   SDValue N0 = N->getOperand(0), N1 = N->getOperand(1);
   SDLoc DL(N);
 
+  // Detect impossible conditions using known bits analysis.
+  if (N1.getOpcode() == ISD::Constant) {
+    ConstantSDNode *N1C = cast<ConstantSDNode>(N1);
+    APInt C1 = N1C->getAPIntValue();
+    KnownBits KnownRHS = KnownBits::makeConstant(C1);
+    
+    // Bail out early if RHS is unknown (shouldn't happen for constants)
+    if (KnownRHS.isUnknown())
+      return SDValue();
+    
+    std::optional<bool> KnownVal;
+    
+    // Handle special cases first (like GlobalISel does)
+    if (KnownRHS.isZero()) {
+      // x >=u 0 -> always true
+      // x <u 0 -> always false  
+      if (Cond == ISD::SETUGE)
+        KnownVal = true;
+      else if (Cond == ISD::SETULT)
+        KnownVal = false;
+    }
+    
+    // If not handled by special cases, use ICmpInst::compare
+    if (!KnownVal) {
+      KnownBits KnownLHS = DAG.computeKnownBits(N0);
+      
+      // Convert ISD::CondCode to CmpInst::Predicate
+      CmpInst::Predicate Pred;
+      switch (Cond) {
+      case ISD::SETEQ:  Pred = CmpInst::ICMP_EQ; break;
+      case ISD::SETNE:  Pred = CmpInst::ICMP_NE; break;
+      case ISD::SETULT: Pred = CmpInst::ICMP_ULT; break;
+      case ISD::SETULE: Pred = CmpInst::ICMP_ULE; break;
+      case ISD::SETUGT: Pred = CmpInst::ICMP_UGT; break;
+      case ISD::SETUGE: Pred = CmpInst::ICMP_UGE; break;
+      case ISD::SETLT:  Pred = CmpInst::ICMP_SLT; break;
+      case ISD::SETLE:  Pred = CmpInst::ICMP_SLE; break;
+      case ISD::SETGT:  Pred = CmpInst::ICMP_SGT; break;
+      case ISD::SETGE:  Pred = CmpInst::ICMP_SGE; break;
+      default: 
+        return SDValue(); // Unsupported predicate
+      }
+      
+      // Use the same logic as GlobalISel: ICmpInst::compare
+      KnownVal = ICmpInst::compare(KnownLHS, KnownRHS, Pred);
+    }
+    
+    // If the comparison result is known, replace with constant
+    if (KnownVal)
+      return DAG.getConstant(*KnownVal ? 1 : 0, DL, VT);
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topperc wrote:

1 isn't the true value on all types for all targets.

https://github.com/llvm/llvm-project/pull/150715