[llvm] [DAG] Remove AssertZext if the input is masked (PR #146052)

[Simon Pilgrim via llvm-commits]

================
@@ -15262,23 +15262,30 @@ SDValue DAGCombiner::visitAssertExt(SDNode *N) {
     }
   }
 
-  // If we have (AssertZext (and (AssertSext X, iX), M), iY) and Y is smaller
-  // than X, and the And doesn't change the lower iX bits, we can move the
-  // AssertZext in front of the And and drop the AssertSext.
   if (Opcode == ISD::AssertZext && N0.getOpcode() == ISD::AND &&
-      N0.hasOneUse() && N0.getOperand(0).getOpcode() == ISD::AssertSext &&
       isa<ConstantSDNode>(N0.getOperand(1))) {
-    SDValue BigA = N0.getOperand(0);
-    EVT BigA_AssertVT = cast<VTSDNode>(BigA.getOperand(1))->getVT();
     const APInt &Mask = N0.getConstantOperandAPInt(1);
-    if (AssertVT.bitsLT(BigA_AssertVT) &&
-        Mask.countr_one() >= BigA_AssertVT.getScalarSizeInBits()) {
-      SDLoc DL(N);
-      SDValue NewAssert =
-          DAG.getNode(Opcode, DL, N->getValueType(0), BigA.getOperand(0), N1);
-      return DAG.getNode(ISD::AND, DL, N->getValueType(0), NewAssert,
-                         N0.getOperand(1));
+
+    // If we have (AssertZext (and (AssertSext X, iX), M), iY) and Y is smaller
+    // than X, and the And doesn't change the lower iX bits, we can move the
+    // AssertZext in front of the And and drop the AssertSext.
+    if (N0.getOperand(0).getOpcode() == ISD::AssertSext && N0.hasOneUse()) {
+      SDValue BigA = N0.getOperand(0);
+      EVT BigA_AssertVT = cast<VTSDNode>(BigA.getOperand(1))->getVT();
+      if (AssertVT.bitsLT(BigA_AssertVT) &&
+          Mask.countr_one() >= BigA_AssertVT.getScalarSizeInBits()) {
+        SDLoc DL(N);
+        SDValue NewAssert =
+            DAG.getNode(Opcode, DL, N->getValueType(0), BigA.getOperand(0), N1);
+        return DAG.getNode(ISD::AND, DL, N->getValueType(0), NewAssert,
+                           N0.getOperand(1));
+      }
     }
+
+    // Remove AssertZext entirely if the mask guarantees the assertion cannot
+    // fail.
+    if (Mask.isIntN(AssertVT.getScalarSizeInBits()))
----------------
RKSimon wrote:

Are we allowed to do this in the more general case using computeKnownBits(...).countMinLeadingZeros()?

https://github.com/llvm/llvm-project/pull/146052