[llvm] [GlobalISel] Implement G_UADDO/G_UADDE/G_SADDO/G_SADDE for computeKnownBits (PR #165497)

[Simon Pilgrim via llvm-commits]

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@@ -615,7 +615,37 @@ void GISelValueTracking::computeKnownBitsImpl(Register R, KnownBits &Known,
   case TargetOpcode::G_UADDO:
   case TargetOpcode::G_UADDE:
   case TargetOpcode::G_SADDO:
-  case TargetOpcode::G_SADDE:
+  case TargetOpcode::G_SADDE: {
+    if (MI.getOperand(1).getReg() == R) {
+      // If we know the result of a compare has the top bits zero, use this
+      // info.
+      if (TL.getBooleanContents(DstTy.isVector(), false) ==
+              TargetLowering::ZeroOrOneBooleanContent &&
+          BitWidth > 1)
+        Known.Zero.setBitsFrom(1);
+      break;
+    }
+
+    assert(MI.getOperand(0).getReg() == R &&
+           "We only compute knownbits for the sum here.");
+    // With [US]ADDE, a carry bit may be added in.
+    KnownBits Carry(1);
+    if (Opcode == TargetOpcode::G_UADDE || Opcode == TargetOpcode::G_SADDE) {
+      computeKnownBitsImpl(MI.getOperand(4).getReg(), Known, DemandedElts,
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RKSimon wrote:

Known -> Carry?

https://github.com/llvm/llvm-project/pull/165497