[libc-commits] [libc] [Support] Add KnownBits::computeForSubBorrow (PR #67788)

[via libc-commits]

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@@ -3764,14 +3768,11 @@ KnownBits SelectionDAG::computeKnownBits(SDValue Op, const APInt &DemandedElts,
     if (Opcode == ISD::ADDE)
       // Can't track carry from glue, set carry to unknown.
       Carry.resetAll();
-    else if (Opcode == ISD::UADDO_CARRY || Opcode == ISD::SADDO_CARRY)
-      // TODO: Compute known bits for the carry operand. Not sure if it is worth
-      // the trouble (how often will we find a known carry bit). And I haven't
-      // tested this very much yet, but something like this might work:
-      //   Carry = computeKnownBits(Op.getOperand(2), DemandedElts, Depth + 1);
-      //   Carry = Carry.zextOrTrunc(1, false);
-      Carry.resetAll();
-    else
+    else if (Opcode == ISD::UADDO_CARRY || Opcode == ISD::SADDO_CARRY) {
+      Carry = computeKnownBits(Op.getOperand(2), DemandedElts, Depth + 1);
+      // Carry has bit width 1
+      Carry = Carry.zextOrTrunc(1);
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goldsteinn wrote:

Likewise just `trunc` here.

https://github.com/llvm/llvm-project/pull/67788