[libunwind] [Support] Add KnownBits::computeForSubBorrow (PR #67788)

[Nikita Popov via cfe-commits]

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@@ -3732,14 +3732,18 @@ KnownBits SelectionDAG::computeKnownBits(SDValue Op, const APInt &DemandedElts,
     assert(Op.getResNo() == 0 &&
            "We only compute knownbits for the difference here.");
 
-    // TODO: Compute influence of the carry operand.
-    if (Opcode == ISD::USUBO_CARRY || Opcode == ISD::SSUBO_CARRY)
-      break;
+    // With UADDO_CARRY and SSUBO_CARRY a borrow bit may be added in.
+    KnownBits Borrow(1);
+    if (Opcode == ISD::USUBO_CARRY || Opcode == ISD::SSUBO_CARRY) {
+      Borrow = computeKnownBits(Op.getOperand(2), DemandedElts, Depth + 1);
+      // Borrow has bit width 1
+      Borrow = Borrow.zextOrTrunc(1);
----------------
nikic wrote:

There are no zero-bit integers in SDAG (or IR). The use of zextOrTrunc in the comment probably dates back to the time when APInt did not allow zext/sext/trunc to the original bit width.

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