[llvm] [GlobalISel] Add G_ADD for computeNumSignBits (PR #159202)

[Yatao Wang via llvm-commits]

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@@ -1959,6 +1959,41 @@ unsigned GISelValueTracking::computeNumSignBits(Register R,
 
     break;
   }
+  case TargetOpcode::G_ADD: {
+    Register Src2 = MI.getOperand(2).getReg();
+    unsigned Src2NumSignBits =
+        computeNumSignBits(Src2, DemandedElts, Depth + 1);
+    if (Src2NumSignBits == 1)
+      return 1; // Early out.
+
+    Register Src1 = MI.getOperand(1).getReg();
+    unsigned Src1NumSignBits =
+        computeNumSignBits(Src1, DemandedElts, Depth + 1);
+    if (Src1NumSignBits == 1)
+      return 1; // Early Out.
+
+    // Special case decrementing a value (ADD X, -1):
+    KnownBits Known2 = getKnownBits(Src2, DemandedElts, Depth);
+    if (Known2.isAllOnes()) {
+      KnownBits Known1 = getKnownBits(Src1, DemandedElts, Depth);
+      // If the input is known to be 0 or 1, the output is 0/-1, which is all
+      // sign bits set.
+      if ((Known1.Zero | 1).isAllOnes())
+        return TyBits;
+
+      // If the input is known to be positive (the sign bit is known clear),
+      // the output of the NEG has the same number of sign bits as the input.
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ningxinr wrote:

Oh good catch! I forgot to change the comment here when I copy it from the SUB case.

Let me fix it.

Although why are you suggesting that we change the comment from "the same number of" to "at least as many"? I thought this comment is referring to the line below that returns the `Src1NumSignBits` when `Known1` is not negative, hence it should be the same. 

https://github.com/llvm/llvm-project/pull/159202