[llvm] [GlobalISel] Add G_SUB for computeNumSignBits (PR #158384)

[Yatao Wang via llvm-commits]

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@@ -1976,6 +1976,42 @@ unsigned GISelValueTracking::computeNumSignBits(Register R,
 
     break;
   }
+  case TargetOpcode::G_SUB: {
+    Register Src1 = MI.getOperand(1).getReg();
+    unsigned Src1NumSignBits =
+        computeNumSignBits(Src1, DemandedElts, Depth + 1);
+    if (Src1NumSignBits == 1)
+      return 1; // Early Out.
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ningxinr wrote:

Functionally it seems both would produce correct results, but I agree that code would look slightly cleaner the way DAG does it. 

https://github.com/llvm/llvm-project/pull/158384