Improved result when RHS is constant

Author: ningxinr

llvm/lib/CodeGen/GlobalISel/GISelValueTracking.cpp

@@ -1977,9 +1977,12 @@ unsigned GISelValueTracking::computeNumSignBits(Register R,
         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.
-      if (Known2.isNonNegative())
-        return Src2NumSignBits;
+      // the output of the NEG has, at worst, the same number of sign bits as
+      // the input.
+      if (Known2.isNonNegative()) {
+        FirstAnswer = Src2NumSignBits;
+        break;
+      }
 
       // Otherwise, we treat this like a SUB.
     }

llvm/test/CodeGen/AArch64/GlobalISel/knownbits-sub.mir

@@ -38,6 +38,18 @@ body:             |
     %2:_(s8) = G_SUB %0, %1
 ...
 ---
+name:            CstNegFour
+body:             |
+  bb.1:
+  ; CHECK-LABEL: name: @CstNegFour
+  ; CHECK-NEXT: %0:_ KnownBits:00000000 SignBits:8
+  ; CHECK-NEXT: %1:_ KnownBits:00000100 SignBits:5
+  ; CHECK-NEXT: %2:_ KnownBits:11111100 SignBits:6
+    %0:_(s8) = G_CONSTANT i8 0
+    %1:_(s8) = G_CONSTANT i8 4
+    %2:_(s8) = G_SUB %0, %1
+...
+---
 name:            CstNeg
 body:             |
   bb.1:

llvm/unittests/Target/AArch64/AArch64SelectionDAGTest.cpp

@@ -204,6 +204,12 @@ TEST_F(AArch64SelectionDAGTest, ComputeNumSignBits_SUB) {
   auto OpNegOne = DAG->getNode(ISD::SUB, Loc, IntVT, N0, N1);
   EXPECT_EQ(DAG->ComputeNumSignBits(OpNegOne), 8u);
 
+  // Neg 5
+  // N0 = 00000000
+  // N5 = 00000101
+  auto OpNegFive = DAG->getNode(ISD::SUB, Loc, IntVT, N0, N5);
+  EXPECT_EQ(DAG->ComputeNumSignBits(OpNegFive), 5u);
+
   // Non negative
   // N0     = 00000000
   // Nsign3 = 000????0