Initial attemp for sub

Author: ningxinr

llvm/lib/CodeGen/GlobalISel/GISelValueTracking.cpp

@@ -1959,6 +1959,20 @@ 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) {
+      Register Src2 = MI.getOperand(2).getReg();
+      unsigned Src2NumSignBits =
+          computeNumSignBits(Src2, DemandedElts, Depth + 1);
+      if (Src2NumSignBits == 1)
+        return 1; // Early out.
+      FirstAnswer = std::min(Src1NumSignBits, Src2NumSignBits) - 1;
+    }
+    break;
+  }
   case TargetOpcode::G_FCMP:
   case TargetOpcode::G_ICMP: {
     bool IsFP = Opcode == TargetOpcode::G_FCMP;

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

@@ -82,7 +82,7 @@ body:             |
   ; CHECK-NEXT: %1:_ KnownBits:00001111 SignBits:4
   ; CHECK-NEXT: %2:_ KnownBits:0000???? SignBits:4
   ; CHECK-NEXT: %3:_ KnownBits:00000000 SignBits:8
-  ; CHECK-NEXT: %4:_ KnownBits:???????? SignBits:1
+  ; CHECK-NEXT: %4:_ KnownBits:???????? SignBits:3
     %0:_(s8) = COPY $b0
     %1:_(s8) = G_CONSTANT i8 15
     %2:_(s8) = G_AND %0, %1
@@ -110,7 +110,7 @@ body:             |
   ; CHECK-NEXT: %1:_ KnownBits:00001111 SignBits:4
   ; CHECK-NEXT: %2:_ KnownBits:0000???? SignBits:4
   ; CHECK-NEXT: %3:_ KnownBits:00000101 SignBits:5
-  ; CHECK-NEXT: %4:_ KnownBits:???????? SignBits:1
+  ; CHECK-NEXT: %4:_ KnownBits:???????? SignBits:3
     %0:_(s8) = COPY $b0
     %1:_(s8) = G_CONSTANT i8 15
     %2:_(s8) = G_AND %0, %1
@@ -154,7 +154,7 @@ body:             |
   ; CHECK-NEXT: %3:_ KnownBits:00000000???????? SignBits:8
   ; CHECK-NEXT: %4:_ KnownBits:0000000000000000 SignBits:16
   ; CHECK-NEXT: %5:_ KnownBits:0000000000000000 SignBits:16
-  ; CHECK-NEXT: %6:_ KnownBits:???????????????? SignBits:1
+  ; CHECK-NEXT: %6:_ KnownBits:???????????????? SignBits:7
     %0:_(<4 x s16>) = COPY $d0
     %1:_(s16) = G_CONSTANT i16 255
     %2:_(<4 x s16>) = G_BUILD_VECTOR %1, %1, %1, %1
@@ -189,7 +189,7 @@ body:             |
   ; CHECK-NEXT: %4:_ KnownBits:0000000000101010 SignBits:10
   ; CHECK-NEXT: %5:_ KnownBits:0000000001001010 SignBits:9
   ; CHECK-NEXT: %6:_ KnownBits:000000000??01010 SignBits:9
-  ; CHECK-NEXT: %7:_ KnownBits:???????????????? SignBits:1
+  ; CHECK-NEXT: %7:_ KnownBits:???????????????? SignBits:7
     %0:_(<4 x s16>) = COPY $d0
     %1:_(s16) = G_CONSTANT i16 255
     %2:_(<4 x s16>) = G_BUILD_VECTOR %1, %1, %1, %1
@@ -208,7 +208,7 @@ body:             |
   ; CHECK-NEXT: %1:_ KnownBits:0000000000000110 SignBits:13
   ; CHECK-NEXT: %2:_ KnownBits:0000000000000?1? SignBits:13
   ; CHECK-NEXT: %3:_ KnownBits:0000000000000?1? SignBits:13
-  ; CHECK-NEXT: %4:_ KnownBits:???????????????? SignBits:1
+  ; CHECK-NEXT: %4:_ KnownBits:???????????????? SignBits:12
     %0:_(s16) = G_CONSTANT i16 3
     %1:_(s16) = G_CONSTANT i16 6
     %2:_(<4 x s16>) = G_BUILD_VECTOR %0, %1, %1, %0