[ValueTracking] Add ComputeNumSignBits support for llvm.abs intrinsic

[craig.topper]

Summary

If absolute value needs turn a negative number into a positive number it reduces the number of sign bits by at most 1.

[spatel]

LGTM

llvm/lib/Analysis/ValueTracking.cpp

3005

Could use 'auto *' here per coding standards.

llvm/lib/Analysis/ValueTracking.cpp

case Instruction::ShuffleVector: {
       // If we don't know anything, early out and try computeKnownBits
       // fall-back.
       if (Tmp == 1)
         break;
       assert(Tmp <= Ty->getScalarSizeInBits() &&
              "Failed to determine minimum sign bits");
       return Tmp;
     }
+    case Instruction::Call: {
+      if (const auto *II = dyn_cast<IntrinsicInst>(U)) {
+        switch (II->getIntrinsicID()) {
+        default: break;
+        case Intrinsic::abs:
+          Tmp = ComputeNumSignBits(U->getOperand(0), Depth + 1, Q);
+          if (Tmp == 1) break;
+
+          // Absolute value reduces number of sign bits by at most 1.
+          return Tmp - 1;
+        }
+      }
+    }
     }
   }
 
   // Finally, if we can prove that the top bits of the result are 0's or 1's,
   // use this information.
 
   // If we can examine all elements of a vector constant successfully, we're
   // done (we can't do any better than that). If not, keep trying.

llvm/test/Transforms/InstCombine/abs-intrinsic.ll

 ; CHECK-NEXT:    [[AND2:%.*]] = and i32 [[ABS]], -4
 ; CHECK-NEXT:    ret i32 [[AND2]]
 ;
   %and = and i32 %x, -2
   %abs = call i32 @llvm.abs.i32(i32 %and, i1 false)
   %and2 = and i32 %abs, -4
   ret i32 %and2
 }
+
+; Make sure we infer this add doesn't overflow. The input to the abs has 3
+; sign bits, the abs reduces this to 2 sign bits.
+define i32 @abs_signbits(i30 %x) {
+; CHECK-LABEL: @abs_signbits(
+; CHECK-NEXT:    [[AND:%.*]] = sext i30 [[X:%.*]] to i32
+; CHECK-NEXT:    [[ABS:%.*]] = call i32 @llvm.abs.i32(i32 [[AND]], i1 false)
+; CHECK-NEXT:    [[ADD:%.*]] = add nsw i32 [[ABS]], 1
+; CHECK-NEXT:    ret i32 [[ADD]]
+;
+  %ext = sext i30 %x to i32
+  %abs = call i32 @llvm.abs.i32(i32 %ext, i1 false)
+  %add = add i32 %abs, 1
+  ret i32 %add
+}