[llvm] [Knowbits] Make nuw and nsw support in computeForAddSub optimal (PR #83382)

[Matt Arsenault via llvm-commits]

================
@@ -63,23 +63,173 @@ KnownBits KnownBits::computeForAddSub(bool Add, bool NSW,
         LHS, RHS, /*CarryZero*/true, /*CarryOne*/false);
   } else {
     // Sum = LHS + ~RHS + 1
-    std::swap(RHS.Zero, RHS.One);
-    KnownOut = ::computeForAddCarry(
-        LHS, RHS, /*CarryZero*/false, /*CarryOne*/true);
+    KnownBits NotRHS = RHS;
+    std::swap(NotRHS.Zero, NotRHS.One);
+    KnownOut = ::computeForAddCarry(LHS, NotRHS, /*CarryZero*/ false,
+                                    /*CarryOne*/ true);
+  }
+  if (!NSW && !NUW)
+    return KnownOut;
+
+  // We truncate out the signbit during nsw handling so just handle this special
+  // case to avoid dealing with it later.
+  if (LHS.getBitWidth() == 1) {
+    return LHS | RHS;
   }
 
-  // Are we still trying to solve for the sign bit?
-  if (!KnownOut.isNegative() && !KnownOut.isNonNegative()) {
+  auto GetMinMaxVal = [Add](bool ForNSW, bool ForMax, const KnownBits &L,
+                            const KnownBits &R, bool &OV) {
+    APInt LVal = ForMax ? L.getMaxValue() : L.getMinValue();
+    APInt RVal = Add == ForMax ? R.getMaxValue() : R.getMinValue();
+
+    if (ForNSW) {
+      LVal = LVal.trunc(LVal.getBitWidth() - 1);
+      RVal = RVal.trunc(RVal.getBitWidth() - 1);
+    }
+    APInt Res = Add ? LVal.uadd_ov(RVal, OV) : LVal.usub_ov(RVal, OV);
+    if (ForNSW)
+      Res = Res.sext(Res.getBitWidth() + 1);
+    return Res;
+  };
+
+  auto GetMaxVal = [&GetMinMaxVal](bool ForNSW, const KnownBits &L,
+                                   const KnownBits &R, bool &OV) {
+    return GetMinMaxVal(ForNSW, /*ForMax*/ true, L, R, OV);
----------------
arsenm wrote:

```suggestion
    return GetMinMaxVal(ForNSW, /*ForMax=*/ true, L, R, OV);
```

https://github.com/llvm/llvm-project/pull/83382