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

[Jay Foad via llvm-commits]

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@@ -54,32 +54,183 @@ KnownBits KnownBits::computeForAddCarry(
       LHS, RHS, Carry.Zero.getBoolValue(), Carry.One.getBoolValue());
 }
 
-KnownBits KnownBits::computeForAddSub(bool Add, bool NSW,
-                                      const KnownBits &LHS, KnownBits RHS) {
+KnownBits KnownBits::computeForAddSub(bool Add, bool NSW, bool NUW,
+                                      const KnownBits &LHS,
+                                      const KnownBits &RHS) {
+  // This can be a relatively expensive helper, so optimistically save some
+  // work.
+  if (LHS.isUnknown() && RHS.isUnknown())
+    return LHS;
   KnownBits KnownOut;
   if (Add) {
     // Sum = LHS + RHS + 0
-    KnownOut = ::computeForAddCarry(
-        LHS, RHS, /*CarryZero*/true, /*CarryOne*/false);
+    KnownOut =
+        ::computeForAddCarry(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);
   }
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jayfoad wrote:

You can skip this whole block if either operand is unknown - just initialize `KnownOut` to unknown. I don't know if that actually saves any time.

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