[llvm] [KnownBits] Make `avg{Ceil,Floor}S` and `{s,u}{add,sub}_sat` optimal (PR #110329)

[via llvm-commits]

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@@ -610,28 +610,78 @@ static KnownBits computeForSatAddSub(bool Add, bool Signed,
                                      const KnownBits &RHS) {
   // We don't see NSW even for sadd/ssub as we want to check if the result has
   // signed overflow.
-  KnownBits Res =
-      KnownBits::computeForAddSub(Add, /*NSW=*/false, /*NUW=*/false, LHS, RHS);
-  unsigned BitWidth = Res.getBitWidth();
-  auto SignBitKnown = [&](const KnownBits &K) {
-    return K.Zero[BitWidth - 1] || K.One[BitWidth - 1];
-  };
-  std::optional<bool> Overflow;
+  unsigned BitWidth = LHS.getBitWidth();
 
+  std::optional<bool> Overflow;
+  // Even if we can't entirely rule out overflow, we may be able to rule out
+  // overflow in one direction. This allows us to potentially keep some of the
+  // add/sub bits. I.e if we can't overflow in the positive direction we won't
+  // clamp to INT_MAX so we can keep low 0s from the add/sub result.
+  bool MayNegClamp = true;
+  bool MayPosClamp = true;
   if (Signed) {
-    // If we can actually detect overflow do so. Otherwise leave Overflow as
-    // nullopt (we assume it may have happened).
-    if (SignBitKnown(LHS) && SignBitKnown(RHS) && SignBitKnown(Res)) {
+    // Easy cases we can rule out any overflow.
+    if (Add && ((LHS.isNegative() && RHS.isNonNegative()) ||
+                (LHS.isNonNegative() && RHS.isNegative())))
+      Overflow = false;
+    else if (!Add && (((LHS.isNegative() && RHS.isNegative()) ||
+                       (LHS.isNonNegative() && RHS.isNonNegative()))))
+      Overflow = false;
+    else {
+      // Check if we may overflow. If we can't rule out overflow then check if
+      // we can rule out a direction at least.
+      KnownBits UnsignedLHS = LHS;
+      KnownBits UnsignedRHS = RHS;
+      UnsignedLHS.One.clearSignBit();
+      UnsignedLHS.Zero.setSignBit();
+      UnsignedRHS.One.clearSignBit();
+      UnsignedRHS.Zero.setSignBit();
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goldsteinn wrote:

Its been an annoyance for a file tbh, ill add a patch with `forceNegative` and `forceNonNegative` API

https://github.com/llvm/llvm-project/pull/110329