[llvm] [ConstantFolding] Add constant folding support for nextafter/nexttoward (PR #167324)

[Joshua Cranmer via llvm-commits]

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@@ -3221,6 +3223,26 @@ static Constant *ConstantFoldLibCall2(StringRef Name, Type *Ty,
     if (TLI->has(Func))
       return ConstantFoldBinaryFP(atan2, Op1V, Op2V, Ty);
     break;
+  case LibFunc_nextafter:
+  case LibFunc_nextafterf:
+  case LibFunc_nexttoward:
+  case LibFunc_nexttowardf:
+    if (TLI->has(Func)) {
+      if (Op1V.isNaN() || Op2V.isNaN()) {
+        return ConstantFP::get(Ty->getContext(),
+                               APFloat::getNaN(Ty->getFltSemantics()));
+      }
+
+      APFloat PromotedOp1V = Op1V.getPromoted(APFloat::IEEEquad());
+      APFloat PromotedOp2V = Op2V.getPromoted(APFloat::IEEEquad());
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jcranmer-intel wrote:

I should also add, C and C++ both guarantee that the set of values for `long double` is a (not necessarily strict) superset of the set of values representable for `double`.

https://github.com/llvm/llvm-project/pull/167324