[clang] [flang] [Flang] Add -ffast-real-mod and direct code for MOD on REAL types (PR #160660)

[Slava Zakharin via cfe-commits]

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@@ -7022,9 +7045,16 @@ mlir::Value IntrinsicLibrary::genMod(mlir::Type resultType,
   if (mlir::isa<mlir::IntegerType>(resultType))
     return mlir::arith::RemSIOp::create(builder, loc, args[0], args[1]);
 
-  // Use runtime.
-  return builder.createConvert(
-      loc, resultType, fir::runtime::genMod(builder, loc, args[0], args[1]));
+  if (useFastRealMod && resultType.isFloat()) {
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vzakhari wrote:

I wonder if we can enable the fast MOD under `afn` FastMathFlag.  Is there a reason to control it via a separate option?

https://github.com/llvm/llvm-project/pull/160660