When the --emit-c++ option is used, the state of the --opt=fast-math option is passed into the generated C++ code.

If --opt=fast-math is used then the generated code contains:
   #define ISPC_FAST_MATH 1
Otherwise it contains:
   #undef ISPC_FAST_MATH

This allows the generic headers to support the user's request.

cbackend.cpp

@@ -12,6 +12,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "ispc.h"
+
 #include <stdio.h>
 
 #ifndef _MSC_VER
@@ -2144,6 +2146,12 @@ bool CWriter::doInitialization(llvm::Module &M) {
   Out << "  #include <alloca.h>\n";
   Out << "#endif\n\n";
 
+  if (g->opt.fastMath) {
+      Out << "#define ISPC_FAST_MATH 1\n"
+  } else {
+      Out << "#undef ISPC_FAST_MATH\n"
+  }
+
   Out << "#include \"" << includeName << "\"\n";
 
   Out << "\n/* Basic Library Function Declarations */\n";