work around modulo failure on 64-bit Windows with MSVC 2008

Something about loading the MinGW-built "londdouble.dll" interferes
with fmod() in some cases --- but only on the first call?

pkgs/racket-test-core/tests/racket/number.rktl

@@ -998,6 +998,8 @@
 (test 0.0 modulo -4.0 2)
 (test 0.0 modulo 4.0 -2)
 (test 0.0 modulo -4.0 -2)
+(test 1.0 modulo 21.0 2)
+(test 1.0 modulo -21.0 2)
 (test 0 remainder 4 2)
 (test 0 remainder -4 2)
 (test 0 remainder 4 -2)
@@ -1006,6 +1008,8 @@
 (test 0.0 remainder -4.0 2)
 (test 0.0 remainder 4.0 -2)
 (test 0.0 remainder -4.0 -2)
+(test 1.0 remainder 21.0 2)
+(test -1.0 remainder -21.0 2)
 (test 0 modulo 0 5.0)
 (test 0 modulo 0 -5.0)
 (test 0 remainder 0 5.0)

racket/collects/compiler/private/xform.rkt

@@ -1762,7 +1762,10 @@
                        e)
                (= 1 (hash-ref used-symbols
 			      (let loop ([e e])
-                                      (if (null? (cddr e))
+				(if (or (null? (cddr e))
+					(and (pair? (cdr e))
+					     (eq? '= (tok-n (cadr e)))
+					     (= (length e) 4)))
 				    (tok-n (car e))
 				    (loop (cdr e))))))))
         

racket/src/racket/src/env.c

@@ -856,6 +856,8 @@ static void make_kernel_env(void)
   scheme_init_print_global_constants();
   scheme_init_variable_references_constants();
 
+  scheme_init_longdouble_fixup();
+
   scheme_defining_primitives = 0;
 }
 

racket/src/racket/src/number.c

@@ -1568,6 +1568,24 @@ void scheme_init_extfl_unsafe_number(Scheme_Env *env)
   scheme_add_global_constant("unsafe-f80vector-set!", p, env);
 }
 
+#ifdef _MSC_VER
+/* Something about loading "longdouble.dll" can interfere with
+   fmod() so that it fails on a first call, but works on subsequent
+   calls. The issue doesn't seem to be as simple as the floating-point
+   control state, it seems to affect mainly MSVC 2008 builds, only
+   the 3m build, etc. Using a non-static variable ensures that the
+   compiler doesn't optimize away the call to fmod(). */
+double scheme_longdouble_fixup_hack = 4;
+void scheme_init_longdouble_fixup(void)
+{
+  scheme_longdouble_fixup_hack = fmod(scheme_longdouble_fixup_hack, 2.0);
+}
+#else
+void scheme_init_longdouble_fixup(void)
+{
+}
+#endif
+
 Scheme_Object *
 scheme_make_integer_value(intptr_t i)
 {

racket/src/racket/src/schpriv.h

@@ -387,6 +387,7 @@ void scheme_init_parameterization();
 void scheme_init_getenv(void);
 void scheme_init_inspector(void);
 void scheme_init_compenv_symbol(void);
+void scheme_init_longdouble_fixup(void);
 
 #ifndef DONT_USE_FOREIGN
 void scheme_init_foreign_globals();