Fix Windows breakage (#5322). When I modified StgRun to use the pure

assembly version as part of the fix for #5250, we inadvertently lost
the Windows magic for extending the stack.  Win32 requires that the
stack is extended a page at a time, otherwise you get a segfault.  The
C compiler knows how to do this, so we now call a C stub to ensure
there's enough stack space at each invocation of the scheduler.

rts/Schedule.c

@@ -581,6 +581,10 @@ static void
 schedulePreLoop(void)
 {
   // initialisation for scheduler - what cannot go into initScheduler()  
+
+#if defined(mingw32_HOST_OS)
+    win32AllocStack();
+#endif
 }
 
 /* -----------------------------------------------------------------------------

rts/StgCRun.c

@@ -192,6 +192,18 @@ StgRunIsImplementedInAssembler(void)
     );
 }
 
+#if defined(mingw32_HOST_OS)
+// On windows the stack has to be allocated 4k at a time, otherwise
+// we get a segfault.  The C compiler knows how to do this (it calls
+// _alloca()), so we make sure that we can allocate as much stack as
+// we need:
+StgWord8 *win32AllocStack(void)
+{
+    StgWord8 stack[RESERVED_C_STACK_BYTES + 16 + 12];
+    return stack;
+}
+#endif
+
 #endif
 
 /* ----------------------------------------------------------------------------

rts/StgRun.h

@@ -11,4 +11,8 @@
 
 RTS_PRIVATE StgRegTable * StgRun (StgFunPtr f, StgRegTable *basereg);
 
+#if defined(mingw32_HOST_OS)
+StgWord8 *win32AllocStack(void);
+#endif
+
 #endif /* STGRUN_H */