X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=ghc%2Fincludes%2FTailCalls.h;h=670da9546f75de9df447311bef7f9edbfd293f0e;hb=e6218fe7eff4e34e1a3c823cd4b7aebe09d2d4fb;hp=13eea0e48043f31c678c78475ebcd97bd2bb6859;hpb=153b9cb9b11e05c4edb1b6bc0a7b972660e41f70;p=ghc-hetmet.git

diff --git a/ghc/includes/TailCalls.h b/ghc/includes/TailCalls.h
index 13eea0e..670da95 100644
--- a/ghc/includes/TailCalls.h
+++ b/ghc/includes/TailCalls.h
@@ -1,5 +1,4 @@
 /* -----------------------------------------------------------------------------
- * $Id: TailCalls.h,v 1.18 2005/01/28 12:55:54 simonmar Exp $
  *
  * (c) The GHC Team, 1998-1999
  *
@@ -65,11 +64,31 @@ extern void __DISCARD__(void);
 
 #if x86_64_HOST_ARCH
 
+/*
+  NOTE about __DISCARD__():
+
+  On x86_64 this is necessary to work around bugs in the register
+  variable support in gcc.  Without the __DISCARD__() call, gcc will
+  silently throw away assignements to global register variables that
+  happen before the jump.
+
+  Here's the example:
+
+  extern void g(void);
+  static void f(void) {
+    R1 = g;
+    __DISCARD__()
+    goto *R1;
+  }
+
+  without the dummy function call, gcc throws away the assignment to R1
+  (gcc 3.4.3) gcc bug #20359.
+*/
+
 #define JMP_(cont)			\
     { 					\
-      void *__target;			\
-      __target = (void *)(cont);    	\
-      goto *__target; 	    	    	\
+      __DISCARD__();			\
+      goto *(void *)(cont); 	    	    	\
     }
 
 #endif /* x86_64_HOST_ARCH */