[project @ 2005-03-30 09:49:55 by simonmar]

Hack around a nasty bug on x86_64, caused by failing to align %rsp
correctly at a C call.  See the comment for details.

diff --git a/ghc/rts/StgCRun.c b/ghc/rts/StgCRun.c
index cf210d2..4d5136f 100644 (file)
--- a/ghc/rts/StgCRun.c
+++ b/ghc/rts/StgCRun.c
@@ -225,7 +225,41 @@ static void StgRunIsImplementedInAssembler(void)
        "addq %0, %%rsp\n\t"
        "retq"
 
-       : : "i"(RESERVED_C_STACK_BYTES+48 /*stack frame size*/));
+       : : "i"(RESERVED_C_STACK_BYTES+48+8 /*stack frame size*/));
+    /* 
+       HACK alert!
+
+       The x86_64 ABI specifies that on a procedure call, %rsp is
+       aligned on a 16-byte boundary + 8.  That is, the first
+       argument on the stack after the return address will be
+       16-byte aligned.  
+       
+       Which should be fine: RESERVED_C_STACK_BYTES+48 is a multiple
+       of 16 bytes.  
+       
+       BUT... when we do a C-call from STG land, gcc likes to put the
+       stack alignment adjustment in the prolog.  eg. if we're calling
+       a function with arguments in regs, gcc will insert 'subq $8,%rsp'
+       in the prolog, to keep %rsp aligned (the return address is 8
+       bytes, remember).  The mangler throws away the prolog, so we
+       lose the stack alignment.
+
+       The hack is to add this extra 8 bytes to our %rsp adjustment
+       here, so that throughout STG code, %rsp is 16-byte aligned,
+       ready for a C-call.  
+
+       A quick way to see if this is wrong is to compile this code:
+
+          main = System.Exit.exitWith ExitSuccess
+
+       And run it with +RTS -sstderr.  The stats code in the RTS, in
+       particular statsPrintf(), relies on the stack alignment because
+       it saves the %xmm regs on the stack, so it'll fall over if the
+       stack isn't aligned, and calling exitWith from Haskell invokes
+       shutdownHaskellAndExit using a C call.
+
+       Future gcc releases will almost certainly break this hack...
+    */
 }
 
 #endif /* x86-64 */