few questions about GHC RTS

[Peter Hercek]

Simon Marlow wrote:
> On 19/08/2009 10:18, Peter Hercek wrote:
>> Why is stg_nofoceIO_info added as last argument to IO actions in
>> unregistered mode? Do I still need to pass it in even when (I think) my
>> IO action does not need it? E.g. is it required for every IO action by
>> some stack walking code or something?
>
> The comment in Interpreter.c says
>
>                   // Note [unreg]: in unregisterised mode, the return
>                   // convention for IO is different.  The
>                   // stg_noForceIO_info stack frame is necessary to
>                   // account for this difference.
>
> However, the return convention for IO is now the same under both 
> registerised and unregisterised builds (I made the change becuase of 
> the proliferation of obscure conditional RTS code like the above), so 
> I'm guessing the stg_noforceIO_info hack, and the above comment, are 
> now redundant.  It needs testing though.

Just for your information. I was curious and tried to remove the 
additional stg_noforceIO_info argument. Breakpoints stopped to be called 
but otherwise ghci seemed to work fine. It was surprising. It looked 
like the call to rts_breakpoint_io_action was somehow optimized away but 
I would not expect any optimizations at this level. Maybe I did not do 
it correctly. The patch I used is at the end of this email.

Thanks,
  Peter.

hunk ./rts/Interpreter.c 879
                   // in a reasonable state for the GC and so that
                   // execution of this BCO can continue when we resume
                   ioAction = (StgClosure *) deRefStablePtr 
(rts_breakpoint_io_action);
-                  Sp -= 9;
-                  Sp[8] = (W_)obj;   $
-                  Sp[7] = (W_)&stg_apply_interp_info;
-                  Sp[6] = (W_)&stg_noforceIO_info;     // see [unreg] below
+                  Sp -= 8;
+                  Sp[7] = (W_)obj;   $
+                  Sp[6] = (W_)&stg_apply_interp_info;
                   Sp[5] = (W_)new_aps;                 // the AP_STACK
                   Sp[4] = (W_)BCO_PTR(arg3_freeVars);  // the info 
about local vars of the breakpoint
                   Sp[3] = (W_)False_closure;            // True <=> a 
breakpoint
hunk ./rts/Interpreter.c 885
-                  Sp[2] = (W_)&stg_ap_pppv_info;
+                  Sp[2] = (W_)&stg_ap_ppp_info;
                   Sp[1] = (W_)ioAction;                // apply the IO 
action to its two arguments above
                   Sp[0] = (W_)&stg_enter_info;         // get ready to 
run the IO action
hunk ./rts/Interpreter.c 888
-                  // Note [unreg]: in unregisterised mode, the return
-                  // convention for IO is different.  The
-                  // stg_noForceIO_info stack frame is necessary to
-                  // account for this difference.
 
                   // set the flag in the TSO to say that we are now
                   // stopping at a breakpoint so that when we resume