middle-end/9997: Coelesce stack slots for disjoint scopes.

[Daniel Jacobowitz]

On Sun, Mar 09, 2003 at 11:32:47AM -0500, Jim Wilson wrote:
> This patch seems more complicated than necessary.  expand_end_bindings 
> already calls pop_temp_slots.  So all you have to do is arrange for 
> locals to have the right temp_slot_level.
> 
> I tried looking at this.  We are calling preserve_stack_temps, and that 
> preserves all temp slots whose address was taken.  This means a local of 
> array type won't be freed when its scope terminates.  The question then 
> is whether this is really necessary to preserve values whose address was 
>      taken.
> 
> This code is used to preserve values that might be needed by an 
> expression statement result.  In this case, the result is const0_rtx. 
> This code has a test for !MEM.  This is apparently to handle the case 
> where the result is in a REG that might contain an address.  However, 
> !MEM also includes constants, and constants can't possibly contain the 
> address of a stack local.  I think this problem would go away if we 
> fixed this code to do nothing for constant results.  The problematic 
> code is here:
>   /* If X is not in memory or is at a constant address, it cannot be in
>      a temporary slot, but it can contain something whose address was
>      taken.  */
>   if (p == 0 && (GET_CODE (x) != MEM || CONSTANT_P (XEXP (x, 0))))
> We should do nothing here if x is a constant, but there is no code for that.

I think I was looking at free_temp_slots instead of pop_temp_slots, so
I just assumed p->keep was causing the problem; I didn't realize that
pop_temp_slots overrode keep.  Thanks.  Your suggestion does fix
Richard's test, so I bootstrapped and tested it on i686-pc-linux-gnu. 
The patch is below; however, it causes one new failure:

FAIL: g++.dg/opt/alias2.C execution test

The interesting segment of the diff is:
-       movl    %eax, -56(%ebp)
-       movl    -24(%ebp), %eax
-       movl    %eax, -44(%ebp)
+       movl    %eax, -24(%ebp)
+       movl    %eax, -12(%ebp)
        call    _ZN11_Deque_baseD2Ev

I'm not quite sure what's going on.  0x123 ends up in eax instead of, I
think, the "this" pointer.

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer

2003-03-09  Daniel Jacobowitz  <drow@mvista.com>

	* function.c (preserve_temp_slots): If X is constant, don't
	preserve temp slots.  Suggested by Jim Wilson.

Index: function.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/function.c,v
retrieving revision 1.405
diff -u -p -r1.405 function.c
--- function.c	4 Mar 2003 06:20:12 -0000	1.405
+++ function.c	9 Mar 2003 16:52:29 -0000
@@ -1132,6 +1132,10 @@ preserve_temp_slots (x)
       return;
     }
 
+  /* If X is a constant, then we don't need to preserve stack slots.  */
+  if (CONSTANT_P (x))
+    return;
+
   /* If X is a register that is being used as a pointer, see if we have
      a temporary slot we know it points to.  To be consistent with
      the code below, we really should preserve all non-kept slots