RFC: eliminate some dead stores

[Dan Nicolaescu <dann at godzilla dot ICS dot UCI dot EDU>]

Stores to structs that are passed by value, or to struct local vars
are dead, but they are not eliminated currently. 

For example: 


struct first {   double d;   char f1;};
struct second {  char f2;  short a;};

void
baz (struct first parmf, struct second parms)
{
  struct first varf;
  parmf.f1 = 0;
  parms.f2 = 0;
  parmf.f1++;
  parms.f2++;
  parmf.f1++;
  parms.f2++;
  varf.f1 = parms.f2 + parmf.f1;
}

is compile on SPARC to: 
(gcc -O1)                              
baz:                                   
        !#PROLOGUE# 0                  
        add     %sp, -128, %sp         
        !#PROLOGUE# 1                  
        stb     %g0, [%o0+8]           
        stb     %g0, [%o1]             
        ldub    [%o0+8], %g1           
        add     %g1, 1, %g1
        stb     %g1, [%o0+8]
        ldub    [%o1], %g1
        add     %g1, 1, %g1
        stb     %g1, [%o1]
        ldub    [%o0+8], %g1
        add     %g1, 1, %g1
        stb     %g1, [%o0+8]
        ldub    [%o1], %g1
        add     %g1, 1, %g1
        stb     %g1, [%o1]
        nop
        retl
        sub     %sp, -128, %sp

The patch below tries the dead stores. The code generated after the
patch is: 

 baz:                          
         !#PROLOGUE# 0         
         add     %sp, -128, %sp
         !#PROLOGUE# 1         
         nop                   
         retl                  
         sub     %sp, -128, %sp  ;;; the stack ajustments are
                                 ;;; redundant, probably an
                                 ;;; unrelated issue

It boostraps/regchecks OK on sparc-solaris, but I've never hacked flow
before, so I would like to get some comments. Does it look correct?
Any suggestions on how to further improve the patch? 


Index: flow.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/flow.c,v
retrieving revision 1.524
diff -c -3 -p -c -r1.524 flow.c
*** flow.c	23 May 2002 19:23:40 -0000	1.524
--- flow.c	3 Jun 2002 07:19:44 -0000
*************** init_propagate_block_info (bb, live, loc
*** 1936,1942 ****
  	  {
  	    rtx mem = SET_DEST (set);
  	    rtx canon_mem = canon_rtx (mem);
! 
  	    /* This optimization is performed by faking a store to the
  	       memory at the end of the block.  This doesn't work for
  	       unchanging memories because multiple stores to unchanging
--- 1936,1943 ----
  	  {
  	    rtx mem = SET_DEST (set);
  	    rtx canon_mem = canon_rtx (mem);
!             tree expr_canon_mem;
!             
  	    /* This optimization is performed by faking a store to the
  	       memory at the end of the block.  This doesn't work for
  	       unchanging memories because multiple stores to unchanging
*************** init_propagate_block_info (bb, live, loc
*** 1944,1954 ****
  	    if (RTX_UNCHANGING_P (canon_mem))
  	      continue;
  
  	    if (XEXP (canon_mem, 0) == frame_pointer_rtx
  		|| (GET_CODE (XEXP (canon_mem, 0)) == PLUS
  		    && XEXP (XEXP (canon_mem, 0), 0) == frame_pointer_rtx
  		    && GET_CODE (XEXP (XEXP (canon_mem, 0), 1)) == CONST_INT))
! 	      add_to_mem_set_list (pbi, canon_mem);
  	  }
      }
  
--- 1945,1986 ----
  	    if (RTX_UNCHANGING_P (canon_mem))
  	      continue;
  
+             expr_canon_mem = MEM_EXPR (canon_mem);
+             
  	    if (XEXP (canon_mem, 0) == frame_pointer_rtx
  		|| (GET_CODE (XEXP (canon_mem, 0)) == PLUS
  		    && XEXP (XEXP (canon_mem, 0), 0) == frame_pointer_rtx
  		    && GET_CODE (XEXP (XEXP (canon_mem, 0), 1)) == CONST_INT))
!               add_to_mem_set_list (pbi, canon_mem);              
! 
!             
!               {
!                 extern tree decl_for_component_ref (tree);
!                 if (expr_canon_mem)
!                   {
!                     /* Stores to structures passed by value, or to
!                        local vars are dead.  */
!                     if (DECL_P (expr_canon_mem))
!                       ;
!                      else if (TREE_CODE (expr_canon_mem) == COMPONENT_REF)
!                        expr_canon_mem = decl_for_component_ref (expr_canon_mem);
!                      
!                      if (expr_canon_mem
! 
!                          && ((TREE_CODE (expr_canon_mem) == PARM_DECL)
!                              || ( !current_function_returns_struct
!                                   /* If the curent function returns a
!                                      struct, we can't guarantee that
!                                      all stores to structures are
!                                      dead.  */
!                                   && (TREE_CODE (expr_canon_mem) == VAR_DECL)
!                                   /* Stores to static vars are not dead. */
!                                   && ! (TREE_STATIC (expr_canon_mem))
!                                   && (DECL_CONTEXT (expr_canon_mem)
!                                       == current_function_decl))))
!                        add_to_mem_set_list (pbi, canon_mem);
!                   }
!               }
  	  }
      }