GCSE considers read only memory clobbered by function calls.

[Mostafa Hagog <MUSTAFA at il dot ibm dot com>]

It appears that GCSE considers "read only memory" as call clobbered, which
is not the case in CSE. I have took the test for read-only memory from CSE
and add it to GCSE where we compute the transparency. Here is a patch that
does so. This patch makes gcse eliminate redundant loads after stores for
the following example. The difference is seen when we compile with the
options:
"-O3 --param max-gcse-passes=3" with/without "-fgcse-las" (on a
powerpc-linux target). If this looks a reasonable change I will
regression-test and bootstrap the patch and ask for commit.

Example:

#define CALL_FPTR(fptr) (*fptr)
#define MY_FOO_CHECK() if (my_foo_var) my_foo_func()

int my_foo_var;

struct my_foo_struct {
    int my_dummy_field;
    int *(*ppaddr)(int);
};

struct my_foo_struct *my_foo_record;

int my_main_foo(int n)
{
    while ((my_foo_record = CALL_FPTR(my_foo_record->ppaddr)(n))) {
        MY_FOO_CHECK();
    }

    return 0;
}

The patch:

2005-05-09 Mostafa Hagog <mustafa@il.ibm.com>

      * gcse.c (compute_transp): use MEM_READONLY_P in case of MEM.

Index: gcse.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/gcse.c,v
retrieving revision 1.340
diff -c -p -r1.340 gcse.c
*** gcse.c      23 Apr 2005 21:27:44 -0000      1.340
--- gcse.c      5 May 2005 12:54:04 -0000
*************** compute_transp (rtx x, int indx, sbitmap
*** 2470,2479 ****
           do any list walking for them.  */
        EXECUTE_IF_SET_IN_BITMAP (blocks_with_calls, 0, bb_index, bi)
          {
!           if (set_p)
!             SET_BIT (bmap[bb_index], indx);
!           else
!             RESET_BIT (bmap[bb_index], indx);
          }

        /* Now iterate over the blocks which have memory modifications
--- 2470,2482 ----
           do any list walking for them.  */
        EXECUTE_IF_SET_IN_BITMAP (blocks_with_calls, 0, bb_index, bi)
          {
!             if (! MEM_READONLY_P (x))
!               {
!                 if (set_p)
!                   SET_BIT (bmap[bb_index], indx);
!                 else
!                   RESET_BIT (bmap[bb_index], indx);
!               }
          }