[Bug rtl-optimization/40314] inefficient address calculation of fields of large struct

[carrot at google dot com]

------- Comment #2 from carrot at google dot com  2009-05-31 02:51 -------
There are a lot of such opportunities in mcf from SPEC CPU 2006.

One possible implementation is to add a pass before cse. In the new pass it
should detect insn patterns like:

(set r200 400)                     # 400 is offset of field1
(set r201 (mem (plus r100 r200)))  # r100 contains struct base
...
(set r300 404)                     # 404 is offset of field2
(set r301 (mem (plus r100 r300)))  # r100 contains struct base

And rewrite them as:

(set r200 400)                     # keep the original insn
(set r250 (plus r100 400))         # r250 is new base
(set r201 (mem (plus r250 0)))
...
(set r300 404)
(set r251 (plus r100 400))         # r251 contains same value as r250
(set r301 (mem (plus r251 4)))

We can let dce and cse remove the redundant code, the final result should look
like:

(set r101 (plus r100 400))
(set r201 (mem (plus r101 0)))
...
(set r301 (mem (plus r101 4)))


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40314