http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49263
--- Comment #22 from Oleg Endo <olegendo at gcc dot gnu.org> ---
(In reply to Oleg Endo from comment #21)
> What happens is that the sequence is expanded to RTL as follows:
>
> (insn 7 4 8 2 (set (reg:SI 163 [ D.1856 ])
> (and:SI (reg/v:SI 162 [ xb ])
> (const_int 33 [0x21]))) sh_tmp.cpp:17 -1
> (nil))
> (insn 8 7 9 2 (set (reg:SI 147 t)
> (eq:SI (reg:SI 163 [ D.1856 ])
> (const_int 0 [0]))) sh_tmp.cpp:17 -1
> (nil))
> (jump_insn 9 8 10 2 (set (pc)
> (if_then_else (eq (reg:SI 147 t)
> (const_int 0 [0]))
> (label_ref:SI 15)
> (pc))) sh_tmp.cpp:17 301 {*cbranch_t}
> (int_list:REG_BR_PROB 3900 (nil))
> -> 15)
> (note 10 9 11 4 [bb 4] NOTE_INSN_BASIC_BLOCK)
> (insn 11 10 12 4 (set (reg:SI 164)
> (const_int 0 [0])) sh_tmp.cpp:18 -1
> (nil))
> (insn 12 11 15 4 (set (mem:SI (reg/v/f:SI 161 [ x ]) [2 *x_5(D)+0 S4 A32])
> (reg:SI 164)) sh_tmp.cpp:18 -1
> (nil))
>
>
> and insn 11 becomes dead code and is eliminated.
> All of that happens long time before combine, so the tst combine patterns
> have no chance to reconstruct the original code.
>
Adding an early peephole pass as described in
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59533#c2 and then adding the
following peephole:
;; Peephole after initial expansion.
(define_peephole2
[(set (match_operand:SI 0 "arith_reg_dest")
(and:SI (match_operand:SI 1 "arith_reg_operand")
(match_operand:SI 2 "logical_operand")))
(set (reg:SI T_REG) (eq:SI (match_dup 0) (const_int 0)))]
"TARGET_SH1 && can_create_pseudo_p ()"
[(set (reg:SI T_REG) (eq:SI (and:SI (match_dup 1) (match_dup 2))
(const_int 0)))
(set (match_dup 0) (and:SI (match_dup 1) (match_dup 2)))])
... fixes the problem and results in more uses of the tst #imm,r0 insn
according to the CSiBE set. On the other hand there is a total code size
increase of 792 bytes on the whole set. Below are some things that get worse
in the Linux source (mm/filemap.c):
mov.b @(15,r1),r0 -> mov.b @(15,r1),r0
cmp/pz r0 tst #128,r0 // cmp/pz has less
bf .L1016 bf .L1001 // pressure on r0
mov.b @(15,r0),r0 -> mov.b @(15,r0),r0
tst #4,r0 shar r0
bf .L107 shar r0
tst #1,r0
add #16,r0 -> add #16,r0
mov.b @(15,r0),r0 mov.b @(15,r0),r0
tst #16,r0 mov #-4,r1
bf/s .L509 shad r1,r0
tst #1,r0
bf/s .L509
