http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53987
--- Comment #2 from Oleg Endo <olegendo at gcc dot gnu.org> ---
As of rev 204180 (4.9) this problem still exists.
As far as I understand, the actual root of the problem is that the 'unsigned
char' mem loads into regs are neither sign nor zero extended. So combine sees
the following:
(insn 11 6 12 2 (set (reg:QI 169 [ *a_3(D) ])
(mem:QI (reg:SI 4 r4 [ a ]) [0 *a_3(D)+0 S1 A8])) {*movqi}
(expr_list:REG_DEAD (reg:SI 4 r4 [ a ])
(nil)))
(insn 12 11 13 2 (set (reg:SI 168 [ *a_3(D)+-3 ])
(zero_extend:SI (reg:QI 169 [ *a_3(D) ]))) {*zero_extendqisi2_compact}
(expr_list:REG_DEAD (reg:QI 169 [ *a_3(D) ])
(nil)))
(insn 13 12 14 2 (set (reg:QI 171 [ *b_5(D) ])
(mem:QI (reg:SI 5 r5 [ b ]) [0 *b_5(D)+0 S1 A8])) {*movqi}
(expr_list:REG_DEAD (reg:SI 5 r5 [ b ])
(nil)))
(insn 14 13 15 2 (set (reg:SI 170 [ *b_5(D)+-3 ])
(zero_extend:SI (reg:QI 171 [ *b_5(D) ]))) {*zero_extendqisi2_compact}
(expr_list:REG_DEAD (reg:QI 171 [ *b_5(D) ])
(nil)))
(insn 15 14 16 2 (set (reg:SI 147 t)
(eq:SI (reg:SI 168 [ *a_3(D)+-3 ])
(reg:SI 170 [ *b_5(D)+-3 ]))) {cmpeqsi_t}
(expr_list:REG_DEAD (reg:SI 170 [ *b_5(D)+-3 ])
(expr_list:REG_DEAD (reg:SI 168 [ *a_3(D)+-3 ])
(nil))))
On the other hand, signed char mem loads are expanded as sign extending.
Since LOAD_EXTEND_OP tells that any mem loads but SImode are sign extending,
one could expect that all mem loads will be automatically expanded as such,
which is not the case.
