[Bug rtl-optimization/65857] New: combine won't generate zero-extend from HImode memory

[ubizjak at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65857

            Bug ID: 65857
           Summary: combine won't generate zero-extend from HImode memory
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ubizjak at gmail dot com
            Target: x86

Following testcase:

--cut here--
extern unsigned char *a;

int test (void)
{
  return (unsigned char) a[0] | (unsigned char) a[1] << 8;
}
--cut here--

compiles (-O2 )nicely to:

        movq    a(%rip), %rax
        movzwl  (%rax), %eax
        ret

However, combine will not combine HImode load with zero-extend:

(insn 5 2 6 2 (set (reg/f:DI 92 [ a ])
        (mem/f/c:DI (symbol_ref:DI ("a") [flags 0x40]  <var_decl 0x2b006b8dfc60
a>) [1 a+0 S8 A64])) 85 {*movdi_internal}
     (nil))
(insn 6 5 11 2 (set (reg:SI 91 [ D.1849 ])
        (zero_extend:SI (mem:HI (reg/f:DI 92 [ a ]) [0 MEM[(unsigned char
*)_2]+0 S2 A8]))) ins.c:5 135 {*zero_extendhisi2}
     (expr_list:REG_DEAD (reg/f:DI 92 [ a ])
        (nil)))

It just chickens out with:

Trying 5 -> 6:
Failed to match this instruction:
(set (reg:SI 91 [ D.1849 ])
    (zero_extend:SI (mem:HI (mem/f/c:DI (symbol_ref:DI ("a") [flags 0x40] 
<var_decl 0x2b006b8dfc60 a>) [1 a+0 S8 A64]) [0 MEM[(unsigned char *)_2]+0 S2 A
8])))

Come on, combine ... the pattern is defined as:

(define_insn "*zero_extend<mode>si2"
  [(set (match_operand:SI 0 "register_operand" "=r")
    (zero_extend:SI
      (match_operand:SWI12 1 "nonimmediate_operand" "<r>m")))]
  "!(TARGET_ZERO_EXTEND_WITH_AND && optimize_function_for_speed_p (cfun))"
  "movz{<imodesuffix>l|x}\t{%1, %0|%0, %1}"
  [(set_attr "type" "imovx")
   (set_attr "mode" "SI")])