[Bug target/44659] New: Combiner fails to match cmp patterns with upper 8bit register

[hjl dot tools at gmail dot com]

After fixing PR 44588, I got

[hjl@gnu-6 divb]$ cat umod-4.c
int
foo (unsigned char x, unsigned char y)
{
   return (x % y) != 0;
}
[hjl@gnu-6 divb]$ /export/build/gnu/gcc/build-x86_64-linux/gcc/xgcc
-B/export/build/gnu/gcc/build-x86_64-linux/gcc/ -Os   -c -o umod-4.o umod-4.c
[hjl@gnu-6 divb]$ cat umod-4.s 
        .file   "umod-4.c"
        .text
.globl foo
        .type   foo, @function
foo:
.LFB0:
        .cfi_startproc
        movzbl  %dil, %eax
        divb    %sil
        movzbl  %ah, %eax
        testb   %al, %al
        setne   %al
        movzbl  %al, %eax
        ret

We can generate

        testb   %ah, %ah

instead of

        movzbl  %ah, %eax
        testb   %al, %al

The combine pass shows

Trying 11 -> 13: 
Failed to match this instruction:
(set (reg:CCZ 17 flags)
    (compare:CCZ (subreg:QI (lshiftrt:SI (subreg:SI (reg:HI 69) 0)
                (const_int 8 [0x8])) 0)
        (const_int 0 [0])))

with

(insn 11 10 13 2 umod-4.c:4 (set (reg:QI 67)
        (subreg:QI (zero_extract:SI (reg:HI 69)
                (const_int 8 [0x8])
                (const_int 8 [0x8])) 0)) 90 {*movqi_extzv_2_rex64}
(expr_list:REG_DEAD (reg:HI 69)
        (expr_list:REG_EQUAL (umod:QI (reg/v:QI 61 [ x ])
                (reg/v:QI 63 [ y ]))
            (nil))))

(insn 13 11 14 2 umod-4.c:4 (set (reg:CCZ 17 flags)
        (compare:CCZ (reg:QI 67)
            (const_int 0 [0]))) 0 {*cmpqi_ccno_1} (expr_list:REG_DEAD (reg:QI
67)
        (nil)))

i386.md has

(define_insn "*cmpqi_ext_2"
  [(set (reg FLAGS_REG)
        (compare
          (subreg:QI
            (zero_extract:SI
              (match_operand 0 "ext_register_operand" "Q")
              (const_int 8)
              (const_int 8)) 0)
          (match_operand:QI 1 "const0_operand" "")))]
  "ix86_match_ccmode (insn, CCNOmode)"
  "test{b}\t%h0, %h0"
  [(set_attr "type" "test")
   (set_attr "length_immediate" "0")
   (set_attr "mode" "QI")])

According to

http://gcc.gnu.org/ml/gcc-patches/2010-06/msg02477.html

(set (reg:CCZ 17 flags)
    (compare:CCZ (subreg:QI (lshiftrt:SI (subreg:SI (reg:HI 69) 0)
                (const_int 8 [0x8])) 0)
        (const_int 0 [0])))

is the standard form within combine.c. The questions are

1. Why doesn't combiner match QI cmp patterns with zero_extract?
2. Should i386.md provide QI cmp patterns with lshiftrt for
combiner?
3. Does i386.md need QI cmp patterns with zero_extract if they aren't
used by combiner?


-- 
           Summary: Combiner fails to match cmp patterns with upper 8bit
                    register
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: hjl dot tools at gmail dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44659