[Bug target/42835] New: Missed merging common code sequence at the end of two basic blocks

["carrot at google dot com" <gcc-bugzilla at gcc dot gnu dot org>]

Compile the following code with options -march=armv7-a -mthumb -Os

int foo(int *p, int i )
{
      return( (i < 0 && *p == 1)
           || (i > 0 && *p == 2) );
}

Gcc generates:

foo:
        cmp     r1, #0
        bge     .L2
        ldr     r0, [r0, #0]
        cmp     r0, #1
        ite     ne         //  A
        movne   r0, #0     //  B
        moveq   r0, #1     //  C
        b       .L3
.L2:
        it      eq
        moveq   r0, r1
        beq     .L3
        ldr     r0, [r0, #0]
        cmp     r0, #2
        ite     ne         // D
        movne   r0, #0     // E
        moveq   r0, #1     // F
.L3:
        bx      lr

Instructions ABC are same as DEF, ideally ABC can be removed and generates
following code:

foo:
        @ args = 0, pretend = 0, frame = 0
        @ frame_needed = 0, uses_anonymous_args = 0
        @ link register save eliminated.
        cmp     r1, #0
        bge     .L2
        ldr     r0, [r0, #0]
        cmp     r0, #1
        b       .L3
.L2:
        it      eq
        moveq   r0, r1
        beq     .L3
        ldr     r0, [r0, #0]
        cmp     r0, #2
.L3:
        ite     ne
        movne   r0, #0
        moveq   r0, #1
        bx      lr

This should be handled by try_crossjump_bb. But a single rtl insn is used to
represent the compare and following IT block, like following. So they looks
different.

(insn:TI 24 22 26 src/t1.c:3 (parallel [
            (set (reg:SI 0 r0 [orig:133 D.2006 ] [133])
                (eq:SI (reg:SI 0 r0 [145])
                    (const_int 2 [0x2])))
            (clobber (reg:CC 24 cc))
        ]) 682 {*thumb2_compare_scc} (expr_list:REG_UNUSED (reg:CC 24 cc)
        (nil)))

(insn:TI 13 11 70 src/t1.c:3 (parallel [
            (set (reg:SI 0 r0 [orig:133 D.2006 ] [133])
                (eq:SI (reg:SI 0 r0 [142])
                    (const_int 1 [0x1])))
            (clobber (reg:CC 24 cc))
        ]) 682 {*thumb2_compare_scc} (expr_list:REG_UNUSED (reg:CC 24 cc)
        (nil)))

Can we break this insn into two separate ones, one is compare insn to set the
cc register, the other is the IT block?


-- 
           Summary: Missed merging common code sequence at the end of two
                    basic blocks
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: carrot at google dot com
 GCC build triplet: i686-linux
  GCC host triplet: i686-linux
GCC target triplet: arm-eabi


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42835