[Bug rtl-optimization/43908] New: Very poor code generation (unnecessary conditionals and reloads) for ARM

[tobias at ringis dot se]

The following code:

    struct A { int r0; int r1; };
    void sigh(struct A *a, const int d)
    {
        int i;
        for (i = 0; i < 8; ++i) {
            if (d & (1U << i))
                a->r0 = 1;
            else
                a->r1 = 1;
        }
    }

compiled using

    arm-none-eabi-gcc -mcpu=arm7tdmi -marm -O3 -S -o- tmp2.c

produces very poor code:

    sigh:
        @ Function supports interworking.
        @ args = 0, pretend = 0, frame = 0
        @ frame_needed = 0, uses_anonymous_args = 0
        @ link register save eliminated.
        tst     r1, #1
        moveq   r3, #1
        movne   r3, #1
        streq   r3, [r0, #4]
        strne   r3, [r0, #0]
        tst     r1, #2
        moveq   r3, #1
        movne   r3, #1
        streq   r3, [r0, #4]
        strne   r3, [r0, #0]
        tst     r1, #4
        moveq   r3, #1
        movne   r3, #1
        streq   r3, [r0, #4]
        strne   r3, [r0, #0]
        tst     r1, #8
        moveq   r3, #1
        movne   r3, #1
        streq   r3, [r0, #4]
        strne   r3, [r0, #0]
        tst     r1, #16
        moveq   r3, #1
        movne   r3, #1
        streq   r3, [r0, #4]
        strne   r3, [r0, #0]
        tst     r1, #32
        moveq   r3, #1
        movne   r3, #1
        streq   r3, [r0, #4]
        strne   r3, [r0, #0]
        tst     r1, #64
        moveq   r3, #1
        movne   r3, #1
        streq   r3, [r0, #4]
        strne   r3, [r0, #0]
        tst     r1, #128
        movne   r3, #1
        moveq   r3, #1
        strne   r3, [r0, #0]
        streq   r3, [r0, #4]
        bx      lr

Note the silly occurrences of:

        moveq   r1, #1
        movne   r1, #1

More importantly, there is no need to load the constant 1 into r3 in every
iteration.  It should be loaded only once before the (unrolled) loop.


-- 
           Summary: Very poor code generation (unnecessary conditionals and
                    reloads) for ARM
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: tobias at ringis dot se
  GCC host triplet: x86_64-redhat-linux
GCC target triplet: arm-none-eabi


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43908