[Bug c/35141] New: ARM: Constant generation inside a loop: Missed optimization opportunity

[alexandre dot nunes at gmail dot com]

On ARM, the following C code:

void whatever(const char *pqp)
{
  volatile unsigned int *uart_thr = (typeof(uart_thr))0xE000C000;
  unsigned int ch;
  while((ch = *pqp++))
    *uart_thr = ch;
}

Generates this assembler output (by means of -mcpu=arm7tdmi -O2):

whatever:
        @ args = 0, pretend = 0, frame = 0
        @ frame_needed = 0, uses_anonymous_args = 0
        @ link register save eliminated.
        ldrb    r2, [r0, #0]    @ zero_extendqisi2
        cmp     r2, #0
        @ lr needed for prologue
        bxeq    lr
.L4:
        mov     r3, #-536870912
        add     r3, r3, #49152
        str     r2, [r3, #0]
        ldrb    r2, [r0, #1]!   @ zero_extendqisi2
        cmp     r2, #0
        bne     .L4
        bx      lr

The relevant part is the bne .L4 ; since r3 is preserved across the loop, it
could optimize for speed without space penality by generating this instead:

.L4:
        mov     r3, #-536870912
        add     r3, r3, #49152
.L5:
        str     r2, [r3, #0]
        ldrb    r2, [r0, #1]!   @ zero_extendqisi2
        cmp     r2, #0
        bne     .L5
        bx      lr

... or, in other words, generating the constant only once, which saves at least
two cycles per iteration.


-- 
           Summary: ARM: Constant generation inside a loop: Missed
                    optimization opportunity
           Product: gcc
           Version: 4.2.3
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: alexandre dot nunes at gmail dot com
  GCC host triplet: i686-unknow-linux
GCC target triplet: arm-*-elf


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35141