[Bug target/40672] New: constant address loads moved into loop unnecessarily

[froydnj at gcc dot gnu dot org]

The testcase below:

typedef unsigned char uint8_t;

#define UART3_LSR (*(volatile uint8_t *)(0x49020000+20))
#define UART3_RBR (*(volatile uint8_t *)(0x49020000+0))

int IsSerialBufferFull(void)
{
  return (UART3_LSR & 0x20) == 0;
}

void SendSerialByte(uint8_t byte)
{
  while (IsSerialBufferFull())
    ;
  UART3_RBR = byte;
}

when compiled with arm-none-eabi-gcc -O2 results in suboptimal code for
SendSerialByte:

SendSerialByte:
        @ Function supports interworking.
        @ args = 0, pretend = 0, frame = 0
        @ frame_needed = 0, uses_anonymous_args = 0
        @ link register save eliminated.
        mov     r3, #1224736768
        add     r3, r3, #131072
.L3:
        ldrb    r2, [r3, #20]   @ zero_extendqisi2
        tst     r2, #32
        mov     r2, #1224736768
        add     r2, r2, #131072
        beq     .L3
        strb    r0, [r2, #0]
        bx      lr
        .size   SendSerialByte, .-SendSerialByte

The load of the constant for UART3_RBR (mov r2/add r2) should not be moved into
the loop, since it's not needed until after the loop.  Furthermore, the
necessary value is already available in r3.

The same problem doesn't happen on other platforms, e.g. mips-sde-elf gives:

        andi    $4,$4,0x00ff
        li      $2,1224867840                   # 0x49020000
.L6:
        lbu     $3,20($2)
        andi    $3,$3,0x20
        beq     $3,$0,.L6
        nop

        sb      $4,0($2)
        j       $31
        nop

or powerpc-eabi:

        lis 9,0x4902
        ori 9,9,20
.L4:
        lbz 0,0(9)
        andi. 11,0,32
        beq+ 0,.L4
        lis 9,0x4902
        stb 3,0(9)
        blr

(which is not quite perfect, but better than the arm-none-eabi code...).


-- 
           Summary: constant address loads moved into loop unnecessarily
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: froydnj at gcc dot gnu dot org
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: arm-none-eabi


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40672