[Bug target/50883] [ARM] Suboptimal optimization for small structures

["sebastian dot huber at embedded-brains dot de" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50883

--- Comment #5 from Sebastian Huber <sebastian.huber@embedded-brains.de> 2011-10-27 15:19:57 UTC ---
If we look at the function f (the function g is similar):

struct s {
  int alignment;
  unsigned char a;
  unsigned char b;
  unsigned char c;
  unsigned char d;
};

unsigned f(struct s x)
{
  return x.a | (x.b << 8) | (x.c << 16) | (x.d << 24);
}

Currently ARM GCC produces this:

f:
    sub    sp, sp, #8
    add    r3, sp, #8
    stmdb    r3, {r0, r1}
    ldrb    r0, [sp, #6]
    ldrb    r3, [sp, #5]
    lsls    r0, r0, #16
    ldrb    r2, [sp, #4]
    orr    r0, r0, r3, lsl #8
    ldrb    r3, [sp, #7]
    orrs    r0, r0, r2
    orr    r0, r0, r3, lsl #24
    add    sp, sp, #8
    bx    lr

According to the ARM EABI, this structure is passed in registers r0 and r1. 
The return value is in r0.  This function may be reduced to:

f:
    mov    r0, r1
    bx    lr

The PowerPC GCC performs this optimization.