Once more THUMB suboptimal code

["Eugene Starokoltsev" <eugene_o at gmx dot net>]

I use arm-ecos-elf-gcc v. 3.1 to compile C++ code. The compiler options are

-g  -Os -mthumb -Wall -Wstrict-prototypes -nostartfiles -Wa,-ahlms=$test.lst
 -fno-rtti -fno-exceptions

For the function:

static void bcpy( char* to, const char* from, int length )
{
	for( int i = 0; i < length; i++ )
		to[i] = from[i];
}

I have got the next compilation result:

  63 0018 10B5     		push	{r4, lr}
  64 001a 041C     		mov	r4, r0
  65 001c 081C     		mov	r0, r1
  66 001e 111C     		mov	r1, r2
  67              	.LM5:
  68              	.LBB3:
  69              	.LBB4:
  70 0020 0022     		mov	r2, #0
  71 0022 8A42     		cmp	r2, r1
  72 0024 04DA     		bge	.L9
  73              	.L7:
  74              	.LM6:
  75 0026 835C     		ldrb	r3, [r0, r2]
  76 0028 A354     		strb	r3, [r4, r2]
  77              	.LM7:
  78 002a 0132     		add	r2, r2, #1
  79 002c 8A42     		cmp	r2, r1
  80 002e FADB     		blt	.L7
  81              	.L9:
  82              	.LM8:
  83              		@ sp needed for prologue
  84 0030 10BD     		pop	{r4, pc}

The result is very strange from my point of view: there are a few excessive
register copying which should be avoided by register scheduler. What do you
think: the gcc register scheduler has some bug or the machine description is
guilty?

Eugene.


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