RE: [PATCH][ARM][thumb1] Reduce lr save for leaf function with non-far jump

["Joey Ye" <joey.ye@arm.com>]

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> -----Original Message-----
> From: Joey Ye
> Sent: Thursday, December 20, 2012 17:53
> To: gcc-patches@gcc.gnu.org; Ramana Radhakrishnan
> Cc: Joey Ye
> Subject: [PATCH][ARM][thumb1] Reduce lr save for leaf function with non-
> far jump
> 
> Current GCC thumb1 has an annoying problem that always assuming far
> branch.
> So it forces to save lr, even when unnecessarily. The most extreme case
> complained by partner is:
> 
> // compiled with "-mthumb -mcpu=cortex-m0 -Os".
> void foo() { for (;;); }
> =>
> foo:
> 	push	{lr}  // Crazy!!!
> .L2:
> 	b	.L2
> 
> The reason is that thumb1 far jump is only resolved in the very late
> pass
> "shorten_branch". Prologue/epilogue pass doesn't actually know a branch
> is
> far or not from its attribute. It has to conservatively save/restore lr
> whenever there is a branch.
> 
> This patch tries to fix it with a simple heuristic, i.e., using function
> size to decide if a far jump will likely be used. Function size
> information
> is meaningful in prologue/epilogue pass. The heuristic uses following
> check
> to decide if lr should be saved for far jump:
> 
> function_size * 3 >= 2048 // yes: save lr for possible far jump. No:
> don't
> save lr for far jump
> 
> The scheme has an issue: if some corner case does break above condition,
> there is no chance to fix-up but to ICE. But the heuristic condition is
> very
> conservative. It is base on the worse normal condition that each
> instruction
> is associated with a 4 byte literal ( (2+4)/2=3, blooming size by 3
> times ).
> I can't think of a real case to trigger the ICE. So I think it should
> work.
> 
> Other approaches than the heuristic scheme are too expensive to
> implement
> for this small size/performance issue. I did explored some but none of
> them
> persuaded myself.
> 
> Tests passed:
> * build libgcc, libstdc++, newlib, libm
> * make check-gcc with cpu=cortex-m0
> * Small and extreme test cases
> 
> ChangeLog:
> 
> 2012-12-20  Joey Ye  <joey.ye@arm.com>
> 
> 	* config/arm/arm.c(thumb1_final_prescan_insn):
> 	Assert lr save for real far jump.
> 	(thumb_far_jump_used_p): Count instruction size and set
>      far_jump_used.
> 
> diff --git a/gcc/config/arm/arm.c b/gcc/config/arm/arm.c
> index 327ef22..ad79451 100644
> --- a/gcc/config/arm/arm.c
> +++ b/gcc/config/arm/arm.c
> @@ -21790,6 +21857,11 @@ thumb1_final_prescan_insn (rtx insn)
>        else if (conds != CONDS_NOCOND)
>  	cfun->machine->thumb1_cc_insn = NULL_RTX;
>      }
> +
> +    /* Check if unexpected far jump is used.  */
> +    if (cfun->machine->lr_save_eliminated
> +        && get_attr_far_jump (insn) == FAR_JUMP_YES)
> +      internal_error("Unexpected thumb1 far jump");
>  }
> 
>  int
> @@ -21815,6 +21887,8 @@ static int
>  thumb_far_jump_used_p (void)
>  {
>    rtx insn;
> +  bool far_jump = false;
> +  unsigned int func_size = 0;
> 
>    /* This test is only important for leaf functions.  */
>    /* assert (!leaf_function_p ()); */
> @@ -21870,6 +21944,26 @@ thumb_far_jump_used_p (void)
>  	  && get_attr_far_jump (insn) == FAR_JUMP_YES
>  	  )
>  	{
> +	  far_jump = true;
> +	}
> +      func_size += get_attr_length (insn);
> +    }
> +
> +  /* Attribute far_jump will always be true for thumb1 before
> shorten_branch
> +     pass. So checking far_jump attribute before shorten_branch isn't
> much
> +     useful.
> +
> +     Following heuristic tries to estimate more accruately if a far
> jump
> may
> +     finally be used. The heuristic is very conservative as there is no
> chance
> +     to roll-back the decision of not to use far jump.
> +
> +     Thumb1 long branch offset is -2048 to 2046. The worst case is each
> 2-byte
> +     insn is assiociated with a 4 byte constant pool. Using function
> size
> +     2048/3 as the threshold is conservative enough.  */
> +  if (far_jump)
> +    {
> +      if ((func_size * 3) >= 2048)
> +        {
>  	  /* Record the fact that we have decided that
>  	     the function does use far jumps.  */
>  	  cfun->machine->far_jump_used = 1;
> 
> 
>