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

["Joey Ye" <joey dot ye at arm dot com>]

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> -----Original Message-----
> From: Joey Ye
> Sent: Saturday, January 05, 2013 3:41 PM
> To: Ramana Radhakrishnan
> Cc: Joey Ye; gcc-patches@gcc.gnu.org
> Subject: RE: [PATCH][ARM][thumb1] Reduce lr save for leaf function with
> non-far jump
> 
> Ping
> 
> > -----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;
> >
> >
> >
> 
>