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



> -----Original Message-----
> From: Ramana Radhakrishnan
> Sent: Thursday, April 11, 2013 4:40 PM
> To: Joey Ye
> Cc: gcc-patches@gcc.gnu.org
> Subject: Re: [PATCH][ARM][thumb1] Reduce lr save for leaf function with
> non-far jump
> 
> On 12/20/12 09:53, Joey Ye wrote:
> > 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 accurately 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 associated 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;
> >
> >
> >
> 
> Check for 80 character line length in the comments above - I can never
tell if
> it is my mail client or yours.
Further shorten the lines.
> 
> Otherwise ok if no regressions..
Make check targeting Cortex-M0/M3 with qemu. No regression.

Committed as 197956

- Joey




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