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

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

[Joey Ye]

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;

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

[Joey Ye]

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;
> 
> 
> 

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

[Joey Ye]

Ping^2

> -----Original Message-----
> From: Joey Ye
> Sent: Saturday, January 05, 2013 15:41
> 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;
> >
> >
> >
> 
> 

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

[Joey Ye]

Ping ^ 2

> -----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;
> >
> >
> >
> 
> 

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

[Ramana Radhakrishnan]

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 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;
>
>
>

Check for 80 character line length in the comments above - I can never 
tell if it is my mail client or yours.

Otherwise ok if no regressions..

regards
Ramana
>

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

[Joey Ye]

> -----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

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

[Joey Ye]

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

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;