Re: [PATCH, ARM] Add a new target hook to compute the frame layout

[Bernd Edlinger <bernd dot edlinger at hotmail dot de>]

Hi Richard,

what do you think of this patch, is it OK (with the suggested wording)?


Thanks
Bernd.

On 08/05/16 16:06, Richard Earnshaw (lists) wrote:
> On 05/08/16 13:49, Bernd Edlinger wrote:
>> On 08/05/16 11:29, Richard Earnshaw (lists) wrote:
>>> On 04/08/16 22:16, Bernd Edlinger wrote:
>>>> Hi,
>>>>
>>>> this patch introduces a new target hook that allows the target's
>>>> INITIAL_ELIMINATION_OFFSET function to use cached values instead of
>>>> re-computing the frame layout every time.
>>>>
>>>> I have updated the documentation a bit and hope it is clearer this time.
>>>>
>>>> It still needs a review by ARM port maintainers.
>>>>
>>>> If the ARM port maintainers find this patch useful, that would be fine.
>>>>
>>>
>>> I need to look into this more, but my first thought was that the
>>> documentation is confusing, or there is a real problem in this patch.
>>>
>>
>> Thanks for your quick response.
>>
>> The documentation is actually the most difficult part for me.
>>
>>> As I understand it the frame has to be re-laid out each time the
>>> contents of the frame changes (an extra register becomes live or another
>>> spill slot is needed).  So saying that it is laid out once can't be
>>> right if (as I read it initially) you mean 'once per function' since I
>>> think it needs to be 'once for each time the frame contents changes'.
>>>
>>> Of course, things might be a bit different with LRA compared with
>>> reload, but I strongly suspect this is never going to be an 'exactly
>>> once per function' operation.
>>>
>>
>> Right.  It will be done 2 or 3 times for each function.
>> LRA and reload behave identical in that respect.
>>
>> But each time reload changes something in the input data the
>> INITIAL_EMIMINATION_OFFSET is called several times, and the results
>> have to be consistent in each iteration.
>>
>> The frame layout function has no way to know if the frame layout
>> is expected to change or not.
>>
>> Many targets use reload_completed as an indication when the frame layout
>> may not change at all, but that is only an approximation.
>>
>>> Can you clarify your meaning in the documentation please?
>>>
>>
>> I meant 'once' in the sense of 'once for each time the frame contents
>> change'.
>>
>> Thus I'd change that sentence to:
>>
>> "This target hook allows the target to compute the frame layout once for
>> each time the frame contents change and make use of the cached frame
>> layout in @code{INITIAL_ELIMINATION_OFFSET} instead of re-computing it
>> on every invocation.  This is particularly useful for targets that have
>> an expensive frame layout function.  Implementing this callback is
>> optional."
>>
>
> Thanks, that's pretty much what I expected would be the case.
>
> Could I suggest:
>
> This target hook is called once each time the frame layout needs to be
> recalculated.  The calculations can be cached by the target and can then
> be used by @code{INITIAL_ELIMINATION_OFFSET} instead of re-computing the
> layout on every invocation of that hook.  This is particularly useful
> for targets that have an expensive frame layout function.  Implementing
> this callback is optional.
>
> R.
>
>>
>> Thanks
>> Bernd.
>>
>>
>>> R.
>>>
>>>>
>>>> Thanks
>>>> Bernd.
>>>>
>>>> On 06/21/16 23:29, Jeff Law wrote:
>>>>> On 06/16/2016 08:47 AM, Bernd Edlinger wrote:
>>>>>> Hi!
>>>>>>
>>>>>>
>>>>>> By the design of the target hook INITIAL_ELIMINATION_OFFSET
>>>>>> it is necessary to call this function several times with
>>>>>> different register combinations.
>>>>>> Most targets use a cached data structure that describes the
>>>>>> exact frame layout of the current function.
>>>>>>
>>>>>> It is safe to skip the computation when reload_completed = true,
>>>>>> and most targets do that already.
>>>>>>
>>>>>> However while reload is doing its work, it is not clear when to
>>>>>> do the computation and when not.  This results in unnecessary
>>>>>> work.  Computing the frame layout can be a simple function or an
>>>>>> arbitrarily complex one, that walks all instructions of the current
>>>>>> function for instance, which is more or less the common case.
>>>>>>
>>>>>>
>>>>>> This patch adds a new optional target hook that can be used
>>>>>> by the target to factor the INITIAL_ELIMINATION_OFFSET-hook
>>>>>> into a O(n) computation part, and a O(1) result function.
>>>>>>
>>>>>> The patch implements a compute_frame_layout target hook just
>>>>>> for ARM in the moment, to show the principle.
>>>>>> Other targets may also implement that hook, if it seems appropriate.
>>>>>>
>>>>>>
>>>>>> Boot-strapped and reg-tested on arm-linux-gnueabihf.
>>>>>> OK for trunk?
>>>>>>
>>>>>>
>>>>>> Thanks
>>>>>> Bernd.
>>>>>>
>>>>>>
>>>>>> changelog-frame-layout.txt
>>>>>>
>>>>>>
>>>>>> 2016-06-16  Bernd Edlinger  <bernd.edlinger@hotmail.de>
>>>>>>
>>>>>>       * target.def (compute_frame_layout): New optional target hook.
>>>>>>       * doc/tm.texi.in (TARGET_COMPUTE_FRAME_LAYOUT): Add hook.
>>>>>>       * doc/tm.texi (TARGET_COMPUTE_FRAME_LAYOUT): Add documentation.
>>>>>>       * lra-eliminations.c (update_reg_eliminate): Call
>>>>>> compute_frame_layout
>>>>>>       target hook.
>>>>>>       * reload1.c (verify_initial_elim_offsets): Likewise.
>>>>>>       * config/arm/arm.c (TARGET_COMPUTE_FRAME_LAYOUT): Define.
>>>>>>       (use_simple_return_p): Call arm_compute_frame_layout if needed.
>>>>>>       (arm_get_frame_offsets): Split up into this ...
>>>>>>       (arm_compute_frame_layout): ... and this function.
>>>>> The ARM maintainers would need to chime in on the ARM specific changes
>>>>> though.
>>>>>
>>>>>
>>>>>
>>>>>> Index: gcc/target.def
>>>>>> ===================================================================
>>>>>> --- gcc/target.def    (Revision 233176)
>>>>>> +++ gcc/target.def    (Arbeitskopie)
>>>>>> @@ -5245,8 +5245,19 @@ five otherwise.  This is best for most machines.",
>>>>>>     unsigned int, (void),
>>>>>>     default_case_values_threshold)
>>>>>>
>>>>>> -/* Retutn true if a function must have and use a frame pointer.  */
>>>>> s/Retutn/Return
>>>>>
>>>>>> +/* Optional callback to advise the target to compute the frame
>>>>>> layout.  */
>>>>>>    DEFHOOK
>>>>>> +(compute_frame_layout,
>>>>>> + "This target hook is called immediately before reload wants to call\n\
>>>>>> +@code{INITIAL_ELIMINATION_OFFSET} and allows the target to cache the
>>>>>> frame\n\
>>>>>> +layout instead of re-computing it on every invocation.  This is
>>>>>> particularly\n\
>>>>>> +useful for targets that have an O(n) frame layout function.
>>>>>> Implementing\n\
>>>>>> +this callback is optional.",
>>>>>> + void, (void),
>>>>>> + hook_void_void)
>>>>> So the docs say "immediately before", but that's not actually reality in
>>>>> lra-eliminations.  I think you can just say "This target hook is called
>>>>> before reload or lra-eliminations calls
>>>>> @code{INITIAL_ELIMINATION_OFFSET} and allows ..."
>>>>>
>>>>>
>>>>> How does this macro interact with INITIAL_FRAME_POINTER_OFFSET?
>>>>>
>>>>> I'm OK with this conceptually.  I think you need a minor doc update and
>>>>> OK from the ARM maintainers before it can be installed though.
>>>>>
>>>>> jeff
>>>>>
>>>>> changelog-frame-layout.txt
>>>>>
>>>>>
>>>>> 2016-06-16  Bernd Edlinger  <bernd.edlinger@hotmail.de>
>>>>>
>>>>> 	* target.def (compute_frame_layout): New optional target hook.
>>>>> 	* doc/tm.texi.in (TARGET_COMPUTE_FRAME_LAYOUT): Add hook.
>>>>> 	* doc/tm.texi (TARGET_COMPUTE_FRAME_LAYOUT): Add documentation.
>>>>> 	* lra-eliminations.c (update_reg_eliminate): Call compute_frame_layout
>>>>> 	target hook.
>>>>> 	* reload1.c (verify_initial_elim_offsets): Likewise.
>>>>> 	* config/arm/arm.c (TARGET_COMPUTE_FRAME_LAYOUT): Define.
>>>>> 	(use_simple_return_p): Call arm_compute_frame_layout if needed.
>>>>> 	(arm_get_frame_offsets): Split up into this ...
>>>>> 	(arm_compute_frame_layout): ... and this function.
>>>>>
>>>>> patch-frame-layout.diff
>>>>>
>>>>>
>>>>> Index: gcc/config/arm/arm.c
>>>>> ===================================================================
>>>>> --- gcc/config/arm/arm.c	(revision 239144)
>>>>> +++ gcc/config/arm/arm.c	(working copy)
>>>>> @@ -81,6 +81,7 @@ static bool arm_const_not_ok_for_debug_p (rtx);
>>>>>    static bool arm_needs_doubleword_align (machine_mode, const_tree);
>>>>>    static int arm_compute_static_chain_stack_bytes (void);
>>>>>    static arm_stack_offsets *arm_get_frame_offsets (void);
>>>>> +static void arm_compute_frame_layout (void);
>>>>>    static void arm_add_gc_roots (void);
>>>>>    static int arm_gen_constant (enum rtx_code, machine_mode, rtx,
>>>>>    			     unsigned HOST_WIDE_INT, rtx, rtx, int, int);
>>>>> @@ -663,6 +664,9 @@ static const struct attribute_spec arm_attribute_t
>>>>>    #undef TARGET_SCALAR_MODE_SUPPORTED_P
>>>>>    #define TARGET_SCALAR_MODE_SUPPORTED_P arm_scalar_mode_supported_p
>>>>>
>>>>> +#undef TARGET_COMPUTE_FRAME_LAYOUT
>>>>> +#define TARGET_COMPUTE_FRAME_LAYOUT arm_compute_frame_layout
>>>>> +
>>>>>    #undef TARGET_FRAME_POINTER_REQUIRED
>>>>>    #define TARGET_FRAME_POINTER_REQUIRED arm_frame_pointer_required
>>>>>
>>>>> @@ -3880,6 +3884,10 @@ use_simple_return_p (void)
>>>>>    {
>>>>>      arm_stack_offsets *offsets;
>>>>>
>>>>> +  /* Note this function can be called before or after reload.  */
>>>>> +  if (!reload_completed)
>>>>> +    arm_compute_frame_layout ();
>>>>> +
>>>>>      offsets = arm_get_frame_offsets ();
>>>>>      return offsets->outgoing_args != 0;
>>>>>    }
>>>>> @@ -19370,7 +19378,7 @@ arm_compute_static_chain_stack_bytes (void)
>>>>>
>>>>>    /* Compute a bit mask of which registers need to be
>>>>>       saved on the stack for the current function.
>>>>> -   This is used by arm_get_frame_offsets, which may add extra registers.  */
>>>>> +   This is used by arm_compute_frame_layout, which may add extra registers.  */
>>>>>
>>>>>    static unsigned long
>>>>>    arm_compute_save_reg_mask (void)
>>>>> @@ -20926,12 +20934,25 @@ any_sibcall_could_use_r3 (void)
>>>>>      alignment.  */
>>>>>
>>>>>
>>>>> +/* Return cached stack offsets.  */
>>>>> +
>>>>> +static arm_stack_offsets *
>>>>> +arm_get_frame_offsets (void)
>>>>> +{
>>>>> +  struct arm_stack_offsets *offsets;
>>>>> +
>>>>> +  offsets = &cfun->machine->stack_offsets;
>>>>> +
>>>>> +  return offsets;
>>>>> +}
>>>>> +
>>>>> +
>>>>>    /* Calculate stack offsets.  These are used to calculate register elimination
>>>>>       offsets and in prologue/epilogue code.  Also calculates which registers
>>>>>       should be saved.  */
>>>>>
>>>>> -static arm_stack_offsets *
>>>>> -arm_get_frame_offsets (void)
>>>>> +static void
>>>>> +arm_compute_frame_layout (void)
>>>>>    {
>>>>>      struct arm_stack_offsets *offsets;
>>>>>      unsigned long func_type;
>>>>> @@ -20943,19 +20964,6 @@ any_sibcall_could_use_r3 (void)
>>>>>
>>>>>      offsets = &cfun->machine->stack_offsets;
>>>>>
>>>>> -  /* We need to know if we are a leaf function.  Unfortunately, it
>>>>> -     is possible to be called after start_sequence has been called,
>>>>> -     which causes get_insns to return the insns for the sequence,
>>>>> -     not the function, which will cause leaf_function_p to return
>>>>> -     the incorrect result.
>>>>> -
>>>>> -     to know about leaf functions once reload has completed, and the
>>>>> -     frame size cannot be changed after that time, so we can safely
>>>>> -     use the cached value.  */
>>>>> -
>>>>> -  if (reload_completed)
>>>>> -    return offsets;
>>>>> -
>>>>>      /* Initially this is the size of the local variables.  It will translated
>>>>>         into an offset once we have determined the size of preceding data.  */
>>>>>      frame_size = ROUND_UP_WORD (get_frame_size ());
>>>>> @@ -21022,7 +21030,7 @@ any_sibcall_could_use_r3 (void)
>>>>>        {
>>>>>          offsets->outgoing_args = offsets->soft_frame;
>>>>>          offsets->locals_base = offsets->soft_frame;
>>>>> -      return offsets;
>>>>> +      return;
>>>>>        }
>>>>>
>>>>>      /* Ensure SFP has the correct alignment.  */
>>>>> @@ -21098,8 +21106,6 @@ any_sibcall_could_use_r3 (void)
>>>>>    	offsets->outgoing_args += 4;
>>>>>          gcc_assert (!(offsets->outgoing_args & 7));
>>>>>        }
>>>>> -
>>>>> -  return offsets;
>>>>>    }
>>>>>
>>>>>
>>>>> Index: gcc/doc/tm.texi
>>>>> ===================================================================
>>>>> --- gcc/doc/tm.texi	(revision 239144)
>>>>> +++ gcc/doc/tm.texi	(working copy)
>>>>> @@ -3693,6 +3693,14 @@ registers.  This macro must be defined if @code{EL
>>>>>    defined.
>>>>>    @end defmac
>>>>>
>>>>> +@deftypefn {Target Hook} void TARGET_COMPUTE_FRAME_LAYOUT (void)
>>>>> +This target hook allows the target to compute the frame layout once and
>>>>> +make use of the cached frame layout in @code{INITIAL_ELIMINATION_OFFSET}
>>>>> +instead of re-computing it on every invocation.  This is particularly
>>>>> +useful for targets that have an expensive frame layout function.
>>>>> +Implementing this callback is optional.
>>>>> +@end deftypefn
>>>>> +
>>>>>    @node Stack Arguments
>>>>>    @subsection Passing Function Arguments on the Stack
>>>>>    @cindex arguments on stack
>>>>> Index: gcc/doc/tm.texi.in
>>>>> ===================================================================
>>>>> --- gcc/doc/tm.texi.in	(revision 239144)
>>>>> +++ gcc/doc/tm.texi.in	(working copy)
>>>>> @@ -3227,6 +3227,8 @@ registers.  This macro must be defined if @code{EL
>>>>>    defined.
>>>>>    @end defmac
>>>>>
>>>>> +@hook TARGET_COMPUTE_FRAME_LAYOUT
>>>>> +
>>>>>    @node Stack Arguments
>>>>>    @subsection Passing Function Arguments on the Stack
>>>>>    @cindex arguments on stack
>>>>> Index: gcc/lra-eliminations.c
>>>>> ===================================================================
>>>>> --- gcc/lra-eliminations.c	(revision 239144)
>>>>> +++ gcc/lra-eliminations.c	(working copy)
>>>>> @@ -1203,6 +1203,10 @@ update_reg_eliminate (bitmap insns_with_changed_of
>>>>>      struct lra_elim_table *ep, *ep1;
>>>>>      HARD_REG_SET temp_hard_reg_set;
>>>>>
>>>>> +#ifdef ELIMINABLE_REGS
>>>>> +  targetm.compute_frame_layout ();
>>>>> +#endif
>>>>> +
>>>>>      /* Clear self elimination offsets.  */
>>>>>      for (ep = reg_eliminate; ep < &reg_eliminate[NUM_ELIMINABLE_REGS]; ep++)
>>>>>        self_elim_offsets[ep->from] = 0;
>>>>> Index: gcc/reload1.c
>>>>> ===================================================================
>>>>> --- gcc/reload1.c	(revision 239144)
>>>>> +++ gcc/reload1.c	(working copy)
>>>>> @@ -3831,6 +3831,7 @@ verify_initial_elim_offsets (void)
>>>>>      {
>>>>>       struct elim_table *ep;
>>>>>
>>>>> +   targetm.compute_frame_layout ();
>>>>>       for (ep = reg_eliminate; ep < &reg_eliminate[NUM_ELIMINABLE_REGS]; ep++)
>>>>>         {
>>>>>           INITIAL_ELIMINATION_OFFSET (ep->from, ep->to, t);
>>>>> @@ -3855,6 +3856,7 @@ set_initial_elim_offsets (void)
>>>>>      struct elim_table *ep = reg_eliminate;
>>>>>
>>>>>    #ifdef ELIMINABLE_REGS
>>>>> +  targetm.compute_frame_layout ();
>>>>>      for (; ep < &reg_eliminate[NUM_ELIMINABLE_REGS]; ep++)
>>>>>        {
>>>>>          INITIAL_ELIMINATION_OFFSET (ep->from, ep->to, ep->initial_offset);
>>>>> Index: gcc/target.def
>>>>> ===================================================================
>>>>> --- gcc/target.def	(revision 239144)
>>>>> +++ gcc/target.def	(working copy)
>>>>> @@ -5269,8 +5269,19 @@ five otherwise.  This is best for most machines.",
>>>>>     unsigned int, (void),
>>>>>     default_case_values_threshold)
>>>>>
>>>>> -/* Retutn true if a function must have and use a frame pointer.  */
>>>>> +/* Optional callback to advise the target to compute the frame layout.  */
>>>>>    DEFHOOK
>>>>> +(compute_frame_layout,
>>>>> + "This target hook allows the target to compute the frame layout once and\n\
>>>>> +make use of the cached frame layout in @code{INITIAL_ELIMINATION_OFFSET}\n\
>>>>> +instead of re-computing it on every invocation.  This is particularly\n\
>>>>> +useful for targets that have an expensive frame layout function.\n\
>>>>> +Implementing this callback is optional.",
>>>>> + void, (void),
>>>>> + hook_void_void)
>>>>> +
>>>>> +/* Return true if a function must have and use a frame pointer.  */
>>>>> +DEFHOOK
>>>>>    (frame_pointer_required,
>>>>>     "This target hook should return @code{true} if a function must have and use\n\
>>>>>    a frame pointer.  This target hook is called in the reload pass.  If its return\n\
>>>
>>
>