Re: [PATCH][expr.c] PR 65358 Avoid clobbering partial argument during sibcall

[Jeff Law <law@redhat.com>]

On 05/11/2015 03:28 AM, Kyrill Tkachov wrote:
>> The more I think about this, the more I think it's an ugly can of
>> worms and maybe we should just disable sibcalls for partial
>> arguments.  I doubt it's a big performance issue in general.
>
> We already have quite a bit of code in calls.c to detect cases with
> partial argument overlap for the
> explicit purpose of allowing sibcalls when partial arguments occur in
> the general case. However, that
> code only detects when a partial argument overlaps with other arguments
> in a call. In this PR the
> partial argument overlaps with itself. It would be a shame to disable
> sibcalls for all partial arguments
> when there is already infrastructure in place to handle them.
I didn't even realize we had support for partial arguments in sibcalls. 
   Ah, Kazu added that in 2005, I totally missed it.  I probably would 
have suggested failing the sibcall for those cases back then too...

Is there any way to re-use that infrastructure to deal with the case at 
hand?



>
>>
>>
>> In addition to the argument/stack direction stuff, I've been pondering
>> the stack/frame/arg pointer issues.  Your approach assumes that the
>> incoming and outgoing areas are always referenced off the same base
>> register.  If they aren't, then the routine returns no overlap.
>>
>> But we'd need to consider the case where we have a reference to the
>> arg or frame pointer which later gets rewritten into a stack pointer
>> relative address.
>>
>> Is it too late at the point were you do the checks to reject the
>> sibling call?  If not, then maybe the overlap routine should return a
>> tri-state.  No overlap, overlap, don't know.  The last would be used
>> when the two addresses use a different register.
>
> Ok, here is my attempt at that. The overlap functions returns -2 when it
> cannot staticall compare the
> two pointers (i.e. when the base registers are different) and the caller
> then disables sibcalls.
> The code in calls.c that calls this code will undo any emitted
> instructions in the meantime if sibcall
> optimisation fails. This required me to change the type of
> emit_push_insn to bool and add an extra
> parameter, so this patch touches a bit more code than the original version.
>
> Bootstrapped on x86_64 and tested on arm. The testcase in this PR still
> performs a sibcall correctly on arm.
>
> What do you think of this?
>
> Thanks,
> Kyrill
>
>
> 2015-05-11  Kyrylo Tkachov <kyrylo.tkachov@arm.com>
>
>      PR target/65358
>      * expr.c (memory_load_overlap): New function.
>      (emit_push_insn): When pushing partial args to the stack would
>      clobber the register part load the overlapping part into a pseudo
>      and put it into the hard reg after pushing.  Change return type
>      to bool.  Add bool argument.
>      * expr.h (emit_push_insn): Change return type to bool.
>      Add bool argument.
>      * calls.c (expand_call): Cancel sibcall optimisation when encountering
>      partial argument on targets with ARGS_GROW_DOWNWARD and
>      !STACK_GROWS_DOWNWARD.
>      (emit_library_call_value_1): Update callsite of emit_push_insn.
>      (store_one_arg): Likewise.
>
>
> 2015-05-11  Honggyu Kim <hong.gyu.kim@lge.com>
>
>      PR target/65358
>      * gcc.dg/pr65358.c: New test.
>
>>
>> Jeff
>>
>>
>
>
> expr.patch
>
>
> commit 5b596f10846b6d3b143442a306801c8262d8b10a
> Author: Kyrylo Tkachov<kyrylo.tkachov@arm.com>
> Date:   Wed Mar 18 13:42:37 2015 +0000
>
>      [expr.c] PR 65358 Avoid clobbering partial argument during sibcall
>
> diff --git a/gcc/calls.c b/gcc/calls.c
> index caa7d60..81ef2c9 100644
> --- a/gcc/calls.c
> +++ b/gcc/calls.c
> @@ -3225,6 +3225,13 @@ expand_call (tree exp, rtx target, int ignore)
>   	    {
>   	      rtx_insn *before_arg = get_last_insn ();
>
> +	     /* On targets with weird calling conventions (e.g. PA) it's
> +		hard to ensure that all cases of argument overlap between
> +		stack and registers work.  Play it safe and bail out.  */
> +#if defined (ARGS_GROW_DOWNWARD) && !defined (STACK_GROWS_DOWNWARD)
> +	      sibcall_failure = 1;
> +	      break;
> +#endif
So we're trying to get away from this kind of conditional compilation.

Instead we want to write

if (ARGS_GROW_DOWNWARD && !STACK_GROWS_DOWNWARD)

ARGS_GROW_DOWNWARD is already a testable value.  But 
STACK_GROWS_DOWNWARD is not.  The way folks have been dealing with this 
is something like this after the #includes:

/* Redefine STACK_GROWS_DOWNWARD in terms of 0 or 1.  */
#ifdef STACK_GROWS_DOWNWARD
# undef STACK_GROWS_DOWNWARD
# define STACK_GROWS_DOWNWARD 1
#else
# define STACK_GROWS_DOWNWARD 0
#endif


With that in place you can change the test into the more desirable
if (ARGS_GROW_DOWNWARD && !STACK_GROWS_DOWNWARD)



> diff --git a/gcc/expr.c b/gcc/expr.c
> index 25aa11f..712fa0b 100644
> --- a/gcc/expr.c
> +++ b/gcc/expr.c
> @@ -4121,12 +4121,35 @@ emit_single_push_insn (machine_mode mode, rtx x, tree type)
>   }
>   #endif
>
> +/* If reading SIZE bytes from X will end up reading from
> +   Y return the number of bytes that overlap.  Return -1
> +   if there is no overlap or -2 if we can't determing
s/determing/determine/

> +   partial argument during a sibcall optimisation (as specified by
s/optimisation/optimization/


I guess I'm still not real comfortable that we've got the issues here 
resolved and if we're going to try and support this case, then re-using 
the existing infrastructure would be better.

I'll approve with the changes noted above, but would ask that you look 
into trying to re-use the existing infrastructure as a follow-up.

Jeff