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Re: RFC: lra-constraints.c and TARGET_HARD_REGNO_CALL_PART_CLOBBERED question/patch


Jeff Law <law@redhat.com> writes:
> On 07/11/2018 02:07 PM, Steve Ellcey wrote:
>> I have a reload/register allocation question and possible patch.  While
>> working on the Aarch64 SIMD ABI[1] I ran into a problem where GCC was
>> saving and restoring registers that it did not need to.  I tracked it
>> down to lra-constraints.c and its use of
>> targetm.hard_regno_call_part_clobbered on instructions that are not
>> calls.  Specifically need_for_call_save_p would check this macro even
>> when the instruction in question (unknown to need_for_call_save_p)
>> was not a call instruction.
>> 
>> This seems wrong to me and I was wondering if anyone more familiar
>> with the register allocator and reload could look at this patch and
>> tell me if it seems reasonable or not.  It passed bootstrap and I
>> am running tests now.  I am just wondering if there is any reason why
>> this target function would need to be called on non-call instructions
>> or if doing so is just an oversight/bug.
>> 
>> Steve Ellcey
>> sellcey@cavium.com
>> 
>> 
>> [1] https://gcc.gnu.org/ml/gcc/2018-07/msg00012.html
>> 
>> 
>> 2018-07-11  Steve Ellcey  <sellcey@cavium.com>
>> 
>> 	* lra-constraints.c (need_for_call_save_p): Add insn argument
>> 	and only check targetm.hard_regno_call_part_clobbered on calls.
>> 	(need_for_split_p): Add insn argument, pass to need_for_call_save_p.
>> 	(split_reg): Pass insn to need_for_call_save_p.
>> 	(split_if_necessary): Pass curr_insn to need_for_split_p.
>> 	(inherit_in_ebb): Ditto.
> Various target have calls which are exposed as INSNs rather than as
> CALL_INSNs.   So we need to check that hook on all insns.
>
> You can probably see this in action with the TLS insns on aarch64.

Not sure whether it's that: I think other code does only consider
hard_regno_call_part_clobbered on calls.  But as it stands
need_for_call_save_p is checking whether there's a call somewhere
inbetween the current instruction and the last use in the EBB:

/* Return true if we need a caller save/restore for pseudo REGNO which
   was assigned to a hard register.  */
static inline bool
need_for_call_save_p (int regno)
{
  lra_assert (regno >= FIRST_PSEUDO_REGISTER && reg_renumber[regno] >= 0);
  return (usage_insns[regno].calls_num < calls_num
...
}

So it only calls targetm.hard_regno_call_part_clobbered if such a
call is known to exist somewhere between the two references to
regno (although we don't have the calls themselves to hand).

Thanks,
Richard


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