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Re: RFC: lra-constraints.c and TARGET_HARD_REGNO_CALL_PART_CLOBBERED question/patch
Jeff Law <firstname.lastname@example.org> 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 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
>>  https://gcc.gnu.org/ml/gcc/2018-07/msg00012.html
>> 2018-07-11 Steve Ellcey <email@example.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).