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


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.

jeff


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