Re: [PATCH] rtl-optimization/110587 - remove quadratic regno_in_use_p

[Vladimir Makarov <vmakarov@redhat.com>]

On 7/25/23 09:40, Richard Biener wrote:
> The following removes the code checking whether a noop copy
> is between something involved in the return sequence composed
> of a SET and USE.  Instead of checking for this special-case
> the following makes us only ever remove noop copies between
> pseudos - which is the case that is necessary for IRA/LRA
> interfacing to function according to the comment.  That makes
> looking for the return reg special case unnecessary, reducing
> the compile-time in LRA non-specific to zero for the testcase.
>
> Bootstrapped and tested on x86_64-unknown-linux-gnu with
> all languages and {,-m32}.
>
> OK?
>
Richard, sorry for the delay with the answer.  I was on vacation.

There is a lot of history of changes of the code.  I believe your change 
is right.  I don't think that RTL will ever contain noop return move 
insn involving the return hard register especially after removing hard 
reg propagation couple years ago, at least IRA/LRA do not generate such 
insns during its work.

So the patch is OK for me.  I specially like that the big part of code 
is removed.  No code, no problem (including performance one).  Thank you 
for the patch.

> 	PR rtl-optimization/110587
> 	* lra-spills.cc (return_regno_p): Remove.
> 	(regno_in_use_p): Likewise.
> 	(lra_final_code_change): Do not remove noop moves
> 	between hard registers.
> ---
>   gcc/lra-spills.cc | 69 +----------------------------------------------
>   1 file changed, 1 insertion(+), 68 deletions(-)
>
> diff --git a/gcc/lra-spills.cc b/gcc/lra-spills.cc
> index 3a7bb7e8cd9..fe58f162d05 100644
> --- a/gcc/lra-spills.cc
> +++ b/gcc/lra-spills.cc
> @@ -705,72 +705,6 @@ alter_subregs (rtx *loc, bool final_p)
>     return res;
>   }