Re: [PATCH] PR tree-optimization/108697 - Create a lazy ssa_cache

[Andrew MacLeod <amacleod@redhat.com>]

On 2/16/23 02:55, Richard Biener wrote:
> On Wed, Feb 15, 2023 at 6:07 PM Andrew MacLeod via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
>> This patch implements the suggestion that we have an alternative
>> ssa-cache which does not zero memory, and instead uses a bitmap to track
>> whether a value is currently set or not.  It roughly mimics what
>> path_range_query was doing internally.
>>
>> For sparsely used cases, expecially in large programs, this is more
>> efficient.  I changed path_range_query to use this, and removed it old
>> bitmap (and a hack or two around PHI calculations), and also utilized
>> this is the assume_query class.
>>
>> Performance wise, the patch doesn't affect VRP (since that still uses
>> the original version).  Switching to the lazy version caused a slowdown
>> of 2.5% across VRP.
>>
>> There was a noticeable improvement elsewhere.,  across 230 GCC source
>> files, threading ran over 12% faster!.  Overall compilation improved by
>> 0.3%  Not sure it makes much difference in compiler.i, but it shouldn't
>> hurt.
>>
>> bootstraps on x86_64-pc-linux-gnu with no regressions.   OK for trunk?
>> or do you want to wait for the next release...
> I see
>
> @@ -365,16 +335,8 @@ path_range_query::compute_ranges_in_phis (basic_block bb)
>
>         Value_Range r (TREE_TYPE (name));
>         if (range_defined_in_block (r, name, bb))
> -       {
> -         unsigned v = SSA_NAME_VERSION (name);
> -         set_cache (r, name);
> -         bitmap_set_bit (phi_set, v);
> -         // Pretend we don't have a cache entry for this name until
> -         // we're done with all PHIs.
> -         bitmap_clear_bit (m_has_cache_entry, v);
> -       }
> +       m_cache.set_global_range (name, r);
>       }
> -  bitmap_ior_into (m_has_cache_entry, phi_set);
>   }
>
>   // Return TRUE if relations may be invalidated after crossing edge E.
>
> which I think is not correct - if we have
>
>   # _1 = PHI <..., _2>
>   # _2 = PHI <..., _1>
>
> then their effects are supposed to be executed in parallel, that is,
> both PHI argument _2 and _1 are supposed to see the "old" version.
> The previous code tried to make sure the range of the new _1 doesn't
> get seen when processing the argument _1 in the definition of _2.
>
> The new version drops this, possibly resulting in wrong-code.

This is dropped because it is actually handled properly in 
range_defined_in_block now.  (which I think Aldy was describing).

It didnt make sense to me why it was handled here like this, so I traced 
through the call chain to find out if it was still actually needed and 
discussed it with Aldy.  I think it was mostly a leftover wart.

>
> While I think it's appropriate to sort out compile-time issues like this
> during stage4 at least the above makes me think it should be defered
> to next stage1.

I am happy to defer it since its a marginal increase anyway.

Andrew