[PATCH RFC] c-family: don't cache large vecs

[Marek Polacek]

On Tue, Nov 16, 2021 at 11:53:14AM -0500, Jason Merrill via Gcc-patches wrote:
> Patrick observed recently that an element of the vector cache could be
> arbitrarily large.  Let's only cache relatively small vecs.
> 
> This has no effect on compiling the libstdc++ stdc++.h, presumably because
> nothing in the library requires a vec that large.  I figure that this makes it
> more likely that a subsequent long list will reuse the same memory when the
> later vec gets expanded.
> 
> Does this make sense to others?

Looks good to me.
 
> gcc/c-family/ChangeLog:
> 
> 	* c-common.c (release_tree_vector): Only cache vecs smaller than
> 	16 elements.
> ---
>  gcc/c-family/c-common.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/gcc/c-family/c-common.c b/gcc/c-family/c-common.c
> index 436df45df68..90e8ec87b6b 100644
> --- a/gcc/c-family/c-common.c
> +++ b/gcc/c-family/c-common.c
> @@ -8213,8 +8213,16 @@ release_tree_vector (vec<tree, va_gc> *vec)
>  {
>    if (vec != NULL)
>      {
> -      vec->truncate (0);
> -      vec_safe_push (tree_vector_cache, vec);
> +      if (vec->allocated () >= 16)
> +	/* Don't cache vecs that have expanded more than once.  On a p64
> +	   target, vecs double in alloc size with each power of 2 elements, e.g
> +	   at 16 elements the alloc increases from 128 to 256 bytes.  */
> +	vec_free (vec);
> +      else
> +	{
> +	  vec->truncate (0);
> +	  vec_safe_push (tree_vector_cache, vec);
> +	}
>      }
>  }
>  
> 
> base-commit: 132f1c27770fa6dafdf14591878d301aedd5ae16
> -- 
> 2.27.0
> 

Marek