Subject: Re: [GSoC Patch 3/3] commit: convert commit->graph_pos to a slab

From: Jakub Narębski <[email-masked]>
Date: Sun, 07 Jun 2020 14:12:21 +0200

Abhishek Kumar <[email-masked]> writes:

> The member graph_pos refers to the integer position used to identify a
> commit in commit-graph files. However, graph_pos is not useful in other
> contexts and bloats the struct.
>
> Let's move it to a commit-slab and shrink the struct by four bytes.
>
> Existing references to graph_pos are replaced using
> 'contrib/coccinelle/graph_pos.cocci'.
>
> Signed-off-by: Abhishek Kumar <[email-masked]>
> ---
>  alloc.c                            |  1 -
>  bloom.c                            |  6 ++--
>  commit-graph.c                     | 50 +++++++++++++++++++++++-------
>  commit-graph.h                     |  3 ++
>  commit.c                           |  2 +-
>  commit.h                           |  2 --
>  contrib/coccinelle/graph_pos.cocci | 12 +++++++
>  7 files changed, 58 insertions(+), 18 deletions(-)
>  create mode 100644 contrib/coccinelle/graph_pos.cocci

I have reordered the chunks to make it easier to review.

> diff --git a/commit.h b/commit.h
> index 01e1c4c3eb..0b10464a10 100644
> --- a/commit.h
> +++ b/commit.h
> @@ -10,8 +10,6 @@
>  #include "pretty.h"
>  #include "commit-slab.h"
>  
> -#define COMMIT_NOT_FROM_GRAPH 0xFFFFFFFF
> -
>  struct commit_list {
>  	struct commit *item;
>  	struct commit_list *next;
> diff --git a/commit-graph.h b/commit-graph.h
> index 653bd041ad..3cb59ba336 100644
> --- a/commit-graph.h
> +++ b/commit-graph.h
> @@ -8,6 +8,7 @@
>  #include "object-store.h"
>  #include "oidset.h"
>  
> +#define COMMIT_NOT_FROM_GRAPH 0xFFFFFFFF
>  #define GENERATION_NUMBER_INFINITY 0xFFFFFFFF
>  #define GENERATION_NUMBER_MAX 0x3FFFFFFF
>  #define GENERATION_NUMBER_ZERO 0

Those two chunks move COMMIT_NOT_FROM_GRAPH from commit.h to
commit-graph.h, because it is no longer needed in init_commit_node()
from alloc.c.  On the other hand the remaining commit-graph #define-s
were moved in first commit in series.

What I DO NOT SEE in this commit is actual *removal* of `graph_pos`
field from the `struct commit`, i.e.:

  diff --git a/commit.h b/commit.h
  index cc610400d5..01e1c4c3eb 100644
  --- a/commit.h
  +++ b/commit.h
  @@ -34,6 +34,5 @@ struct commit {
   	 */
   	struct tree *maybe_tree;
  -	uint32_t graph_pos;
   	unsigned int index;
   };

> diff --git a/alloc.c b/alloc.c
> index cbed187094..f37fb3b8b6 100644
> --- a/alloc.c
> +++ b/alloc.c
> @@ -108,7 +108,6 @@ void init_commit_node(struct repository *r, struct commit *c)
>  {
>  	c->object.type = OBJ_COMMIT;
>  	c->index = alloc_commit_index(r);
> -	c->graph_pos = COMMIT_NOT_FROM_GRAPH;
>  }
>
>  void *alloc_commit_node(struct repository *r)

This removes commit->graph_pos initialization from init_commit_node(),
and thus from alloc_commit_node(); the handling of COMMIT_NOT_FROM_GRAPH
is moved to setter and getter "methods", see below.

> diff --git a/commit-graph.c b/commit-graph.c
> index 9ce7d4acb1..7ff460b442 100644
> --- a/commit-graph.c
> +++ b/commit-graph.c
> @@ -87,6 +87,34 @@ static int commit_pos_cmp(const void *va, const void *vb)
>  	       commit_pos_at(&commit_pos, b);
>  }
>  
> +define_commit_slab(graph_pos_slab, uint32_t);
> +static struct graph_pos_slab graph_pos_slab = COMMIT_SLAB_INIT(1, graph_pos_slab);
> +
> +uint32_t graph_pos(const struct commit *c)
> +{
> +	uint32_t *pos = graph_pos_slab_peek(&graph_pos_slab, c);
> +
> +	return pos ? *pos : COMMIT_NOT_FROM_GRAPH;
> +}
> +
> +static void set_graph_pos(const struct commit *c, const uint32_t position)
> +{
> +	unsigned int i = graph_pos_slab.slab_count;
> +	uint32_t *pos = graph_pos_slab_at(&graph_pos_slab, c);
> +
> +	/*
> +	 * commit-slab initializes with zero, overwrite this with
> +	 * COMMIT_NOT_FROM_GRAPH
> +	 */
> +	for (; i < graph_pos_slab.slab_count; ++i)
> +	{
> +		memset(graph_pos_slab.slab[i], COMMIT_NOT_FROM_GRAPH,
> +		       graph_pos_slab.slab_size * sizeof(uint32_t));
> +	}
> +
> +	*pos = position;
> +}
> +
>  define_commit_slab(generation_slab, uint32_t);
>  static struct generation_slab generation_slab = COMMIT_SLAB_INIT(1, generation_slab);
>  
[...]
> @@ -142,4 +143,6 @@ void free_commit_graph(struct commit_graph *);
>  void disable_commit_graph(struct repository *r);
>  
>  uint32_t generation(const struct commit *c);
> +
> +uint32_t graph_pos(const struct commit *c);
>  #endif

I wonder why those helper functions: graph_pos() and set_graph_pos()
were not introduced in the 1st patch of this series, together with
generation() and set_generation().

The same comments as for previous patch apply: if graph_pos was not
explicitely set, we want for it to be COMMIT_NOT_FROM_GRAPH (like
init_commit_node() did before this change).  If it is not on
commit-slab, it is not set -- this is handled by graph_pos() function.
However we allocate memory on slab in chunks, and set_graph_pos()
ensures that those extra allocated `graph_pos` values on commit-slab are
properly initialized to COMMIT_NOT_FROM_GRAPH, like init_commit_node()
did.

Note that we always have COMMIT_NOT_FROM_GRAPH for `graph_pos` if and
only if there is GENERATION_NUMBER_INFINITY for `generation`, so perhaps
putting those two together in `struct commit_graph_info` would make
sense.  But whether doing more work in "setter" set_*() functions or
doing extra conditional in "getter" *() would give better performance
needs (micro-)benchmarking.

> diff --git a/contrib/coccinelle/graph_pos.cocci b/contrib/coccinelle/graph_pos.cocci
> new file mode 100644
> index 0000000000..0929164bdf
> --- /dev/null
> +++ b/contrib/coccinelle/graph_pos.cocci
> @@ -0,0 +1,12 @@
> +@@
> +struct commit *c;
> +expression E;
> +@@
> +- c->graph_pos = E
> ++ set_graph_pos(c, E)
> +
> +@@
> +struct commit *c;
> +@@
> +- c->graph_pos
> ++ graph_pos(c)

This is semantic patch that generates the rest of changes.  (If any of
those needs manual improvement, it is better left for a separate "manual
fixup" patch, in my opinion.)

> diff --git a/bloom.c b/bloom.c
> index 9b86aa3f59..5bee5bb0c1 100644
> --- a/bloom.c
> +++ b/bloom.c
> @@ -34,14 +34,14 @@ static int load_bloom_filter_from_graph(struct commit_graph *g,
>  {
>  	uint32_t lex_pos, start_index, end_index;
>  
> -	while (c->graph_pos < g->num_commits_in_base)
> +	while (graph_pos(c) < g->num_commits_in_base)
>  		g = g->base_graph;
>  
>  	/* The commit graph commit 'c' lives in doesn't carry bloom filters. */
>  	if (!g->chunk_bloom_indexes)
>  		return 0;
>  
> -	lex_pos = c->graph_pos - g->num_commits_in_base;
> +	lex_pos = graph_pos(c) - g->num_commits_in_base;
>  
>  	end_index = get_be32(g->chunk_bloom_indexes + 4 * lex_pos);
>

Here graph_pos(c) is used twice.

It probably needs to be checked (with benchmark) if it matters.

> @@ -188,7 +188,7 @@ struct bloom_filter *get_bloom_filter(struct repository *r,
>  
>  	if (!filter->data) {
>  		load_commit_graph_info(r, c);
> -		if (c->graph_pos != COMMIT_NOT_FROM_GRAPH &&
> +		if (graph_pos(c) != COMMIT_NOT_FROM_GRAPH &&
>  			r->objects->commit_graph->chunk_bloom_indexes) {
>  			if (load_bloom_filter_from_graph(r->objects->commit_graph, filter, c))
>  				return filter;

> diff --git a/commit-graph.c b/commit-graph.c
> index 9ce7d4acb1..7ff460b442 100644
> --- a/commit-graph.c
> +++ b/commit-graph.c
> @@ -697,7 +725,7 @@ static struct commit_list **insert_parent_or_die(struct repository *r,
>  	c = lookup_commit(r, &oid);
>  	if (!c)
>  		die(_("could not find commit %s"), oid_to_hex(&oid));
> -	c->graph_pos = pos;
> +	set_graph_pos(c, pos);
>  	return &commit_list_insert(c, pptr)->next;
>  }
>  
> @@ -711,7 +739,7 @@ static void fill_commit_graph_info(struct commit *item, struct commit_graph *g,
>  
>  	lex_index = pos - g->num_commits_in_base;
>  	commit_data = g->chunk_commit_data + GRAPH_DATA_WIDTH * lex_index;
> -	item->graph_pos = pos;
> +	set_graph_pos(item, pos);
>  	set_generation(item, get_be32(commit_data + g->hash_len + 8) >> 2);
>  }
>  
> @@ -741,7 +769,7 @@ static int fill_commit_in_graph(struct repository *r,
>  	 * Store the "full" position, but then use the
>  	 * "local" position for the rest of the calculation.
>  	 */
> -	item->graph_pos = pos;
> +	set_graph_pos(item, pos);
>  	lex_index = pos - g->num_commits_in_base;
>  
>  	commit_data = g->chunk_commit_data + (g->hash_len + 16) * lex_index;

> @@ -786,8 +814,8 @@ static int fill_commit_in_graph(struct repository *r,
>  
>  static int find_commit_in_graph(struct commit *item, struct commit_graph *g, uint32_t *pos)
>  {
> -	if (item->graph_pos != COMMIT_NOT_FROM_GRAPH) {
> -		*pos = item->graph_pos;
> +	if (graph_pos(item) != COMMIT_NOT_FROM_GRAPH) {
> +		*pos = graph_pos(item);
>  		return 1;

Here graph_pos(item) is used twice.

>  	} else {
>  		struct commit_graph *cur_g = g;
> @@ -843,11 +871,11 @@ static struct tree *load_tree_for_commit(struct repository *r,
>  	struct object_id oid;
>  	const unsigned char *commit_data;
>  
> -	while (c->graph_pos < g->num_commits_in_base)
> +	while (graph_pos(c) < g->num_commits_in_base)
>  		g = g->base_graph;
>  
>  	commit_data = g->chunk_commit_data +
> -			GRAPH_DATA_WIDTH * (c->graph_pos - g->num_commits_in_base);
> +			GRAPH_DATA_WIDTH * (graph_pos(c) - g->num_commits_in_base);
>  
>  	hashcpy(oid.hash, commit_data);
>  	set_commit_tree(c, lookup_tree(r, &oid));

Here graph_pos(c) is used twice.

> @@ -861,7 +889,7 @@ static struct tree *get_commit_tree_in_graph_one(struct repository *r,
>  {
>  	if (c->maybe_tree)
>  		return c->maybe_tree;
> -	if (c->graph_pos == COMMIT_NOT_FROM_GRAPH)
> +	if (graph_pos(c) == COMMIT_NOT_FROM_GRAPH)
>  		BUG("get_commit_tree_in_graph_one called from non-commit-graph commit");
>  
>  	return load_tree_for_commit(r, g, (struct commit *)c);
> @@ -1247,7 +1275,7 @@ static void close_reachable(struct write_commit_graph_context *ctx)
>  			continue;
>  		if (ctx->split) {
>  			if ((!parse_commit(commit) &&
> -			     commit->graph_pos == COMMIT_NOT_FROM_GRAPH) ||
> +			     graph_pos(commit) == COMMIT_NOT_FROM_GRAPH) ||
>  			    flags == COMMIT_GRAPH_SPLIT_REPLACE)
>  				add_missing_parents(ctx, commit);
>  		} else if (!parse_commit_no_graph(commit))
> @@ -1493,7 +1521,7 @@ static uint32_t count_distinct_commits(struct write_commit_graph_context *ctx)
>  			if (ctx->split) {
>  				struct commit *c = lookup_commit(ctx->r, &ctx->oids.list[i]);
>  
> -				if (!c || c->graph_pos != COMMIT_NOT_FROM_GRAPH)
> +				if (!c || graph_pos(c) != COMMIT_NOT_FROM_GRAPH)
>  					continue;
>  			}
>  
> @@ -1527,7 +1555,7 @@ static void copy_oids_to_commits(struct write_commit_graph_context *ctx)
>  		ctx->commits.list[ctx->commits.nr] = lookup_commit(ctx->r, &ctx->oids.list[i]);
>  
>  		if (ctx->split && flags != COMMIT_GRAPH_SPLIT_REPLACE &&
> -		    ctx->commits.list[ctx->commits.nr]->graph_pos != COMMIT_NOT_FROM_GRAPH)
> +		    graph_pos(ctx->commits.list[ctx->commits.nr]) != COMMIT_NOT_FROM_GRAPH)
>  			continue;
>  
>  		if (ctx->split && flags == COMMIT_GRAPH_SPLIT_REPLACE)
> diff --git a/commit.c b/commit.c
> index 8dad0f8446..da6de08b2b 100644
> --- a/commit.c
> +++ b/commit.c
> @@ -339,7 +339,7 @@ struct tree *repo_get_commit_tree(struct repository *r,
>  	if (commit->maybe_tree || !commit->object.parsed)
>  		return commit->maybe_tree;
>  
> -	if (commit->graph_pos != COMMIT_NOT_FROM_GRAPH)
> +	if (graph_pos(commit) != COMMIT_NOT_FROM_GRAPH)
>  		return get_commit_tree_in_graph(r, commit);
>  
>  	return NULL;

Best,
-- 
Jakub Narębski