In the next commit this will allow to reduce intermediate calls, when
recursing into subtrees - at that stage we know only subtree sha1, and
it is natural for tree walker to start from that phase. For now we do

    diff_tree
        show_path
            diff_tree_sha1
                diff_tree
                    ...

and the change will allow to reduce it to

    diff_tree
        show_path
            diff_tree

Also, it will allow to omit allocating strbuf for each subtree, and just
reuse the common strbuf via playing with its len.

The above-mentioned improvements go in the next 2 patches.

The downside is that try_to_follow_renames(), if active, we cause
re-reading of 2 initial trees, which was negligible based on my timings,
and which is outweighed cogently by the upsides.

NOTE To keep with the current interface and semantics, I needed to
rename the function. It will probably be renamed once more later, when
its semantic will change to just generate paths for a diff, instead of
producing it. So the current name is appropriate, but probably temporary.

Signed-off-by: Kirill Smelkov <k...@mns.spb.ru>
---
 diff.h      |  4 ++--
 tree-diff.c | 60 ++++++++++++++++++++++++++++--------------------------------
 2 files changed, 30 insertions(+), 34 deletions(-)

diff --git a/diff.h b/diff.h
index a24a767..4994d15 100644
--- a/diff.h
+++ b/diff.h
@@ -189,8 +189,8 @@ const char *diff_line_prefix(struct diff_options *);
 
 extern const char mime_boundary_leader[];
 
-extern int diff_tree(struct tree_desc *t1, struct tree_desc *t2,
-                    const char *base, struct diff_options *opt);
+extern int __diff_tree_sha1(const unsigned char *old, const unsigned char *new,
+                           const char *base, struct diff_options *opt);
 extern int diff_tree_sha1(const unsigned char *old, const unsigned char *new,
                          const char *base, struct diff_options *opt);
 extern int diff_root_tree_sha1(const unsigned char *new, const char *base,
diff --git a/tree-diff.c b/tree-diff.c
index 7688402..dd6c760 100644
--- a/tree-diff.c
+++ b/tree-diff.c
@@ -139,12 +139,17 @@ static void skip_uninteresting(struct tree_desc *t, 
struct strbuf *base,
        }
 }
 
-int diff_tree(struct tree_desc *t1, struct tree_desc *t2,
-             const char *base_str, struct diff_options *opt)
+int __diff_tree_sha1(const unsigned char *old, const unsigned char *new,
+                    const char *base_str, struct diff_options *opt)
 {
+       struct tree_desc t1, t2;
+       void *t1tree, *t2tree;
        struct strbuf base;
        int baselen = strlen(base_str);
 
+       t1tree = fill_tree_descriptor(&t1, old);
+       t2tree = fill_tree_descriptor(&t2, new);
+
        /* Enable recursion indefinitely */
        opt->pathspec.recursive = DIFF_OPT_TST(opt, RECURSIVE);
 
@@ -157,39 +162,41 @@ int diff_tree(struct tree_desc *t1, struct tree_desc *t2,
                if (diff_can_quit_early(opt))
                        break;
                if (opt->pathspec.nr) {
-                       skip_uninteresting(t1, &base, opt);
-                       skip_uninteresting(t2, &base, opt);
+                       skip_uninteresting(&t1, &base, opt);
+                       skip_uninteresting(&t2, &base, opt);
                }
-               if (!t1->size && !t2->size)
+               if (!t1.size && !t2.size)
                        break;
 
-               cmp = tree_entry_pathcmp(t1, t2);
+               cmp = tree_entry_pathcmp(&t1, &t2);
 
                /* t1 = t2 */
                if (cmp == 0) {
                        if (DIFF_OPT_TST(opt, FIND_COPIES_HARDER) ||
-                           hashcmp(t1->entry.sha1, t2->entry.sha1) ||
-                           (t1->entry.mode != t2->entry.mode))
-                               show_path(&base, opt, t1, t2);
+                           hashcmp(t1.entry.sha1, t2.entry.sha1) ||
+                           (t1.entry.mode != t2.entry.mode))
+                               show_path(&base, opt, &t1, &t2);
 
-                       update_tree_entry(t1);
-                       update_tree_entry(t2);
+                       update_tree_entry(&t1);
+                       update_tree_entry(&t2);
                }
 
                /* t1 < t2 */
                else if (cmp < 0) {
-                       show_path(&base, opt, t1, /*t2=*/NULL);
-                       update_tree_entry(t1);
+                       show_path(&base, opt, &t1, /*t2=*/NULL);
+                       update_tree_entry(&t1);
                }
 
                /* t1 > t2 */
                else {
-                       show_path(&base, opt, /*t1=*/NULL, t2);
-                       update_tree_entry(t2);
+                       show_path(&base, opt, /*t1=*/NULL, &t2);
+                       update_tree_entry(&t2);
                }
        }
 
        strbuf_release(&base);
+       free(t2tree);
+       free(t1tree);
        return 0;
 }
 
@@ -204,7 +211,7 @@ static inline int diff_might_be_rename(void)
                !DIFF_FILE_VALID(diff_queued_diff.queue[0]->one);
 }
 
-static void try_to_follow_renames(struct tree_desc *t1, struct tree_desc *t2, 
const char *base, struct diff_options *opt)
+static void try_to_follow_renames(const unsigned char *old, const unsigned 
char *new, const char *base, struct diff_options *opt)
 {
        struct diff_options diff_opts;
        struct diff_queue_struct *q = &diff_queued_diff;
@@ -242,7 +249,7 @@ static void try_to_follow_renames(struct tree_desc *t1, 
struct tree_desc *t2, co
        diff_opts.break_opt = opt->break_opt;
        diff_opts.rename_score = opt->rename_score;
        diff_setup_done(&diff_opts);
-       diff_tree(t1, t2, base, &diff_opts);
+       __diff_tree_sha1(old, new, base, &diff_opts);
        diffcore_std(&diff_opts);
        free_pathspec(&diff_opts.pathspec);
 
@@ -303,23 +310,12 @@ static void try_to_follow_renames(struct tree_desc *t1, 
struct tree_desc *t2, co
 
 int diff_tree_sha1(const unsigned char *old, const unsigned char *new, const 
char *base, struct diff_options *opt)
 {
-       void *tree1, *tree2;
-       struct tree_desc t1, t2;
-       unsigned long size1, size2;
        int retval;
 
-       tree1 = fill_tree_descriptor(&t1, old);
-       tree2 = fill_tree_descriptor(&t2, new);
-       size1 = t1.size;
-       size2 = t2.size;
-       retval = diff_tree(&t1, &t2, base, opt);
-       if (!*base && DIFF_OPT_TST(opt, FOLLOW_RENAMES) && 
diff_might_be_rename()) {
-               init_tree_desc(&t1, tree1, size1);
-               init_tree_desc(&t2, tree2, size2);
-               try_to_follow_renames(&t1, &t2, base, opt);
-       }
-       free(tree1);
-       free(tree2);
+       retval = __diff_tree_sha1(old, new, base, opt);
+       if (!*base && DIFF_OPT_TST(opt, FOLLOW_RENAMES) && 
diff_might_be_rename())
+               try_to_follow_renames(old, new, base, opt);
+
        return retval;
 }
 
-- 
1.9.rc1.181.g641f458

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