This is just half-a-review (bottom half of the file).
> +static int histogram_diff(xpparam_t const *xpp, xdfenv_t *env,
> + int line1, int count1, int line2, int count2)
> +{
What does this function return?
> + struct histindex index;
> + struct region lcs;
> + int sz;
> + int result = -1;
> +
> + if (count1 <= 0 && count2 <= 0)
> + return 0;
> +
> + if (LINE_END(1) >= MAX_PTR)
> + return -1;
> +
> + if (!count1) {
> + while(count2--)
> + env->xdf2.rchg[line2++ - 1] = 1;
> + return 0;
> + } else if (!count2) {
> + while(count1--)
> + env->xdf1.rchg[line1++ - 1] = 1;
> + return 0;
> + }
> +
> + memset(&index, 0, sizeof(index));
> +
> + index.env = env;
> + index.xpp = xpp;
> +
> + index.records = NULL;
> + index.line_map = NULL;
> + /* in case of early xdl_cha_free() */
> + index.rcha.head = NULL;
> +
> + index.table_bits = xdl_hashbits(count1);
> + sz = index.records_size = 1 << index.table_bits;
> + sz *= sizeof(struct record *);
> + if (!(index.records = (struct record **) xdl_malloc(sz)))
> + goto cleanup;
> + memset(index.records, 0, sz);
> +
> + sz = index.line_map_size = count1;
> + sz *= sizeof(struct record *);
> + if (!(index.line_map = (struct record **) xdl_malloc(sz)))
> + goto cleanup;
> + memset(index.line_map, 0, sz);
> +
> + sz = index.line_map_size;
> + sz *= sizeof(unsigned int);
> + if (!(index.next_ptrs = (unsigned int *) xdl_malloc(sz)))
> + goto cleanup;
> + memset(index.next_ptrs, 0, sz);
> +
> + /* lines / 4 + 1 comes from xprepare.c:xdl_prepare_ctx() */
> + if (xdl_cha_init(&index.rcha, sizeof(struct record), count1 / 4 + 1) < 0)
> + goto cleanup;
> +
> + index.ptr_shift = line1;
> + index.max_chain_length = 64;
> +
> + memset(&lcs, 0, sizeof(lcs));
> + if (find_lcs(&index, &lcs, line1, count1, line2, count2))
> + result = fall_back_to_classic_diff(&index, line1, count1, line2, count2);
> + else {
> + result = 0;
> + if (lcs.begin1 == 0 && lcs.begin2 == 0) {
> + int ptr;
> + for (ptr = 0; ptr < count1; ptr++)
> + env->xdf1.rchg[line1 + ptr - 1] = 1;
> + for (ptr = 0; ptr < count2; ptr++)
> + env->xdf2.rchg[line2 + ptr - 1] = 1;
> + } else {
> + result = histogram_diff(xpp, env,
> + line1, lcs.begin1 - line1,
> + line2, lcs.begin2 - line2);
> + result = histogram_diff(xpp, env,
> + lcs.end1 + 1, LINE_END(1) - lcs.end1,
> + lcs.end2 + 1, LINE_END(2) - lcs.end2);
The result from the first half before lcs is discarded?
> + result *= -1;
Again, what does this function (called recursively) return, and what does
flipping the sign of it do?
> + }
> + }
> +
> +cleanup:
> + xdl_free(index.records);
> + xdl_free(index.line_map);
> + xdl_free(index.next_ptrs);
> + xdl_cha_free(&index.rcha);
> +
> + return result;
> +}
> +
> +int xdl_do_histogram_diff(mmfile_t *file1, mmfile_t *file2,
> + xpparam_t const *xpp, xdfenv_t *env)
> +{
> + int line1, line2, count1, count2;
> +
> + if (xdl_prepare_env(file1, file2, xpp, env) < 0)
> + return -1;
> +
> + line1 = line2 = 1;
> + count1 = env->xdf1.nrec;
> + count2 = env->xdf2.nrec;
> +
> + reduce_common_start_end(xpp, env, &line1, &count1, &line2, &count2);
What this does is logically not specific to histogram algorithm but can be
applied to other backends, no? And I vaguely recall that Linus did try
something like this once, found some issues with it when context is set to
non zero, and stopped doing it (sorry, I do not have any more details).
I am not suggesting you to remove this call or hoist the call to one level
up to xdl_do_diff(), but I do have to wonder how much of the performance
improvement you reported is due to this common head/tail reduction.
> + return histogram_diff(xpp, env, line1, count1, line2, count2);
> +}
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