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bpo-41972: Tweak fastsearch.h string search algorithms #27091

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Merged
ambv merged 21 commits into from
Jul 19, 2021
Merged

bpo-41972: Tweak fastsearch.h string search algorithms #27091

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90a6a0c
Don't force inlining
sweeneyde Jul 6, 2021
19fa661
Use a Boyer-Moore skip table
sweeneyde Jul 7, 2021
d2219d5
Switch to two-way in more cases
sweeneyde Jul 8, 2021
3b8760f
Tighter comments
sweeneyde Jul 9, 2021
5d86b8d
Fix the direction of the cutoff inequality :)
sweeneyde Jul 10, 2021
2e9a9b1
Separate a gap loop
sweeneyde Jul 10, 2021
c84314c
update cutoffs
sweeneyde Jul 10, 2021
7517c60
tweak cutoffs, fix i - cut + 1 jump
sweeneyde Jul 10, 2021
e5ef33b
Refactor into smaller functions
sweeneyde Jul 11, 2021
a01fb70
Focus the table on the last character of the pattern, giving a tighte…
sweeneyde Jul 11, 2021
6be0c32
tweak cutoffs
sweeneyde Jul 11, 2021
f9fe807
📜🤖 Added by blurb_it.
blurb-it[bot] Jul 12, 2021
25f9b56
Fix sanitizer warnings
sweeneyde Jul 12, 2021
d3f8438
Merge branch 'fastersearch' of https://github.com/sweeneyde/cpython i…
sweeneyde Jul 12, 2021
4379ea6
Refactor for sanitizer
sweeneyde Jul 12, 2021
e336b79
Don't goto declarations
sweeneyde Jul 12, 2021
b8df3e1
Emphasize to the compiler that the loop is a loop
sweeneyde Jul 13, 2021
869f7d0
Use Py_LOCAL_INLINE
sweeneyde Jul 15, 2021
747a39a
Use Py_LOCAL_INLINE again
sweeneyde Jul 15, 2021
72143ef
Tweak thresholds based on most recent comparison.
sweeneyde Jul 16, 2021
4d7d102
Fix greater/less than sign.
sweeneyde Jul 16, 2021
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Separate a gap loop
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@sweeneyde
sweeneyde committed Jul 10, 2021
commit 2e9a9b1520494c78104b8a42640567f3db9b5200
22 changes: 15 additions & 7 deletions Objects/stringlib/fastsearch.h
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Original file line number Diff line number Diff line change
Expand Up @@ -340,7 +340,6 @@ STRINGLIB(_preprocess)(const STRINGLIB_CHAR *needle, Py_ssize_t len_needle,
}
}
}

// Fill up a compressed Boyer-Moore "Bad Character" table
Py_ssize_t not_found_shift = Py_MIN(len_needle, MAX_SHIFT);
for (Py_ssize_t i = 0; i < TABLE_SIZE; i++) {
Expand Down Expand Up @@ -424,6 +423,7 @@ STRINGLIB(_two_way)(const STRINGLIB_CHAR *haystack, Py_ssize_t len_haystack,
LOG("Needle is not periodic.\n");
assert(cut < len_needle);
STRINGLIB_CHAR needle_cut = needle[cut];
Py_ssize_t gap_jump_end = Py_MIN(len_needle, cut + gap);
windowloop:
while (window_last < haystack_end) {
LOG_LINEUP();
Expand All @@ -438,11 +438,19 @@ STRINGLIB(_two_way)(const STRINGLIB_CHAR *haystack, Py_ssize_t len_haystack,
break;
}
const STRINGLIB_CHAR *window = window_last - len_needle + 1;
for (Py_ssize_t i = cut; i < len_needle; i++) {
for (Py_ssize_t i = cut; i < gap_jump_end; i++) {
if (needle[i] != window[i]) {
Py_ssize_t two_way_shift = i - cut + 1;
LOG("Right half does not match.\n");
window_last += Py_MAX(two_way_shift, gap);
LOG("Early right half mismatch: jump by gap.\n");
assert(gap >= i - cut + 1);
window_last += gap;
goto windowloop;
}
}
for (Py_ssize_t i = gap_jump_end; i < len_needle; i++) {
if (needle[i] != window[i]) {
LOG("Late right half mismatch.\n");
assert(i - cut + 1 > gap);
window_last += gap;
goto windowloop;
}
}
Expand Down Expand Up @@ -550,7 +558,7 @@ FASTSEARCH(const STRINGLIB_CHAR* s, Py_ssize_t n,
mask = 0;

if (mode != FAST_RSEARCH) {
if (m < w / 4 && ((m >= 100 && w >= 1000) || (m >= 6 && w >= 30000))) {
if (m < w / 8 && ((m >= 100 && w >= 3000) || (m >= 6 && w >= 30000))) {
/* For larger problems where the needle isn't a huge
percentage of the size of the haystack, the relatively
expensive O(m) startup cost of the two-way algorithm
Expand Down Expand Up @@ -578,7 +586,7 @@ FASTSEARCH(const STRINGLIB_CHAR* s, Py_ssize_t n,
/* process pattern[-1] outside the loop */
STRINGLIB_BLOOM_ADD(mask, p[mlast]);

if (m >= 100 && w >= 1000) {
if (m >= 100 && w >= 3000) {
/* To ensure that we have good worst-case behavior,
here's an adaptive version of the algorithm, where if
we match O(m) characters without any matches of the
Expand Down