Use `memchr` in SRE's prefix scanner

Feature or enhancement

Proposal:

Python's regex prefix scanning loops advance one byte at a time to find the first character of a literal prefix. For SIZEOF_SRE_CHAR == 1, memchr is a drop-in replacement that uses SIMD. The idea is to skip say 32 bytes per iteration in the negative case (no match).

The two loops are at

https://github.com/python/cpython/blob/7ce737ea11919aebf7eef174f910759e74d0ea50/Modules/_sre/sre_lib.h#L1756-L1759

https://github.com/python/cpython/blob/7ce737ea11919aebf7eef174f910759e74d0ea50/Modules/_sre/sre_lib.h#L1789-L1792

Similar issues/prs are gh-145797 (memchr in str.split) and building on gh-57343, gh-57345.

Proposed patch:

--- a/Modules/_sre/sre_lib.h
+++ b/Modules/_sre/sre_lib.h
@@ -1753,10 +1753,19 @@
         end = (SRE_CHAR *)state->end;
         state->must_advance = 0;
         while (ptr < end) {
+#if SIZEOF_SRE_CHAR == 1
+            {
+                SRE_CHAR *found = memchr(ptr, c, end - ptr);
+                if (!found)
+                    return 0;
+                ptr = found;
+            }
+#else
             while (*ptr != c) {
                 if (++ptr >= end)
                     return 0;
             }
+#endif
             TRACE(("|%p|%p|SEARCH LITERAL\n", pattern, ptr));
             state->start = ptr;
@@ -1786,10 +1795,19 @@
         while (ptr < end) {
             SRE_CHAR c = (SRE_CHAR) prefix[0];
+#if SIZEOF_SRE_CHAR == 1
+            {
+                SRE_CHAR *found = memchr(ptr, c, end - ptr);
+                if (!found)
+                    return 0;
+                ptr = found + 1;
+            }
+#else
             while (*ptr++ != c) {
                 if (ptr >= end)
                     return 0;
             }
+#endif
             if (ptr >= end)
                 return 0;

I ran into this while profiling a log parser (pretty much grepping for error and warning strings) that applies a handful of literal-prefix regexes to each line of a ~5MB log file. With the patch, the runtime is reduced by ~14% end-to-end. A similar use case in the same application is scanning a binary executable/library for strings with a pattern /common/prefix/(foo|bar|baz) where I would benefit even more from memchr to find the / anchors.

Microbenchmark on my M4 macbook (without PGO):

Single-char prefix re.compile(r"X(A|B)"), match at position N against aaa...aXA

Multi-char prefix re.compile(r"XXXX(A|B)"), match at position N against aaaa...aXXXXA

The reason the second benchmark is not as good as the first is that the apple clang compiler seems to have unrolled the second loop 4x while the first loop is not unrolled at all. Neither of them were auto-vectorized.

"""Synthetic benchmark for SRE prefix scanning at various string lengths."""

import re
import time

def bench(pat, text, iterations):
    for _ in range(1000):
        pat.search(text)
    t0 = time.perf_counter()
    for _ in range(iterations):
        pat.search(text)
    dt = time.perf_counter() - t0
    ns_per_call = dt / iterations * 1e9
    print(f"  {ns_per_call:8.1f} ns/call")

# Single-char prefix: X(A|B) has prefix "X" (len=1)
pat1 = re.compile(r"X(A|B)")
print("Single-char prefix: X(A|B), match at position N")
for n in [0, 5, 10, 50, 100, 500, 1000, 10000, 100000]:
    text = "a" * n + "XA"
    print(f"  match at pos {n:>6d} (len={len(text):>6d})", end="")
    bench(pat1, text, max(100000, 1000000 // max(n, 1)))

# Multi-char prefix: XXXX(A|B) has prefix "XXXX" (len=4)
pat4 = re.compile(r"XXXX(A|B)")
print()
print("Multi-char prefix: XXXX(A|B), match at position N")
for n in [0, 5, 10, 50, 100, 500, 1000, 10000, 100000]:
    text = "a" * n + "XXXXA"
    print(f"  match at pos {n:>6d} (len={len(text):>6d})", end="")
    bench(pat4, text, max(100000, 1000000 // max(n, 1)))

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

Linked PRs

• gh-148733

Start in Modules/_sre/sre_lib.h at the two prefix-scanning loops linked in the issue, and review the proposed SIZEOF_SRE_CHAR == 1 branches using memchr. Run the supplied regex microbenchmark and compare behavior and performance with the existing loops; done means the scanner uses memchr for single-byte characters without changing other character-width paths.