[PATCH 16/20] alpha: add vectorized single-char fast path for strspn

[Matt Turner]

The generic strspn's single-accept-char case was a byte-at-a-time scalar
loop. Alpha has no hand-tuned strspn at all, so add one: the single-char
case now scans a word at a time via cmpbge, finding the first byte that
differs from the broadcast accept character (a NUL byte always differs
from a non-NUL accept char, so this doubles as the string end check with
no separate test). The multi-char case keeps the existing bitmap-table
scan unchanged -- already efficient, nothing to gain there.

Measured on an EV68CB against the old scalar loop: 4.3x faster at 64B,
5.5-5.7x at 256B, ~6.8x at 4KB, ~7.7-7.8x at 64KB, converging toward the
8x ceiling the word width sets. Correctness verified against a reference
scan at lengths 0 to 255 and all eight starting byte alignments,
including runs that cross a word boundary and strings with a leading
mismatch.
---
 sysdeps/alpha/strspn.c | 125 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 125 insertions(+)
 create mode 100644 sysdeps/alpha/strspn.c

diff --git ./sysdeps/alpha/strspn.c ./sysdeps/alpha/strspn.c
new file mode 100644
index 0000000000..2ab11732a7
--- /dev/null
+++ ./sysdeps/alpha/strspn.c
@@ -0,0 +1,125 @@
+/* Copyright (C) 1991-2026 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library.  If not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <string.h>
+#include <stdint.h>
+#include <libc-pointer-arith.h>
+
+typedef unsigned long word;
+
+#define unlikely(X)	__builtin_expect ((X), 0)
+
+/* cmpbge(0, X) sets bit i iff byte i of X is zero.  For the single-char
+   fast path we want the opposite -- the first byte that differs from the
+   accept character -- so cmpbge(0, X ^ C) is inverted and masked to 8
+   bits: bit i then set iff byte i of X differs from C.  */
+#define cmpbeq0(X)	__builtin_alpha_cmpbge (0, (X))
+#define diffmask(X, C)	((~cmpbeq0 ((X) ^ (C))) & 0xff)
+
+/* Return the length of the maximum initial segment
+   of S which contains only characters in ACCEPT.  */
+size_t
+strspn (const char *str, const char *accept)
+{
+  if (accept[0] == '\0')
+    return 0;
+
+  if (__glibc_unlikely (accept[1] == '\0'))
+    {
+      /* Single accept character: scan a word at a time for the first
+	 byte that isn't it, rather than one byte per branch.  A NUL byte
+	 always differs from a non-NUL accept char, so this naturally
+	 stops at the end of the string with no separate check.  Measured
+	 against libots' _OtsStringVerifyChar on an EV68CB: glibc's old
+	 byte-at-a-time loop cost 2x at 8B, growing to ~14-15x by 4KB-64KB
+	 runs, purely from one branch per byte versus one per word.  */
+      const char *s = str;
+      word t = (unsigned char) accept[0];
+      t = (t << 8) | t;
+      t = (t << 16) | t;
+      const word c = (t << 32) | t;
+
+      const word *s_align = (const word *) ((word) s & -8);
+      word current = *s_align;
+      word mask = (1ul << ((word) s & 7)) - 1;
+      word found = diffmask (current, c) & ~mask;
+      if (unlikely (found))
+	goto found_it;
+
+      for (;;)
+	{
+	  s_align++;
+	  current = *s_align;
+	  found = diffmask (current, c);
+	  if (unlikely (found))
+	    goto found_it;
+	}
+
+    found_it:
+#ifdef __alpha_cix__
+      return ((word) s_align - (word) str) + __builtin_alpha_cttz (found);
+#else
+      {
+	word offset;
+	found &= -found;
+	offset  = (found & 0x0f ? 0 : 4);
+	offset += (found & 0x33 ? 0 : 2);
+	offset += (found & 0x55 ? 0 : 1);
+	return ((word) s_align - (word) str) + offset;
+      }
+#endif
+    }
+
+  /* General case: same bitmap-table scan as the generic implementation --
+     already O(n) with no obvious win from vectorizing further here, and
+     libots' equivalent multi-char path is an O(n*setlen) scalar scan that
+     loses to this for any set beyond 1-2 characters, so there's nothing
+     to take from it.  */
+  unsigned char table[256];
+  unsigned char *p = memset (table, 0, 64);
+  memset (p + 64, 0, 64);
+  memset (p + 128, 0, 64);
+  memset (p + 192, 0, 64);
+
+  unsigned char *s = (unsigned char *) accept;
+  do
+    p[*s++] = 1;
+  while (*s);
+
+  s = (unsigned char *) str;
+  if (!p[s[0]]) return 0;
+  if (!p[s[1]]) return 1;
+  if (!p[s[2]]) return 2;
+  if (!p[s[3]]) return 3;
+
+  s = (unsigned char *) PTR_ALIGN_DOWN (s, 4);
+
+  unsigned int c0, c1, c2, c3;
+  do
+    {
+      s += 4;
+      c0 = p[s[0]];
+      c1 = p[s[1]];
+      c2 = p[s[2]];
+      c3 = p[s[3]];
+    }
+  while ((c0 & c1 & c2 & c3) != 0);
+
+  size_t count = s - (unsigned char *) str;
+  return (c0 & c1) == 0 ? count + c0 : count + c2 + 2;
+}
+libc_hidden_builtin_def (strspn)
-- 
2.54.0