[php-src] PR #24774: ext/standard: Decode base64 four characters at a time in the character loop

Pull Request: https://github.com/php/php-src/pull/24774 Author: ArtUkrainskiy base64_decode() runs a character loop in php_base64_decode_impl(): one character per iteration, with a switch on its position in the group. That loop is the whole decoder for input under 45 bytes — tokens, ids, keys — and for everything after the first space or line break, where the vector decoders (SSSE3/AVX2/AVX-512/NEON) stop. This PR decodes whole groups of four alphabet characters at once while the input is clean, and keeps the one-character step for whitespace, padding, invalid bytes and the last partial group. The vector decoders are untouched. Output is unchanged, including every strict-mode rejection. Release builds, master vs this PR. Instructions per call (perf stat, deterministic); the range spans six compilers — GCC 13/14 and clang 18/20 on Intel, GCC 15 and clang 21 on AMD: | base64_decode() input | instructions | |---|---| | 24 bytes (a token) | 41–46% fewer | | 44 bytes (a 32-byte key) | 45–50% fewer | | 1 KB, clean | 29–31% fewer | | 64 KB, clean | unchanged | | 64 KB in 76-column lines (MIME) | 69–71% fewer | | 64 KB in 64-column lines (PEM) | 68–71% fewer | | 64 KB MIME, strict | 73–76% fewer | In CPU cycles on the AMD host the MIME and PEM cases are 64–81% fewer, short input 37–64% fewer. base64_encode() is not touched. Clean 64 KB barely moves: the vector loop does the bulk, and only the ≤44-byte tail reaches this loop. The short rows are any base64 under ~45 bytes, decoded entirely by this loop. The line-wrapped rows apply when a whole wrapped blob goes to base64_decode() in one call: it skips the line breaks, even in strict mode, and runs the rest through this loop. Email base64 is wrapped at 76 columns (RFC 2045), PEM at 64. A parser that strips the breaks first stays on the vector path and is unaffected. Why it is safe: a group is decoded only when all four characters are in the alphabet, so whitespace, padding and invalid bytes still take the one-character path and every strict-mode rejection is preserved. A differential test against master decoded 30,000 random inputs on each of the six compilers — clean, MIME and PEM lines, whitespace and invalid bytes anywhere, padding in every position, both modes — with identical output; the same test flags a deliberately broken build, so it is not toothless. The change also matches master under a forced-scalar build and under ASan + UBSan (80,000 decodes). A new .phpt covers a line break at every position of short input in both modes. Not tested: 32-bit and ZTS by reading only; the AVX-512 and NEON paths are unchanged and not re-run here. Full numbers and reproduction: https://github.com/ArtUkrainskiy/php-src-bench/tree/main/reports/base64-decode-quads A follow-up could resume the vector loop after a line break instead of handing the rest to this loop — about 3× more on MIME and PEM — but that touches every vector variant, so I will send it separately.