[clang] [clang] Provide an SSE4.2 implementation of identifier token lexer (PR #68962)

[via cfe-commits]

https://github.com/serge-sans-paille created https://github.com/llvm/llvm-project/pull/68962

The _mm_cmpistri instruction can be used to quickly parse identifiers.

With this patch activated, clang pre-processes <iostream> 1.8% faster, and sqlite3.c amalgametion 1.5% faster, based on time measurements and number of executed instructions as measured by valgrind.

The introduction of an extra helper function in the regular case has no impact on performance, see

    https://llvm-compile-time-tracker.com/compare.php?from=30240e428f0ec7d4a6d1b84f9f807ce12b46cfd1&to=12bcb016cde4579ca7b75397762098c03eb4f264&stat=instructions:u

>From ccf33cafada526241a3fb1aca9c2d280444b589b Mon Sep 17 00:00:00 2001
From: serge-sans-paille <sguelton at mozilla.com>
Date: Thu, 12 Oct 2023 22:30:30 +0200
Subject: [PATCH] [clang] Provide an SSE4.2 implementation of identifier token
 lexer

The _mm_cmpistri instruction can be used to quickly parse identifiers.

With this patch activated, clang pre-processes <iostream> 1.8% faster, and
sqlite3.c amalgametion 1.5% faster, based on time measurements and
number of executed instructions as measured by valgrind.

The introduction of an extra helper function in the regular case has no
impact on performance, see

    https://llvm-compile-time-tracker.com/compare.php?from=30240e428f0ec7d4a6d1b84f9f807ce12b46cfd1&to=12bcb016cde4579ca7b75397762098c03eb4f264&stat=instructions:u
---
 clang/lib/Lex/Lexer.cpp | 45 ++++++++++++++++++++++++++++++++++-------
 1 file changed, 38 insertions(+), 7 deletions(-)

diff --git a/clang/lib/Lex/Lexer.cpp b/clang/lib/Lex/Lexer.cpp
index feed1b9ecd71a8d..f2d9eb3a8af4e3d 100644
--- a/clang/lib/Lex/Lexer.cpp
+++ b/clang/lib/Lex/Lexer.cpp
@@ -47,6 +47,10 @@
 #include <tuple>
 #include <utility>
 
+#ifdef __SSE4_2__
+#include <nmmintrin.h>
+#endif
+
 using namespace clang;
 
 //===----------------------------------------------------------------------===//
@@ -1847,19 +1851,46 @@ bool Lexer::LexUnicodeIdentifierStart(Token &Result, uint32_t C,
   return true;
 }
 
+static const char *fastParseASCIIIdentifier(const char *CurPtr, const char* BufferEnd) {
+#ifdef __SSE4_2__
+  static constexpr char AsciiIdentifierRange[16] = {
+      '_', '_', 'A', 'Z', 'a', 'z', '0', '9',
+  };
+  constexpr ssize_t BytesPerRegister = 16;
+
+  while (LLVM_LIKELY(BufferEnd - CurPtr >= BytesPerRegister)) {
+    __m128i AsciiIdentifierRangeV = _mm_loadu_si128((const __m128i *)AsciiIdentifierRange);
+
+      __m128i Cv = _mm_loadu_si128((const __m128i *)(CurPtr));
+      int Consumed =
+          _mm_cmpistri(AsciiIdentifierRangeV, Cv,
+                       _SIDD_LEAST_SIGNIFICANT | _SIDD_CMP_RANGES |
+                           _SIDD_UBYTE_OPS | _SIDD_NEGATIVE_POLARITY);
+      CurPtr += Consumed;
+      if (Consumed == BytesPerRegister)
+        continue;
+      return CurPtr;
+  }
+#else
+  (void)BufferEnd;
+#endif
+
+  unsigned char C = *CurPtr;
+  while (isAsciiIdentifierContinue(C))
+    C = *++CurPtr;
+  return CurPtr;
+}
+
 bool Lexer::LexIdentifierContinue(Token &Result, const char *CurPtr) {
   // Match [_A-Za-z0-9]*, we have already matched an identifier start.
+
   while (true) {
-    unsigned char C = *CurPtr;
-    // Fast path.
-    if (isAsciiIdentifierContinue(C)) {
-      ++CurPtr;
-      continue;
-    }
+
+    CurPtr = fastParseASCIIIdentifier(CurPtr, BufferEnd);
 
     unsigned Size;
     // Slow path: handle trigraph, unicode codepoints, UCNs.
-    C = getCharAndSize(CurPtr, Size);
+    unsigned char C = getCharAndSize(CurPtr, Size);
     if (isAsciiIdentifierContinue(C)) {
       CurPtr = ConsumeChar(CurPtr, Size, Result);
       continue;