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

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: None (serge-sans-paille)

<details>
<summary>Changes</summary>

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

---
Full diff: https://github.com/llvm/llvm-project/pull/68962.diff


1 Files Affected:

- (modified) clang/lib/Lex/Lexer.cpp (+38-7) 


``````````diff
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;

``````````

</details>


https://github.com/llvm/llvm-project/pull/68962