[Clang] Add a warning on invalid UTF-8 in comments.

Introduce an off-by default `-Winvalid-utf8` warning
that detects invalid UTF-8 code units sequences in comments.

Invalid UTF-8 in other places is already diagnosed,
as that cannot appear in identifiers and other grammar constructs.

The warning is off by default as its likely to be somewhat disruptive
otherwise.

This warning allows clang to conform to the yet-to be approved WG21
"P2295R5 Support for UTF-8 as a portable source file encoding"
paper.

Reviewed By: aaron.ballman, #clang-language-wg

Differential Revision: https://reviews.llvm.org/D128059

clang/docs/ReleaseNotes.rst

@@ -282,6 +282,8 @@ Improvements to Clang's diagnostics
 - ``-Wself-assign``, ``-Wself-assign-overloaded`` and ``-Wself-move`` will
   suggest a fix if the decl being assigned is a parameter that shadows a data
   member of the contained class.
+- Added ``-Winvalid-utf8`` which diagnoses invalid UTF-8 code unit sequences in
+  comments.
 
 Non-comprehensive list of changes in this release
 -------------------------------------------------

clang/include/clang/Basic/DiagnosticLexKinds.td

@@ -113,6 +113,8 @@ def warn_four_char_character_literal : Warning<
 // Unicode and UCNs
 def err_invalid_utf8 : Error<
   "source file is not valid UTF-8">;
+def warn_invalid_utf8_in_comment : Extension<
+  "invalid UTF-8 in comment">, InGroup<DiagGroup<"invalid-utf8">>;
 def err_character_not_allowed : Error<
   "unexpected character <U+%0>">;
 def err_character_not_allowed_identifier : Error<

clang/lib/Lex/Lexer.cpp

@@ -2392,13 +2392,37 @@ bool Lexer::SkipLineComment(Token &Result, const char *CurPtr,
   //
   // This loop terminates with CurPtr pointing at the newline (or end of buffer)
   // character that ends the line comment.
+
+  // C++23 [lex.phases] p1
+  // Diagnose invalid UTF-8 if the corresponding warning is enabled, emitting a
+  // diagnostic only once per entire ill-formed subsequence to avoid
+  // emiting to many diagnostics (see http://unicode.org/review/pr-121.html).
+  bool UnicodeDecodingAlreadyDiagnosed = false;
+
   char C;
   while (true) {
     C = *CurPtr;
     // Skip over characters in the fast loop.
-    while (C != 0 &&                // Potentially EOF.
-           C != '\n' && C != '\r')  // Newline or DOS-style newline.
+    while (isASCII(C) && C != 0 &&   // Potentially EOF.
+           C != '\n' && C != '\r') { // Newline or DOS-style newline.
       C = *++CurPtr;
+      UnicodeDecodingAlreadyDiagnosed = false;
+    }
+
+    if (!isASCII(C)) {
+      unsigned Length = llvm::getUTF8SequenceSize(
+          (const llvm::UTF8 *)CurPtr, (const llvm::UTF8 *)BufferEnd);
+      if (Length == 0) {
+        if (!UnicodeDecodingAlreadyDiagnosed && !isLexingRawMode())
+          Diag(CurPtr, diag::warn_invalid_utf8_in_comment);
+        UnicodeDecodingAlreadyDiagnosed = true;
+        ++CurPtr;
+      } else {
+        UnicodeDecodingAlreadyDiagnosed = false;
+        CurPtr += Length;
+      }
+      continue;
+    }
 
     const char *NextLine = CurPtr;
     if (C != 0) {
@@ -2665,6 +2689,12 @@ bool Lexer::SkipBlockComment(Token &Result, const char *CurPtr,
   if (C == '/')
     C = *CurPtr++;
 
+  // C++23 [lex.phases] p1
+  // Diagnose invalid UTF-8 if the corresponding warning is enabled, emitting a
+  // diagnostic only once per entire ill-formed subsequence to avoid
+  // emiting to many diagnostics (see http://unicode.org/review/pr-121.html).
+  bool UnicodeDecodingAlreadyDiagnosed = false;
+
   while (true) {
     // Skip over all non-interesting characters until we find end of buffer or a
     // (probably ending) '/' character.
@@ -2673,14 +2703,22 @@ bool Lexer::SkipBlockComment(Token &Result, const char *CurPtr,
         // doesn't check for '\0'.
         !(PP && PP->getCodeCompletionFileLoc() == FileLoc)) {
       // While not aligned to a 16-byte boundary.
-      while (C != '/' && ((intptr_t)CurPtr & 0x0F) != 0)
+      while (C != '/' && (intptr_t)CurPtr % 16 != 0) {
+        if (!isASCII(C))
+          goto MultiByteUTF8;
         C = *CurPtr++;
-
+      }
       if (C == '/') goto FoundSlash;
 
 #ifdef __SSE2__
       __m128i Slashes = _mm_set1_epi8('/');
-      while (CurPtr+16 <= BufferEnd) {
+      while (CurPtr + 16 < BufferEnd) {
+        int Mask = _mm_movemask_epi8(*(const __m128i *)CurPtr);
+        if (LLVM_UNLIKELY(Mask != 0)) {
+          CurPtr += llvm::countTrailingZeros<unsigned>(Mask);
+          goto MultiByteUTF8;
+        }
+        // look for slashes
         int cmp = _mm_movemask_epi8(_mm_cmpeq_epi8(*(const __m128i*)CurPtr,
                                     Slashes));
         if (cmp != 0) {
@@ -2693,21 +2731,39 @@ bool Lexer::SkipBlockComment(Token &Result, const char *CurPtr,
         CurPtr += 16;
       }
 #elif __ALTIVEC__
+      __vector unsigned char LongUTF = {0x80, 0x80, 0x80, 0x80, 0x80, 0x80,
+                                        0x80, 0x80, 0x80, 0x80, 0x80, 0x80,
+                                        0x80, 0x80, 0x80, 0x80};
       __vector unsigned char Slashes = {
         '/', '/', '/', '/',  '/', '/', '/', '/',
         '/', '/', '/', '/',  '/', '/', '/', '/'
       };
-      while (CurPtr + 16 <= BufferEnd &&
-             !vec_any_eq(*(const __vector unsigned char *)CurPtr, Slashes))
+      while (CurPtr + 16 < BufferEnd) {
+        if (LLVM_UNLIKELY(
+                vec_any_ge(*(const __vector unsigned char *)CurPtr, LongUTF)))
+          goto MultiByteUTF8;
+        if (vec_any_eq(*(const __vector unsigned char *)CurPtr, Slashes)) {
+          C = *CurPtr++;
+          break;
+        }
         CurPtr += 16;
+      }
+
 #else
-      // Scan for '/' quickly.  Many block comments are very large.
-      while (CurPtr[0] != '/' &&
-             CurPtr[1] != '/' &&
-             CurPtr[2] != '/' &&
-             CurPtr[3] != '/' &&
-             CurPtr+4 < BufferEnd) {
-        CurPtr += 4;
+      while (CurPtr + 16 < BufferEnd) {
+        bool HasNonASCII = false;
+        for (unsigned I = 0; I < 16; ++I)
+          HasNonASCII |= !isASCII(CurPtr[I]);
+
+        if (LLVM_UNLIKELY(HasNonASCII))
+          goto MultiByteUTF8;
+
+        bool HasSlash = false;
+        for (unsigned I = 0; I < 16; ++I)
+          HasSlash |= CurPtr[I] == '/';
+        if (HasSlash)
+          break;
+        CurPtr += 16;
       }
 #endif
 
@@ -2715,9 +2771,31 @@ bool Lexer::SkipBlockComment(Token &Result, const char *CurPtr,
       C = *CurPtr++;
     }
 
-    // Loop to scan the remainder.
-    while (C != '/' && C != '\0')
+    // Loop to scan the remainder, warning on invalid UTF-8
+    // if the corresponding warning is enabled, emitting a diagnostic only once
+    // per sequence that cannot be decoded.
+    while (C != '/' && C != '\0') {
+      if (isASCII(C)) {
+        UnicodeDecodingAlreadyDiagnosed = false;
         C = *CurPtr++;
+        continue;
+      }
+    MultiByteUTF8:
+      // CurPtr is 1 code unit past C, so to decode
+      // the codepoint, we need to read from the previous position.
+      unsigned Length = llvm::getUTF8SequenceSize(
+          (const llvm::UTF8 *)CurPtr-1, (const llvm::UTF8 *)BufferEnd);
+      if (Length == 0) {
+        if (!UnicodeDecodingAlreadyDiagnosed && !isLexingRawMode())
+          Diag(CurPtr-1, diag::warn_invalid_utf8_in_comment);
+        UnicodeDecodingAlreadyDiagnosed = true;
+      }
+      else {
+        UnicodeDecodingAlreadyDiagnosed = false;
+        CurPtr += Length - 1;
+      }
+      C = *CurPtr++;
+    }
 
     if (C == '/') {
   FoundSlash:

clang/test/Lexer/comment-invalid-utf8.c

@@ -0,0 +1,27 @@
+// RUN: %clang_cc1 -fsyntax-only %s -Winvalid-utf8 -verify=expected
+// RUN: %clang_cc1 -fsyntax-only %s -verify=nowarn
+// nowarn-no-diagnostics
+
+// This file is purposefully encoded as windows-1252
+// be careful when modifying.
+
+//€
+// expected-warning@-1 {{invalid UTF-8 in comment}}
+
+// € ‚ƒ„…†‡ˆ‰ Š ‹ Œ Ž
+// expected-warning@-1 6{{invalid UTF-8 in comment}}
+
+/*€*/
+// expected-warning@-1 {{invalid UTF-8 in comment}}
+
+/*€ ‚ƒ„…†‡ˆ‰ Š ‹ Œ Ž*/
+// expected-warning@-1 6{{invalid UTF-8 in comment}}
+
+/*
+€
+*/
+// expected-warning@-2 {{invalid UTF-8 in comment}}
+
+// abcd
+// €abcd
+// expected-warning@-1 {{invalid UTF-8 in comment}}

clang/test/Lexer/comment-utf8.c

@@ -0,0 +1,20 @@
+// RUN: %clang_cc1 -fsyntax-only %s -Winvalid-utf8 -verify
+// expected-no-diagnostics
+
+
+//§ § § 😀 你好 ©
+
+/*§ § § 😀 你好 ©*/
+
+/*
+§ § § 😀 你好 ©©©
+*/
+
+/* § § § 😀 你好 © */
+/*
+    a longer comment to exerce the vectorized code path
+    ----------------------------------------------------
+    αααααααααααααααααααααα      // here is some unicode
+    ----------------------------------------------------
+    ----------------------------------------------------
+*/

clang/test/SemaCXX/static-assert.cpp

@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -fsyntax-only -verify %s -std=c++11 -pedantic -triple=x86_64-linux-gnu
+// RUN: %clang_cc1 -fsyntax-only -verify %s -std=c++11 -pedantic -triple=x86_64-linux-gnu -Wno-invalid-utf8
 
 int f(); // expected-note {{declared here}}
 

llvm/include/llvm/Support/ConvertUTF.h

@@ -181,6 +181,8 @@ Boolean isLegalUTF8Sequence(const UTF8 *source, const UTF8 *sourceEnd);
 
 Boolean isLegalUTF8String(const UTF8 **source, const UTF8 *sourceEnd);
 
+unsigned getUTF8SequenceSize(const UTF8 *source, const UTF8 *sourceEnd);
+
 unsigned getNumBytesForUTF8(UTF8 firstByte);
 
 /*************************************************************************/

llvm/lib/Support/ConvertUTF.cpp

@@ -417,6 +417,16 @@ Boolean isLegalUTF8Sequence(const UTF8 *source, const UTF8 *sourceEnd) {
     return isLegalUTF8(source, length);
 }
 
+/*
+ * Exported function to return the size of the first utf-8 code unit sequence,
+ * Or 0 if the sequence is not valid;
+ */
+unsigned getUTF8SequenceSize(const UTF8 *source, const UTF8 *sourceEnd) {
+  int length = trailingBytesForUTF8[*source] + 1;
+  return (length <= sourceEnd - source && isLegalUTF8(source, length)) ? length
+                                                                         : 0;
+}
+
 /* --------------------------------------------------------------------- */
 
 static unsigned