[clang] Fix preprocessor output from #embed

[Fznamznon]

When bytes with negative signed char values appear in the data, make sure to use raw bytes from the data string when preprocessing, not char values.

Fixes #102798

[llvmbot]

@llvm/pr-subscribers-clang

Author: Mariya Podchishchaeva (Fznamznon)

Changes

When bytes with negative signed char values appear in the data, make sure to use raw bytes from the data string when preprocessing, not char values.

Fixes #102798

---

Full diff: https://github.com/llvm/llvm-project/pull/126742.diff

3 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+3)
• (modified) clang/lib/Frontend/PrintPreprocessedOutput.cpp (+2-3)
• (modified) clang/test/Preprocessor/embed_preprocess_to_file.c (+8)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 369d9e9de7d16..5da929c2a2143 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -141,6 +141,9 @@ Improvements to Coverage Mapping
 Bug Fixes in This Version
 -------------------------
 
+- Clang now outputs correct values when #embed data contains bytes with negative
+  signed char values (#GH102798).
+
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
diff --git a/clang/lib/Frontend/PrintPreprocessedOutput.cpp b/clang/lib/Frontend/PrintPreprocessedOutput.cpp
index 1005825441b3e..2ae355fb33885 100644
--- a/clang/lib/Frontend/PrintPreprocessedOutput.cpp
+++ b/clang/lib/Frontend/PrintPreprocessedOutput.cpp
@@ -974,11 +974,10 @@ static void PrintPreprocessedTokens(Preprocessor &PP, Token &Tok,
       // Loop over the contents and print them as a comma-delimited list of
       // values.
       bool PrintComma = false;
-      for (auto Iter = Data->BinaryData.begin(), End = Data->BinaryData.end();
-           Iter != End; ++Iter) {
+      for (unsigned char Byte : Data->BinaryData.bytes()) {
         if (PrintComma)
           *Callbacks->OS << ", ";
-        *Callbacks->OS << static_cast<unsigned>(*Iter);
+        *Callbacks->OS << static_cast<int>(Byte);
         PrintComma = true;
       }
     } else if (Tok.isAnnotation()) {
diff --git a/clang/test/Preprocessor/embed_preprocess_to_file.c b/clang/test/Preprocessor/embed_preprocess_to_file.c
index 9895d958cf96d..b3c99d36f784a 100644
--- a/clang/test/Preprocessor/embed_preprocess_to_file.c
+++ b/clang/test/Preprocessor/embed_preprocess_to_file.c
@@ -37,3 +37,11 @@ const char even_more[] = {
 // DIRECTIVE-NEXT: #embed <jk.txt> prefix(4, 5,) suffix(, 6, 7) /* clang -E -dE */
 // DIRECTIVE-NEXT:  , 8, 9, 10
 // DIRECTIVE-NEXT: };
+
+constexpr char big_one[] = {
+#embed <big_char.txt>
+};
+
+// EXPANDED: constexpr char big_one[] = {255
+// DIRECTIVE: constexpr char big_one[] = {
+// DIRECTIVE-NEXT: #embed <big_char.txt>

[AaronBallman]

LGTM!

[AaronBallman]

This seems like a reasonable one to backport, so I'm adding it to the 20.x milestone.

[Fznamznon]

/cherry-pick 3be48a6

[llvmbot]

Failed to cherry-pick: 3be48a6

https://github.com/llvm/llvm-project/actions/runs/13326798464

Please manually backport the fix and push it to your github fork. Once this is done, please create a pull request

[Fznamznon]

Manual cherry-pick #127222