[C23] #embed bytes with negative signed char values are wrapped around to unsigned int when output with -E

[MitalAshok]

For example:

https://godbolt.org/z/8Tf5ncYWf

//é
constexpr char t[] = {
#embed __FILE__
};

Compiled with -std=c23 -E outputs:

# 1 "/app/example.c"
# 1 "<built-in>" 1
# 1 "<built-in>" 3
# 432 "<built-in>" 3
# 1 "<command line>" 1
# 1 "<built-in>" 2
# 1 "/app/example.c" 2

constexpr char t[] = {47, 47, 4294967235, 4294967209, 10, 99, 111, 110, <<snipped>> };

Which doesn't compile when fed back into the compiler

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Mital Ashok (MitalAshok)

For example:

https://godbolt.org/z/8Tf5ncYWf

//é
constexpr char t[] = {
#embed __FILE__
};

Compiled with -std=c23 -E outputs:

# 1 "/app/example.c"
# 1 "<built-in>" 1
# 1 "<built-in>" 3
# 432 "<built-in>" 3
# 1 "<command line>" 1
# 1 "<built-in>" 2
# 1 "/app/example.c" 2

constexpr char t[] = {47, 47, 4294967235, 4294967209, 10, 99, 111, 110, <<snipped>> };

Which doesn't compile when fed back into the compiler

[zygoloid]

Should -funsigned-char affect the preprocessed output for this case?

[Fznamznon]

Hmm, since the intention now is to yield int (not unsigned int) values by #embed, I think this is a bug.
I think this cast might be guilty

llvm-project/clang/lib/Frontend/PrintPreprocessedOutput.cpp

Line 980 in 4915fdd

*Callbacks->OS << static_cast<unsigned>(*Iter);

[AaronBallman]

Should -funsigned-char affect the preprocessed output for this case?

The standard wording says:

The values of the integer constant expressions in the expanded sequence are determined by an
implementation-defined mapping of the resource’s data. Each integer constant expression’s value is
in the range from 0 to (2^embed element width) − 1, inclusive. If:
— the list of integer constant expressions is used to initialize an array of a type compatible with
unsigned char, or compatible with char if char cannot hold negative values; and,
— the embed element width is equal to CHAR_BIT (5.2.5.3.2),
then the contents of the initialized elements of the array are as-if the resource’s binary data is fread
(7.23.8.1) into the array at translation time.

With an attached footnote:

For example, an embed element width of 8 will yield a range of values from 0 to 255, inclusive.