llvm, char16_t, and target windows

[franta]

I have a C++ program

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// a.cpp
typedef char16_t mychar;

and compile it with clang++:

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franta@here ~ $clang++ -fno-PIE --target=x86_64-unknown-windows -c a.cpp 
a.cpp:2:9: error: unknown type name 'char16_t'
typedef char16_t mychar;
        ^
1 error generated.

whereas, with linux target, it compiles just fine:

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franta@here ~ $ clang++ -fno-PIE --target=x86_64-unknown-linux -c a.cpp
franta@here ~ $

The versions of clang is

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franta@here ~ $ clang++ --version
clang version 16.0.5
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/lib/llvm/16/bin
Configuration file: /etc/clang/clang++.cfg

I cannot figure out why clang does not recognise char16_t with that architecture, it does on other distros I tried.
Is this a bug, a feature, something else?

However, if I change char16_t to built-in __char16_t:

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// b.cpp
typedef __char16_t mychar;

then it's compiled just fine:

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franta@here ~ $ clang++ -fno-PIE --target=x86_64-unknown-linux -c b.cpp
franta@here ~ $

My working assumption is that there is essentially a bug in Gentoo llvm headers as opposed to
upstream and that there is a typedef missing in one of the headers that are loaded implicitly.
I don't understand the whole interplay of the pieces involved enough though to figure out,
why it works with one triple, but not the other.

Any suggestions how to track this down?

[GDH-gentoo]

I have a C++ program

Code: Select all

// a.cpp
typedef char16_t mychar;

and compile it with clang++:

Code: Select all

franta@here ~ $clang++ -fno-PIE --target=x86_64-unknown-windows -c a.cpp 
a.cpp:2:9: error: unknown type name 'char16_t'
typedef char16_t mychar;
        ^
1 error generated.

Post the output of that command when adding the --verbose option.

[franta]

The failing windows triple:

franta@here ~ $ clang++ --std=c++2b -fno-PIE --target=x86_64-unknown-windows -c --verbose a.cpp
clang version 16.0.5
Target: x86_64-unknown-windows-msvc
Thread model: posix
InstalledDir: /usr/lib/llvm/16/bin
Configuration file: /etc/clang/clang++.cfg
System configuration file directory: /etc/clang
(in-process)
"/usr/lib/llvm/16/bin/clang-16" -cc1 -triple x86_64-unknown-windows-msvc -emit-obj -mrelax-all -mincremental-linker-compatible -disable-free -clear-ast-before-backend -disable-llvm-verifier -discard-value-names -main-file-name a.cpp -mrelocation-model pic -pic-level 2 -pic-is-pie -mframe-pointer=none -fmath-errno -ffp-contract=on -fno-rounding-math -mconstructor-aliases -funwind-tables=2 -target-cpu x86-64 -tune-cpu generic -mllvm -treat-scalable-fixed-error-as-warning -debugger-tuning=gdb -v -fcoverage-compilation-dir=/home/franta -resource-dir /usr/lib/llvm/16/bin/../../../../lib/clang/16 -include /usr/include/gentoo/fortify.h -include /usr/include/gentoo/maybe-stddefs.h -internal-isystem /usr/local/include -internal-isystem /usr/lib/llvm/16/bin/../../../../lib/clang/16/include -internal-externc-isystem /usr/include --std=c++2b -fdeprecated-macro -fdebug-compilation-dir=/home/franta -ferror-limit 19 -stack-protector 2 -fno-use-cxa-atexit -fms-extensions -fms-compatibility -fno-threadsafe-statics -fdelayed-template-parsing -fno-implicit-modules -fcxx-exceptions -fexceptions -fcolor-diagnostics -faddrsig -o a.o -x c++ a.cpp
clang -cc1 version 16.0.5 based upon LLVM 16.0.5 default target x86_64-pc-linux-gnu
ignoring nonexistent directory "/usr/local/include"
#include "..." search starts here:
#include <...> search starts here:
/usr/lib/llvm/16/bin/../../../../lib/clang/16/include
/usr/include
End of search list.
a.cpp:2:9: error: unknown type name 'char16_t'
typedef char16_t mychar;
^
1 error generated.

with linux triple:

franta@here ~ $ clang++ --std=c++2b -fno-PIE --target=x86_64-unknown-linux -c --verbose a.cpp
clang version 16.0.5
Target: x86_64-unknown-linux
Thread model: posix
InstalledDir: /usr/lib/llvm/16/bin
Configuration file: /etc/clang/clang++.cfg
System configuration file directory: /etc/clang
Selected GCC installation: /usr/lib/gcc/x86_64-pc-linux-gnu/12
Candidate multilib: .;@m64
Selected multilib: .;@m64
(in-process)
"/usr/lib/llvm/16/bin/clang-16" -cc1 -triple x86_64-unknown-linux -emit-obj -mrelax-all -disable-free -clear-ast-before-backend -disable-llvm-verifier -discard-value-names -main-file-name a.cpp -mrelocation-model static -mframe-pointer=all -fmath-errno -ffp-contract=on -fno-rounding-math -mconstructor-aliases -funwind-tables=2 -target-cpu x86-64 -tune-cpu generic -mllvm -treat-scalable-fixed-error-as-warning -debugger-tuning=gdb -v -fcoverage-compilation-dir=/home/franta -resource-dir /usr/lib/llvm/16/bin/../../../../lib/clang/16 -include /usr/include/gentoo/fortify.h -include /usr/include/gentoo/maybe-stddefs.h -internal-isystem /usr/lib/gcc/x86_64-pc-linux-gnu/12/include/g++-v12 -internal-isystem /usr/lib/gcc/x86_64-pc-linux-gnu/12/include/g++-v12/x86_64-pc-linux-gnu -internal-isystem /usr/lib/gcc/x86_64-pc-linux-gnu/12/include/g++-v12/backward -internal-isystem /usr/lib/llvm/16/bin/../../../../lib/clang/16/include -internal-isystem /usr/local/include -internal-isystem /usr/lib/gcc/x86_64-pc-linux-gnu/12/../../../../x86_64-pc-linux-gnu/include -internal-externc-isystem /include -internal-externc-isystem /usr/include --std=c++2b -fdeprecated-macro -fdebug-compilation-dir=/home/franta -ferror-limit 19 -stack-protector 2 -fstack-clash-protection -fgnuc-version=4.2.1 -fno-implicit-modules -fcxx-exceptions -fexceptions -fcolor-diagnostics -D__GCC_HAVE_DWARF2_CFI_ASM=1 -o a.o -x c++ a.cpp
clang -cc1 version 16.0.5 based upon LLVM 16.0.5 default target x86_64-pc-linux-gnu
ignoring nonexistent directory "/usr/local/include"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-pc-linux-gnu/12/../../../../x86_64-pc-linux-gnu/include"
ignoring nonexistent directory "/include"
#include "..." search starts here:
#include <...> search starts here:
/usr/lib/gcc/x86_64-pc-linux-gnu/12/include/g++-v12
/usr/lib/gcc/x86_64-pc-linux-gnu/12/include/g++-v12/x86_64-pc-linux-gnu
/usr/lib/gcc/x86_64-pc-linux-gnu/12/include/g++-v12/backward
/usr/lib/llvm/16/bin/../../../../lib/clang/16/include
/usr/include
End of search list.

[GDH-gentoo]

The failing windows triple:

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franta@here ~ $ clang++ --std=c++2b -fno-PIE --target=x86_64-unknown-windows -c --verbose a.cpp 
clang version 16.0.5
Target: x86_64-unknown-windows-msvc
...
 "/usr/lib/llvm/16/bin/clang-16" -cc1 -triple x86_64-unknown-windows-msvc ... -o a.o -x c++ a.cpp

I have slightly different results compiling your example with Clang 15.0.7 straight from a stage3 archive from some months ago:

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$ clang++ -v -c -fno-pie --target=x86_64-unknown-windows a.cpp 
clang version 15.0.7
Target: x86_64-unknown-windows-msvc
...
 "/usr/lib/llvm/15/bin/clang-15" -cc1 -triple x86_64-unknown-windows-msvc19.20.0 ... -o a.o -x c++ a.cpp

First, notice that target x86_64-unknown-windows means x86_64-unknown-windows-msvc, i.e. compatibility with Microsoft Visual C and C++ (MSVC). Second, notice that my clang-15 gets a triplet that has an MSVC version appended by default, 19.20.0, and your clang-16 doesn't.

So I tried passing complete targets, and found that the example compiles with --target=x86_64-unknown-windows-msvc19, but I get the same error passing --target=x86_64-unknown-windows-msvc18.99. So a) Clang seems to think that there is a compatibility problem with certain versions of MSVC when a program uses keyword char16_t, and b) I don't know why your Clang doesn't get a triplet with an MSVC version.