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GCC - Clang system headers incompatibilities

Table of Contents

GCC fixincludes

A little-known part of GCC's build process is a script called "fixincludes", or fixinc.sh. Purportedly, the purpose of this script is to fix "non-ANSI system header files" which GCC "cannot compile" (https://ewontfix.com/12/). Some examples of these fixes are:

  • Changing AIX's _LARGE_FILES redirection of open to open64, etc. to use GCC's __asm__ keyword rather than #define, as the latter breaks C++.
  • Exposing the long double math functions in math.h on Mac OS 10.3.9, which inexplicably omitted declarations for them.
  • Adding workaround for Linux 2.2 and earlier kernel bug with direction flag to FD_ZERO macros.

The fixincludes process iterates over each header file it finds under /usr/include (or whatever the configured include directory is), applies a set of heuristics based on the filename, machine tuple, and a regular expression matched against the file contents, and if these rules match, it applies a sequence of sed commands to the file. As of this writing, there are 228 such "hacks" that may be applied. The output is then stored in GCC's private include-fixed directory (roughly /usr/lib/gcc/$MACH/$VER/include-fixed), which GCC searches before the system include directory, thus "replacing" the original header when the new GCC is used.

Fixincludes is a bad idea and a bad hack, the Gentoo Linux distribution simply removes the whole generated directory. Consequently, Clang (from version 3.8) skips the include-fixed include path even if it is built with GCC and configured to use GCC's libstdc++ (https://bugs.launchpad.net/ubuntu/+source/llvm-toolchain-3.8/+bug/1573778).

References:

System headers for Intel vector instructions

GCC and Clang support native vector operations differently. The corresponding Intel intrinsics (for MMX, SSE, AVX, etc) have their own headers in a compiler specific include directory. For instance, the _mm_add_ss for SSE can be found in xmmintrin.h. GCC implements the intrinsic by delegating to it's own builtin intrinsic:

__m128 _mm_add_ss (__m128 __A, __m128 __B)
{
  return (__m128) __builtin_ia32_addss ((__v4sf)__A, (__v4sf)__B);
}

Meanwhile Clang handles vector types commonly, therefore there is no such compiler intrinsic as __builtin_ia32_addss in Clang. Clang uses the common operators on the vector types to implement _mm_add_ss:

__m128 _mm_add_ss(__m128 __a, __m128 __b)
{
  __a[0] += __b[0];
  return __a;
}

Because of this implementation difference Clang simply skips the system header path for GCC intrinsics even if Clang is built with GCC and configured to use GCC's libstdc++. For example:

$ clang -E -x c -v -
...
#include <...> search starts here:
 /usr/local/include
 /usr/lib/llvm-4.0/bin/../lib/clang/4.0.0/include
 /usr/include/x86_64-linux-gnu
 /usr/include

$ gcc -E -x c -v -
#include <...> search starts here:
 /usr/lib/gcc/x86_64-linux-gnu/7/include
 /usr/local/include
 /usr/lib/gcc/x86_64-linux-gnu/7/include-fixed
 /usr/include/x86_64-linux-gnu
 /usr/include

References:

Other system headers in GCC

GCC implements many other features which are not implemented in Clang. These headers are in the same directory where the Intel vector intrinsic headers are. E.g. in:

/usr/lib/gcc/x86_64-linux-gnu/7/include

Some of these headers may be used with Clang, but the users must be aware that by adding GCC's internal header path to the include search path would result compilation error if vector instructions are used as well.

backtrace.h, backtrace-supported.h

Part of libbacktrace. It uses the GCC unwind interface to collect a stack trace, and parses DWARF debug info to get file/line/function information. The library may be linked into a program or library and used to produce symbolic backtraces. Sample uses would be to print a detailed backtrace when an error occurs or to gather detailed profiling information. We can use the library with Clang by adding gcc's intrinsic system include, for instance:

clang -lbactrace -I
/usr/lib/gcc/x86_64-pc-linux-gnu/6.2.1/include bt.c

However, using the library this way together with vector instructions (e.g. from xmmintrin.h) would fail the compilation and the analysis.

cross-stdarg.h

Contains macros to handle functions with variadic arguments (vararg, va_list, va_start, etc) during cross platform compilation for non x64 platforms. It defines ms_abi and sysv_abi specific macros like __builtin_ms_va_list. There is no need to use this header directly, rather function attributes should be used as described in both compilers' documentation:

omp.h

Part of libgomp. As of today within mainline GCC, the libgomp library, which up to now has been known as the GNU OpenMP Runtime Library, has been renamed to GNU Offloading and Multi Processing Runtime Library. The libgomp library has grown beyond just OpenMP to now being utilized for OpenMP 4 offloading capabilities, OpenACC support, AMD HSA support, etc.

According to one stackoverflow post it may work with Clang. We can use the library with Clang by adding gcc's intrinsic system include, for instance:

clang++ -I/usr/lib/gcc/x86_64-linux-gnu/4.9/include
-fopenmp=libiomp5 -o test test.cpp

However, using the library this way together with vector instructions (e.g. from xmmintrin.h) would fail the compilation and the analysis.

openacc.h

Part of libgomp. OpenACC (for open accelerators) is a programming standard for parallel computing developed by Cray, CAPS, Nvidia and PGI. The standard is designed to simplify parallel programming of heterogeneous CPU/GPU systems.

quadmath.h, quadmath_weak.h

Part of the GCC Quad-Precision Math Library Application Programming Interface (API). All math functions in this lib (acosq, asinq) uses the type __float128. We can use the library with Clang by adding gcc's intrinsic system include, for instance:

clang -O0 -m32 -lquadmath -I
/usr/lib/gcc/x86_64-pc-linux-gnu/6.2.1/include float128.c

However, using the library this way together with vector instructions (e.g. from xmmintrin.h) would fail the compilation and the analysis.

Related: Problem about 128bit floating-point operations in x86 machines

sgxintrin.h

Support for Intel SGX extension. Contains inline assembly macros. This Intel technology is for application developers who are seeking to protect select code and data from disclosure or modification. Intel® SGX makes such protections possible through the use of enclaves, which are protected areas of execution in memory.

We can use the library with Clang by adding gcc's intrinsic system include. However, using the library this way together with vector instructions (e.g. from xmmintrin.h) would fail the compilation and the analysis.

stdfix.h

Fixed-Point Arithmetic Support. Digital Signal Processors have traditionally supported fixed-point arithmetic in hardware. But more recently, many DSP-enhanced RISC processors are starting to support fixed-point data types as part of their native instruction set. When the precision requirements of the application can be met with fixed-point arithmetic, then this is preferred since it can be smaller and more efficient than floating-point hardware. DSP algorithms often represent the data samples and the coefficients used in the computation as fractional numbers (between -1 and +1) to avoid magnitude growth of a multiplication product. Fractional data type, where there are zero integer bits, is a subset of the more general fixed-point data type.

The types _Fract, _Accum, _Sat are not supported by Clang. Related: https://groups.google.com/forum/#!topic/llvm-dev/G7MDIj4Pq6w

stdint-gcc.h

Fixed size integers (e.g uint64_t). Use <stdint.h> instead of this header.