gcc – Make Me Engineer

How can I mitigate the impact of the Intel jcc erratum on gcc?

By compiler: GCC: -Wa,-mbranches-within-32B-boundaries clang (10+): -mbranches-within-32B-boundaries compiler option directly, not -Wa. MSVC: /QIntel-jcc-erratum See Intel JCC Erratum – what is the effect of prefixes used for mitigation? ICC: TODO, look for docs. The GNU toolchain does mitigation in the assembler, with as -mbranches-within-32B-boundaries, which enables (GAS manual: x86 options): -malign-branch-boundary=32 (care about 32-byte boundaries). … Read more

How can I add a default include path for GCC in Linux?

Try setting C_INCLUDE_PATH (for C header files) or CPLUS_INCLUDE_PATH (for C++ header files). As Ciro mentioned, CPATH will set the path for both C and C++ (and any other language). More details in GCC’s documentation.

Why can’t the size of a static array be made variable?

Since the size of the array you declare is not constant, what you have is an Variable Length Array(VLA). VLA are allowed by the c99 standard but there are some limitations associated with it. You cannot have an variable length array with static or extern storage class specifier. You have an VLA with static storage … Read more

Is there a compiler bug exposed by my implementation of an is_complete type trait?

The problem appears to be with the definition of void_t. Defining it as template<typename… Ts> struct make_void { typedef void type;}; template<typename… Ts> using void_t = typename make_void<Ts…>::type; instead yields the correct result (10) on both compilers (Demo). I believe this is the same issue noted in section 2.3 of N3911, the paper proposing void_t, … Read more

How to compile my own glibc C standard library from source and use it?

Setup 1: glibc without dedicated GCC This setup might work and is quick as it does not recompile the whole GCC toolchain, just glibc. The only problem I have with this setup is that I haven’t found a nice way to use runtime objects such as crt1.o, crti.o, and crtn.o provided by our glibc, and … Read more

GCC left shift overflow

Short answer: the Intel processor masks the shift count to 5 bits (maximum 31). In other words, the shift actually performed is 32 & 31, which is 0 (no change). The same result appears using gcc on a Linux 32-bit PC. I assembled a shorter version of this program because I was puzzled by why … Read more

How to include omp.h in OS X?

This command can help you brew install libomp brew info libomp libomp: stable 6.0.1 (bottled) LLVM’s OpenMP runtime library https://openmp.llvm.org/ /usr/local/Cellar/libomp/6.0.1 (12 files, 1.2MB) * Poured from bottle on 2018-11-20 at 16:12:22 From: https://github.com/Homebrew/homebrew-core/blob/master/Formula/libomp.rb ==> Dependencies Build: cmake ✘ ==> Requirements Required: macOS >= 10.10 ✔ ==> Caveats On Apple Clang, you need to add … Read more

GCC: Array type has incomplete element type

It’s the array that’s causing trouble in: void print_graph(g_node graph_node[], double weight[][], int nodes); The second and subsequent dimensions must be given: void print_graph(g_node graph_node[], double weight[][32], int nodes); Or you can just give a pointer to pointer: void print_graph(g_node graph_node[], double **weight, int nodes); However, although they look similar, those are very different internally. … Read more

gcc, strict-aliasing, and casting through a union

The fact that GCC is warning about unions doesn’t necessarily mean that unions don’t currently work. But here’s a slightly less simple example than yours: #include <stdio.h> struct B { int i1; int i2; }; union A { struct B b; double d; }; int main() { double d = 3.0; #ifdef USE_UNION ((union A*)&d)->b.i2 … Read more

Detecting 64bit compile in C

Since you tagged this “gcc”, try #if __x86_64__ /* 64-bit */ #endif

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