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GCC

Options for Linking

  • -l<library> when searching, shared libraries have preference unless the -static option is set
    • the linker searches and processes libraries and object files in the order they are specified
    • so the libraries with symbol definitions should come last

Options That Control Optimization

  • -fif-conversion attempt to transform conditional jumps into branch-less equivalents (e.g. conditional moves)

  • -fomit-frame-pointer don't keep the frame pointer in a register (RBX) for functions that don't need one

    • this can make RBX be used as general purpose register and spare the room for optimizations
    • but make the debugger harder for back tracing (don't know where the stack frames begin and end)

  • -ffunction-sections / -fdata-sections place each function or data item into its own section in the output file

  • -march tells the compiler that it should produce code for a certain kind of CPU. For instance, to take benefit of AVX instructions

    • it's an ISA selection option; it tells the compiler that it may use the instructions from the ISA

  • -mtune generate more generic code than -march; though it will tune code for a certain CPU, it does not take into account available instruction sets and ABI

    • only use this flag when there is no available -march option

  • -ftree-vectorize attempts to vectorize loops using the selected ISA if possible (default at -O3 and -Ofast)

    • note that, this option doesn't always improve code, and usually makes the code larger

Warning: Do not use -march=native or -mtune=native in CLFAGS or CXXFLAGS variables when compiling with distcc

Options for Code Generation Conventions

  • -fcommon whether to put uninitialized global variables into COMMON block, the default is -fno-common (put them in .bss section)
    • this behavior is inconsistent with C++ and may have speed and code size penalty

Options for Developers

  • -fdump-class-hierarchy output a representation of each class's hierarchy and virtual function table layout to a file <source>.class

Options Controlling C++ Dialect

  • -fvisibility-inlines-hidden marks the inline functions with __attribute__ ((visibility ("hidden"))) so that they do not appear in the export table of a DSO and do not require a PLT indirection when used within the DSO
    • Enable this switch can dramatically reduce load and link time
    • This switch also means the user does not attempt to compare pointers to inline functions or methods where the addresses of the two functions are taken in different shared objects

Precompiled Headers

  • gcc xxx.h will generate a precompiled headers that accelerate following compilations

Reference