https://gcc.gnu.org/onlinedocs/gcc/Option-Index.html#Option-Index

flags

• basic: -pedantic -Wall -Wextra
• error: -Werror -Wl,--fatal-warnings
• math: -Wcast-align -Wfloat-equal -Wsign-compare
• use gold linker: -fuse-ld=gold
• build id: -Wl,--build-id
• compile options record: -frecord-gcc-switches
• generate compile dependency: -MM

spec file

format

• customize linker behavior
  • link to a different set of libraries
    • newlib, newlib_nano
  • change link flag

semihosting

• --specs=rdimon.specs: printf is redirected to host machine using jtag

reduce binary size

• function remove:
  • mark with: -ffunction-sections -fdata-sections
  • remove with: -Wl,--gc-sections
• remove rtti -fno-rtti

performance flags

• link time optimization -lto
  • optimize across compilation unit
• profile guided optimization
  • hot cold splitting

function attibute

strict-aliasing

an object of one type is assumed never to reside at the same address as an object of a different type

• optimization (such as re-order) might took place against our intention.

type aliasing in c++

Dereferencing a pointer with incompatible type is a UB. More details here

https://en.cppreference.com/w/cpp/memory/shared_ptr/pointer_cast

memcpy, bit_cast, union, aggregate?

frame-pointer

In a stack frame, we have:

• top
• local variable
• return address
• parameter
• bottom

And

• stack pointer, which points to the top of the stack
• frame pointer, which points to the return address of function
  • a frame pointer is needed when we have variable stack length ex. alloca

With the flag,

• -fno-omit-frame-pointer
  • compiler will always generate frame pointer
  • can help with debug because variable position can be easily determined
• -fomit-frame-pointer
  • can reduce code size when frame pointer is not necessary
  • it might still get generated when compiler determine it is necessary

overloaded virtual

class base
{
public:

    virtual void exception()
    {
    cerr << "unknown exception" << endl;
    }

    virtual void exception(const char* msg)
    {
    cerr << "exception: " << msg << endl;
    }
};

class derived : public base
{
public:
    // method 1
    // using base::execption;
    virtual void exception()
    {
        intermediate::exception("derived:unknown exception");
    }
    // method 2
    // private:
    // void exception(const char* tmp);
};

If we have a overloaded virtual function and only partially inherit it, the other overload will be shadowed. In the example, only exception() can be called through derived

This flag will show the shadowing as a warning.

hardening

Reference

-Wformat -Wformat-security -Werror=format-security # warn about uses of format function
-D_FORTIFY_SOURCE=2 # replace unlimited length buffer function call
-fstack-protector-strong # add check against stack overwrites
-fPIE -pie # ASLR
-Wl,-z,relro # turn several elf section to be read only
-Wl,-z,now # resolve all symbol now so that the complete GOT will be read only

executable stack

Reference

-Wa,--noexecstack
-Wl,-z,noexecstack
-Wl,--warn-execstack

pre-compile header

• avoid header being processed multiple times in larger project
• generate .gch file which take precedence over normal header file
• only one pre-compile header can be used in one translation unit
• ASLR can generate not binary identical .gch file
  • gch is a dump of compiler parsing state?
• any macro must be defined the same way when compiling the header
  • cannot use compiled header over c / c++
• debug info must be the same format, but can be ignored

gcc path

• -print-file-name: print file path to certain file
  • very helpful when use library from gcc
• --sysroot: change gcc base folder for searching headers -print-sysroot: print current sysroot
• gcc -E -Wp,-v -xc /dev/null: print what the include path is
  • https://stackoverflow.com/questions/17939930/finding-out-what-the-gcc-include-path-is

makefile

make has a very stupid relationship with tabs. All actions of every rule are identified by tabs. And, no, four spaces don’t make a tab. Only a tab makes a tab.

FILES := $(shell ...)  # expand now; FILES is now the result of $(shell ...)
FILES = $(shell ...)   # expand later: FILES holds the syntax $(shell ...)

FILES != ... # same as FILES := $(shell ...)

all:
    # once inside of a indented block, each line is run by a separate shell
    FILES="$(shell ls)"; echo $$FILES

# print all variables
test:
    $(foreach var,$(.VARIABLES),$(info $(var) = $($(var))))

# print
$(MAKECMDGOALS)

todo

sized-deallocation
-rdynamic
-Wl,--hash-style=gnu
-Wl,-Bsymbolic-functions
--dynamic-list

coverage

-dM : generate a list of ‘#define’ directives for all the macros defined during the execution of the preprocessor, including predefined macros

-Wl,–no-undefined does not work with weak symbol?

-Wl,–no-allow-shlib-undefined

https://gcc.gnu.org/onlinedocs/libstdc++/manual/parallel_mode_using.html