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PDR: Docs: Useful Compiler Flags

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This page is intended to summarize the various compiler flags that we will be learning throughout the semester. There are hundreds (if not thousands!) of such options; we'll only be dealing with a few of them.

These flags are for the clang compiler, but the are mostly the same for the g++ compiler (the only difference, of the ones listed below, are the flags to generate the particualr assembly flavor).

  • -O2: Creates an optimized executable. Note that if you are using the -c command (below), then you should call -O2 for those as well as the final linker call.
  • -c <filename.cpp>: This flag will compile BUT NOT LINK the passed .cpp file. It will create a filename.o file. To create the executable, you must call the compiler with all the .o files (i.e. clang *.o)
  • -o <filename>: This will save the output executable into .exe (or, in Linux/Unix, just ). For example, clang -o foo foo.cpp will compile the foo.cpp file and name the executable 'foo'. If you do out specify this flag, the output is saved to a.exe (or a.out in Linux/Unix). The 'o' in -o stands for output (as in output file).
  • -g: Include debugging information in the executable file. This is needed to debug the file in gdb.
  • -Wall: Display all warning messages. An error will prevent the program from being compiled, whereas a warning will not. There are many types of warnings that can be displayed, some of which are rather obscure. The 'all' part means to display all of them. It is a good idea to use this, as warnings are often bugs in your program.
  • -S: generate assembly output, and then stop (i.e. does not compile the program beyond the x86 assembly). Note that the assembly format that is created as a result of this flag has a different format than what we have seen in lecture -- the idea is the same, but the register specification is different, and the destination/source order is reversed for the commands.
  • -mllvm --x86-asm-syntax=intel: sets the assembly output format to the flavor that we are used to in (this is the only flag on this list that is not the same in g++)
  • -MM: generates dependencies in the format used in Makefiles

If you want to see all of the clang options, enter man clang at the Linux prompt. It's quite a list!