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I studied Option Summary for gfortran but found no compiler option to detect integer overflow. Then I found the GCC (GNU Compiler Collection) flag option -fsanitize=signed-integer-overflow here and used it when invoking gfortran. It works--integer overflow can be detected at run time!

So what does -fsanitize=signed-integer-overflow do here? Just adding to the machine code generated by gfortran some machine-level pieces that check integer overflow?

What is the relation between GCC (GNU Compiler Collection) flag options and gfortran compiler options ? What gcc compiler options can I use for gfortran, g++ etc ?

Kevin Powell
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2 Answers2

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There is the GCC - GNU Compiler Collection. It shares the common backend and middleend and has frontends for different languages. For example frontends for C, C++ and Fortran which are usually invoked by commands gcc, g++ and gfortran.

It is actually more complicated, you can call gcc on a Fortran source and gfortran on a C source and it will work almost the same with the exceptions of libraries being linked (there are some other fine points). The appropriate frontend will be called based on the file extension or the language requested.

You can look almost all GCC (not just gcc) flags for all of the mentioned frontends. There are certain flags which are language specific. Normally you will get a warning like 

gfortran -fcheck=all source.c
cc1: warning: command line option ‘-fcheck=all’ is valid for Fortran but not for C

but the file will compile fine, the option is just ignored and you will get a warning about that. Notice it is a C file and it is compiled by the gfortran command just fine.

The sanitization options are AFAIK not that language specific and work for multiple languages implemented in GCC, maybe with some exceptions for some obviously language specific checks. Especially -fsanitize=signed-integer-overflow which you ask about works perfectly fine for both C and C++. Signed integer overwlow is undefined behaviour in C and C++ and it is not allowed by the Fortran standard (which effectively means the same, Fortran just uses different words).

Vladimir F Героям слава
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  • Sorry for the confusion, by `gcc` I really mean GCC (questions already updated). It is great that the compiler will give warning if a wrong flag option is used. However, for those options that do not complain, some does not work !!! For example, both `-fsanitize=bounds` and `-fsanitize=integer-divide-by-zero` do not work for `gfortran`. So I guess one can still use non-language-specific flag options from GCC when invoking, say `gfortran`, compiler, but should not rely on those flag options. Only those compiler options documented for a specific compiler will guarantee to work. – Kevin Powell Sep 07 '17 at 13:36
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    The options mentioned in the GFortran manual are implemented in the GFortran frontend and are thus specific for Fortran. The GCC manual contains options that are specific to C, C++, options that are generic, and options that are valid for a specific target (backend) only. Some of these options may work perfectly fine with GFortran, others might not. – janneb Sep 07 '17 at 18:10
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This isn't a terribly precise answer to your question, but an aha! moment, when learning about compilers, is learning that gcc (the GNU Compiler Collection), like llvm, is an example of a three-stage compiler.

  • The ‘front end’ parses the syntax of whichever language you're interested, and spits out an Abstract Syntax Tree (AST), which represents your program in a language-independent way.
  • Then the ‘middle end’ (terrible name, but ‘the clever bit’) reorganises that AST into another AST which is semantically equivalent but easier to turn into machine code.
  • Then the ‘back end’ turns that reorganised AST into assembler for one-or-other processor, possibly doing platform-specific micro-optimisations along the way.

That's why the (huge number of) gcc/llvm options are unexpectedly common to (apparently wildly) different languages. A few of the options are specific to C, or Fortran, or Objective-C, or whatever, but the majority of them (probably) are concerned with the middle and last bits, and so are common to all of the languages that gcc/llvm supports.

Thus the various options are specific to stage 1, 2 or 3, but may not be conveniently labelled as such; with this in mind, however, you might reasonably intuit what is and isn't relevant to the particular language you're interested in.

(It's for this sort of reason that I will dogmatically claim that CC++FortranJavaPerlPython is essentially a single language, with only trivial syntactical and library minutiae to distinguish between dialects).

Norman Gray
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  • Thanks for the in-depth explanation! Well, the bad thing is that when invoking a specific language compiler , `gfortran` for example, using certain GCC flag options, the compiler does not give warning and the flag options do NOT work. – Kevin Powell Sep 07 '17 at 13:40