How does GCC behave if passed conflicting compiler flags?

Question

I know that if you execute GCC as such:

gcc -O3 -O2 foo.c

GCC will use the last optimization flag passed (in this case O2). However, is this true for all flags? For example, if I execute GCC like so:

gcc -mno-sse -msse bar.c

Will it support SSE since that was the last flag passed, or would this result in undefined behavior? My initial experimentation seems to indicate that it will support SSE, but I'm not sure if this is true for all cases.

Answer 1

Normally later options on the line override ones passed previously, as you mention in your first example. I haven't personally come across any different behaviour for -m or -f flags, but I don't know of a specific reference in the documentation.

Note that some options don't behave this way:

$ gcc example.c -DABC -DABC=12
<command-line>: warning: "ABC" redefined
<command-line>: warning: this is the location of the previous definition

So there would need to be a -UABC in between there to shut that warning up.

As an aside, clang is particularly good at solving this problem - it will produce a warning if it ignores a command line option, which can help you out.