On 3 September 2015 at 09:45, Mason wrote: > I noticed something strange in gcc -v output: > > $ gcc -v -c foo.c > ... > gcc version 4.8.2 (Ubuntu 4.8.2-19ubuntu1) > COLLECT_GCC_OPTIONS='-v' '-c' '-mtune=generic' '-march=x86-64' > /usr/lib/gcc/x86_64-linux-gnu/4.8/cc1 -quiet -v -imultiarch x86_64-linux-gnu foo.c -quiet -dumpbase foo.c -mtune=generic -march=x86-64 -auxbase foo -version -fstack-protector -Wformat -Wformat-security -o /tmp/ccFwNaX5.s > > > -mtune=generic -march=x86-64 > Won't the -march option override the -mtune option? No, I don't think so. I assume the explicit -mtune overrides the implicit one. > Another thing. I was under the impression that when the target > platform is x86_64 (so not the same x86-64 as above) then some x86-64 is the argument to -march and -mtune that means "the generic x86_64 instruction set" so when the target platform is x86_64 and there is no (implicit or explicit) -march then the instruction set used is as if -march=x86-64 was specified. > options are enabled by default, such as -msse and -msse2. > Is that correct? Yes. You can easily check by using -S -fverbose-asm and looking at the assembly output, which will list the active options. > Or is just the use of SSE for FP? i.e. -mfpmath=sse > "For the i386 compiler, you must use -march=cpu-type, -msse or -msse2 > switches to enable SSE extensions and make this option effective. > For the x86-64 compiler, these extensions are enabled by default." > > > gcc -v does not print what instruction-set extensions it will > be using? Is there a way to know that gcc used e.g. SSE3 > instructions in the code generation? -fverbose-asm
