I know that it's generally recommended to use -march=native (if you're compiling for the machine you're on), so that gcc determines your arch and cputype and generates the most machine-specific code, but how does it do that?
Does it use cpuid (on either arm or x86)? What techniques are used on platforms without a cpuid-like instruction?
Good question.
My intuition was that it would go check /proc/cpuinfo. Actually it depends on the architecture it has been compiled to run on. It seems like host_detect_local_cpu is the function responsible for that. Its job is to replace -march=native by either the good -march=<...> or a set of flags (-mmmx, -mno-avx, etc) that match as closely as possible the current cpu. Examples for i386 and arm. i386 uses cpuid directly to check for each and every possible feature. arm checks in /proc/cpuinfo for the CPU part line and has a table that maps from a CPU part value to a generation of the architecture and use this directly in -march=<...>.
Just for fun I check on other architectures (I am not familiar with them).
sparc: uses the kstat interfacesparc: uses /proc/cpuinfoalpha: uses the implver instructionrs6000: uses the hw.cpusubtype system callrs6000: uses hardcoded powerpcrs6000: checks in /proc/self/auxv for the platform value in the elf interpreter of its own processrs6000: uses _system_configuration (apparently a global structure)