On 12/23/2015 05:02 AM, Yuri D'Elia wrote:
I couldn't find anything on the matter, but what happens if -march is repeated more than once? I would assume the usual behavior that the last flag "wins".
This is correct for all -march= arguments except for native. Some -march= option processing takes place in the compiler driver and some in the compiler itself. For -march=native, unless the handling is overridden in the GCC spec file, the i386 compiler driver determines the host CPU model and substitutes its name and a set of other options for "native" in the -march option passed to the compiler. This can be seen in the driver output by invoking it with -march=native -v. For example, on my machine, -march=native ends up replaced with -march=sandybridge -mmmx -mno-3dnow -msse and a few dozen other options. All other -march= options are passed to the compiler unchanged. The compiler then takes the last -march= option and ignores any other -march= options that may have preceded it.
On a haswell host, the following: gcc -march=native -march=opteron or gcc -march=opteron -march=native both emit code which is illegal for the opteron ISA, as if -march=native had special treatment. Specifying -march=opteron alone works as intended.
Because of the special handling, when -march=native and another, more specific -march= option is specified, the latter option is always appended to the cc1 command line by the driver. This has the effect of -march=opteron overriding the -march= option substituted for -march=native by the driver regardless of which comes first. (See also bug 64534.)
Since I generally override the default flags in makefiles by appending exceptions where needed, I'd sort of expected the regular behavior. Is this intended? If not, should I try to generate a small test case?
A test case with the -v output of the compiler and its version is always helpful. Based on your description it sounds to me as though one of the other (non-march) options substituted for -march=native by the driver is interfering with the subsequent -march=opteron option. In any event, I would be inclined to consider this a bug since the details above aren't documented anywhere and are far from intuitive. At the very least, the special treatment of -march=native should be documented. Ideally, though, I would expect the driver to avoid doing the substitution when the -march=native is subsequently overridden on the command line. Martin
