On Wed, 3 Apr 2013 20:49:40 -0700 (PDT) Hei Chan <structurechart@xxxxxxxxx> wrote: > Hi, > > I noticed that I can specify -march=native and -mtune=native. > > I have few questions: > - On some boxes, I noticed that if I specify -march=native and -mtune=native, > g++ might "convert" -march=native into -march=corei7-avx along with a list of > flags such as -mcx16 -msahf -mavx -msse4.2 --param l1-cache-size=32 --param > l1-cache-line-size=64 --param l2-cache-size=10240 (there are a lot more flags > but I have to take them out as GCC mailing server marked my mail as spam if I > listed all), and -mtune=generic. how come gcc recognize the CPU type as > corei7-avx but it can't recognize the CPU type for -mtune? It turns out that for some cpus -mtune=generic can produce better code than -mtune=<cpu>. Mostly for cpus that don't have a specifically tuned cost model IIUC. > - Let's say g++ figures that my CPU support sse4.1, and how does it choose > between sse4.1 instructions and other set of instructions (assuming both > achieve the same task)? Does g++ have a database that it computes the > numbers of CPU cycles for all the possible combinations before it decides? Yes there are cost models that are used to decide what instructions to use. See gcc/config/i386/i386.c for example. > - Also, how does the l1-cache-size and l2-cache-size parameters affect the > optimization? Only affecting the decision of inline? Or there are more? Not sure on that one. > - Is there any tool that will tell me what CPU specific instructions are used > (other than going through the object files manually with objdump one by one)? https://github.com/dirtyepic/scripts/blob/master/analyze-x86 is a clumsy attempt. It probably misses a lot and I haven't updated it in a couple releases though. If there's another tool like this out there I'd definitely be interested. -- gcc-porting toolchain, wxwidgets by design, by neglect @ gentoo.org for a fact or just for effect
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