On Tue, 9 Oct 2007, Franck Bui-Huu wrote: > > What would be the gain for the kernel from using "-march=4ksd" rather > > than "-march=mips32r2"? > > > > It actually results in a kernel image ~30kbytes smaller for the former > case. It has been discussed sometimes ago on this list. I'm sorry but > I don't know why... Perhaps the pipeline description for the 4KSd CPU is different from the default for the MIPS32r2 ISA. Barring a study of GCC sources, if that really troubles you, you could build the same version of the kernel with these options: 1. "-march=mips32r2" 2. "-march=4ksd" 3. "-march=mips32r2 -mtune=4ksd" and compare the results. I expect the results of #2 and #3 to be the same and it would just back up my suggestion about keeping CPU-specific optimisations separate from the CPU selection. Please also note that our optimisation model is for speed (-O2) rather than size (-Os), so if "-mtune=4ksd" yields smaller code than "-mtune=mips32r2", it just means it is safe for this CPU to shrink code where appropriate without losing performance. One obvious place for such a choice is the use of the hardware multiplier vs shifts and additions where one multiplicand is a constant. > > What if you want to run a single kernel image regardless of the CPU > > installed in the system. Rebuilding the kernel (or having to keep a large > > collection of binaries) just because you want to swap the CPU does not > > seem like a terribly attractive idea. Some systems come with their CPU(s) > > on a daughtercard (each), you know... > > > > ok, I wasn't aware about this. You could have started by this point ;) Well, daughtercards for CPUs are so common for me -- the vast majority of MIPS-based systems I use have them -- that I have assumed, obviously incorrectly, that you see a benefit from such a rewrite of the TLB exception handlers which is large enough to justify the inconvenience of limiting the kernel to a given CPU card. > So now I think the right direction is to stick with tlbex.c and > make it smaller like Ralf did. That is certainly a good idea. Maciej
