Le 03/10/2010 17:36, J. William Campbell a écrit : > Hi All, > It is for sure that -fPIC/-fPIE programs will contain more executable > instructions than programs compiled without these options. > The program will also contain more data space for the got. If -fPIC > actually produced a fully position-independent executable, the extra > overhead would perhaps be tolerable. However, since it does not do this, > (problems with initialized data etc.) there is really no advantage in > using these compile-time options. The executable code and required data > space for the program without these switches will "always" be smaller > and faster than with them. In order to fix the remaining issues even > when using -fPIC, a relocation loop must exist in the u-boot code, > either one global one or a bunch of user written specific ones. Also, > the -pie switch will be needed anyway at link time to build the > relocation table for the remaining relocation requirements. > Programs compiled without -fPIC will have a larger .rel.dyn table than > those compiled with -fPIC. However, the table entries in the relocation > table occupy about the same storage as the code generated by the > compiler to relocate a reference to the symbol at run time. So this is > probably a almost a wash. Also, the dynamic relocation data need not be > copied into the run-time object, as it is no longer needed. So the > likely outcome is that the "flash" image is about the same size/slightly > larger than the one compiled by -fPIC, and that the ram footprint after > relocation is slightly smaller. > If one is REALLY pressed for space, the size of the dynamic relocation > area can be reduced by a post-processor program that would re-format the > relocation entries. This re-formatting is possible because 1) ELF is a > very general format and we only need a small subset of it, and 2) u-boot > code will never occupy say 16 MB of space, so each relocation can > probably be compressed into a 32 bit word. I doubt anyone is that > desperate, but it IS possible. > It will be interesting to see what the results of this comparison are. > For me, the no user awareness of relocation is worth a lot, and the fact > that the difference/overhead of relocation will all be in exactly one > place is very appealing. > > Best Regards, > Bill Campbell
Hi Bill, Thanks for the explanations. I am experimenting with ELF relocation right now, replacing -fPIe with -pie, and this generates .rel.dyn, but also many other sections. I'm trying to get rid of them; apparently /DISCARD/ing them in the linker file seems to reduce this to a minimum (I still have a .got.plt section which seems useless but I cannot remove it lest the linker segfaults). But the .rel.dyn generated by the linker section does not provide symbols to mark its start and end, and I have found no documentation in binutils ld which would describe how to rewrite the .rel.dyn section and add these symbols myself. How did you manage that for i386? I did not see a linker file in the i386 part of u-boot. Amicalement, -- Albert. _______________________________________________ U-Boot mailing list U-Boot@lists.denx.de http://lists.denx.de/mailman/listinfo/u-boot
