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,