On Thu, Apr 04, 2013 at 10:27:31AM +0200, Florian Weimer wrote: > On 04/04/2013 09:47 AM, Jakub Jelinek wrote: > >On Thu, Apr 04, 2013 at 09:39:18AM +0200, Paolo Bonzini wrote: > >>>I'm willing to agree that PIE on x86 is going to be very slow due to > >>>register pressure. > >> > >>Yes, but not on x86-64 which has %rip-relative addressing. It is > >>probably a wash there. > > On x86_64, GCC uses %rip-relative addressing even in non-PIC mode. Only rarely, when it is less expensive or a wash. Whenever you need more complex addressing modes, we don't emit %rip addressing for non-pic and have to do for pic. Say we emit: movzwl local_symbol(%rsi,%rsi), %edx for non-pic, but have to emit: leaq local_symbol(%rip), %rax movzwl (%rax,%rsi,2), %edx for pic/pie code (and only when the symbol in static/hidden, otherwise there would need to be an extra indirection). > I think a similar optimization would be possible for access to > global variables because ld could compute the final layout of all > global variables in the binary itself, just as in the non-PIE case. Nope. The thing is, depending on if the variable is known to bind locally (for PIC that is essentially static or hidden visibility, for PIE you can add to that global vars defined in the current CU), you either emit code that avoids the indirection (say %rip addressing, GOTOFF etc.), or emit code that does the indirection, there is no linker relaxation that could turn (albeit with worse generated code, increased register pressure etc.) code that uses indirection (loads from GOT) into one that doesn't, we'd need extra relocations and as that there are just too many forms of the instructions on i?86/x86_64, it would be pretty hard. So, PIE is definitely not for free, not even on x86_64. Jakub -- devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxxx https://admin.fedoraproject.org/mailman/listinfo/devel
