On Wed, May 6, 2015 at 10:35 AM, Rich Felker <dal...@libc.org> wrote: > On Wed, May 06, 2015 at 07:43:58PM +0300, Alexander Monakov wrote: >> On Wed, 6 May 2015, Jakub Jelinek wrote: >> > The linker would know very well what kind of relocations are used for >> > particular PLT slot, and for the new relocations which would resolve to the >> > address of the .got.plt slot it could just tweak corresponding 3rd insn >> > in the slot, to not jump to first plt slot - 16, but a few bytes before >> > that >> > that would just load the address of _G_O_T_ into %ebx and then fallthru >> > into the 0x4c2b7310 snippet above. The lazy binding would be a few ticks >> > slower in that case, but no requirement on %ebx to contain _G_O_T_. >> >> No, %ebx is callee-saved, so you can't outright overwrite it in the PLT stub. > > Indeed. And the situation is the same on almost all targets. The only > exceptions are those with direct PC-relative addressing (like x86_64) > and those with reserved inter-procedural linkage registers and > efficient PC-relative address loading via them (like ARM and AArch64). > MIPS (o32) is also an interesting exception in that the normal ABI is > already PLT-free, and while callees need a PIC register loaded, it's a > call-clobbered register, not a call-saved one, so it doesn't make the > same kind of trouble, > > I really don't see a need to make no-PLT code gen support lazy binding > when it's necessarily going to be costly to do so, and precludes most > of the benefits of the no-PLT approach. Anyone still wanting/needing > lazy binding semantics can use PLT, and can even choose on a per-TU > basis (or maybe even more fine-grained with pragmas/attributes?). > Those of us who are suffering the cost of PLT with no benefits > (because we use -Wl,-z,relro -Wl,-z,now) can just be rid of it (by > adding -fno-plt) and enjoy something like a 10% performance boost in > PIC/PIE. >
There are things compiler can do for performance and correctness if it is told what options will be passed to linker. -z now is one and -Bsymbolic is another one: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65886 I think we should add -fnow and -fsymbolic. Together with LTO, we can generate faster executables as well as shared libraries. -- H.J.
