On Sat, May 16, 2015 at 12:03 PM, Rich Felker <dal...@libc.org> wrote: > On Sat, May 16, 2015 at 11:59:56AM -0700, H.J. Lu wrote: >> On Sat, May 16, 2015 at 7:19 AM, H.J. Lu <hjl.to...@gmail.com> wrote: >> > On Fri, May 15, 2015 at 4:49 PM, Rich Felker <dal...@libc.org> wrote: >> >> On Fri, May 15, 2015 at 04:34:57PM -0700, H.J. Lu wrote: >> >>> On Fri, May 15, 2015 at 4:30 PM, H.J. Lu <hjl.to...@gmail.com> wrote: >> >>> > On Fri, May 15, 2015 at 4:14 PM, H.J. Lu <hjl.to...@gmail.com> wrote: >> >>> >> My relax branch proposal works even without LTO. >> >>> >> >> >>> > >> >>> > I will borrow GOTPCREL from x86-64 and do >> >>> > >> >>> > [hjl@gnu-6 relax-4]$ cat b.S >> >>> > call *foo@GOTPCREL(%eax) >> >>> >> >>> call *foo@GOTPLT(%eax) >> >>> >> >>> is a better choice. >> >> >> >> foo@GOTPCREL is preferable (but does not yet exist for ia32, so the >> >> reloc type would have to be added) since it saves a useless add. >> >> Instead of: >> >> >> >> call __x86.get_pc_thunk.ax >> >> addl $_GLOBAL_OFFSET_TABLE_, %eax >> >> call *foo@GOTPLT(%eax) >> >> >> >> you can just do: >> >> >> >> call __x86.get_pc_thunk.ax >> >> call *foo@GOTPCREL(%eax) >> >> >> >> Note that it also works to have extra instructions between: >> >> >> >> call __x86.get_pc_thunk.ax >> >> 1: ... >> >> call *foo@GOTPCREL+(1b-.)(%eax) >> >> >> >> I may not have gotten the syntax quite right, but hopefully yoy get >> >> the idea. This same approach (with GOTPCREL) can be used for _all_ GOT >> >> accesses, including global data, to eliminate the useless add. >> >> >> > >> > This is a good idea. But I'd like to use something for both i386 and >> > x86-64. I am proposing >> > >> > call/jmp *foo@GOTPCRELAX+addend(%reg) >> > >> > It is similar to @GOTPCREL, but with a new relax relocation. Before >> > I can do that, I need to fix >> >> It doesn't work. REG must hold GOT base for other GOT relocations. >> We need to keep >> >> addl $_GLOBAL_OFFSET_TABLE_, %eax > > Like I just said, all foo@GOT(%gotreg) can be replaced with > foo@GOTPCREL+[label-.](%labelreg) where %labelreg is a register > pointing to the referenced label (the point at which the program > counter was saved). This is a minor but useful optimization that can > be made for all GOT accesses, not just ones for [relaxable] function > calls.
There is also foo@GOTOFF(%reg). Remove addl is independent of relax branch. I will leave it out. Relax branch will support call/jmp *bar@GOTRELAX(%reg) for both i386 and x86-64. -- H.J.
