On Tue, May 12, 2015 at 11:42 AM, H.J. Lu <hjl.to...@gmail.com> wrote: > On Mon, May 11, 2015 at 8:52 AM, H.J. Lu <hjl.to...@gmail.com> wrote: >> >> I will clarify in the spec language. Yes, that is the intention for both >> R_X86_64_RELAX_PC32 and R_X86_64_RELAX_PLT32. That is what >> is implemented on users/hjl/relax branch. >> > > Here is the updated proposal. I changed nop prefix from 0x48 > to 0x67 and clarified how foo@GOTPCREL(%rip) should be > resolved. >
Another update. I replaced R_X86_64_RELAX_PLT32 with R_X86_64_RELAX_GOTPCREL so that I can apply the same relaxation to i386. H.J. ---- To remove one direct branch to PLT for external function calls: https://gcc.gnu.org/ml/gcc-patches/2015-05/msg00001.html I am proposing to add 2 new relocations, R_X86_64_RELAX_PC32 and R_X86_64_RELAX_GOTPCREL: 1. R_X86_64_RELAX_PC32 can only be used on 32-bit direct call/jmp instructions with a relax prefix, 0x67, which is the address size prefix and is ignored by 32-bit direct call/jmp instructions in both 32-bit and 64-bit modes. 2. Compiler may generate a. call/jmp *foo@GOTRELAX(%rip) which generates R_X86_64_RELAX_GOTPCREL relocaton, instead of call/jmp foo@PLT when branching to an external function, foo, in PIC mode. b. relax call/jmp foo which generates R_X86_64_RELAX_PC32 relocaton, instead of call/jmp foo when branching to an external function, foo, in non-PIC mode. 3. Linker may treat them as R_X86_64_PC32 and R_X86_64_GOTPCREL respectively. 4. Optionally, a. For 0x67 call/jmp foo linker may convert it to call/jmp *foo@GOTRELAX(%rip) when function foo is externally defined. b. For call/jmp *foo@GOTRELAX(%rip) linker may convert it to 0x67 call/jmp foo when function foo is locally defined. This relaxation effectively turns off lazy binding on function, foo. R_X86_64_RELAX_PC32 is defined as 39, which was the deprecated R_X86_64_PC32_BND for bnd call/jmp foo R_X86_64_RELAX_GOTPCREL is defined as 41. Since the current linkers treat R_X86_64_PC32_BND as R_X86_64_PC32, they can handle R_X86_64_RELAX_PC32, correctly, but not R_X86_64_RELAX_GOTPCREL.
