On Thu, Apr 30, 2015 at 05:31:30PM -0700, Sriraman Tallam wrote:
> This comes with caveats. This cannot be generally done for all
> functions marked extern as it is impossible for the compiler to say if
> a function is "truly extern" (defined in a shared library). If a
> function is not truly extern(ends up defined in the final executable),
> then calling it indirectly is a performance penalty as it could have
> been a direct call. Further, the newly created GOT entries are fixed
> up at start-up and do not get lazily bound.
I've considered something similar for PowerPC (but didn't consider
doing do so for a subset of calls). Losing lazy symbol resolution is
a real problem. The other problem you cite of indirect calls that
could be direct can be fixed in the linker relatively easily.
Edit this code
0: ff 15 00 00 00 00 callq *0x0(%rip) # 0x6
2: R_X86_64_GOTPCREL foo-0x4
6: ff 25 00 00 00 00 jmpq *0x0(%rip) # 0xc
8: R_X86_64_GOTPCREL foo-0x4
to this
c: e8 00 00 00 00 callq 0x11
d: R_X86_64_PC32 foo-0x4
11: 90 nop
12: e9 00 00 00 00 jmpq 0x17
13: R_X86_64_PC32 foo-0x4
17: 90 nop
You may need to have gcc or gas add a marker reloc to say exactly
where an instruction starts.
--
Alan Modra
Australia Development Lab, IBM
