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Re: [PATCH i386] Allow sibcalls in no-PLT PIC

On Fri, May 15, 2015 at 01:08:15PM -0700, H.J. Lu wrote:
> With relax branch in 32-bit, there are 2 cases:
> 1. PIC or PIE:  We generate
> set up EBX
> relax call foo@PLT
> It is almost the same as we do now, except for the relax prefix.
> If foo is defined in another shared library or may be preempted,
> linker will generate
> call *foo@GOTPLT(%ebx)
> If foo turns out local, linker will output
> relax call foo

This does not address the initial and primary motivation for no-plt on
32-bit: eliminating the awful codegen constraint costs of the
GOT-register (ebx, and equivalent on other targets) ABI for calling
PLT entries. If instead you generated code that sets up an expression
for the GOT slot using arbitrary registers, and relaxed it to a direct
call (possibly rendering the register setup useless), it would be
comparable to the no-plt approach. So for example:

set up ecx (or whatever register)
relax call *foo@GOT(%ecx)

and relax to:

set up ecx (or whatever register; now useless)
relax call foo

But the no-plt approach is still superior in that the address load
from the GOT can be hoisted out of loops, etc., resulting in something

call *%esi

This could be valuable in loops calling a math function repeatedly,
for example.

Overall I'm still not a fan of the relaxation approach. There are very
few places it would actually help that couldn't already be improved
better with use of visibility, and it can't give codegen as good as
no-plt option.


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