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Re: [RFC][PATCH][X86_64] Eliminate PLT stubs for specified external functions via -fno-plt=


Sriraman Tallam <tmsriram@google.com> writes:
>
> 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.

This means you need to make it depend on -fno-semantic-interposition ?

> Given this, I propose adding a new option called
> -fno-plt=<function-name> to the compiler.  This tells the compiler
> that we know that the function is truly extern and we want the
> indirect call only for these call-sites.  I have attached a patch that
> adds -fno-plt= to GCC.  Any number of "-fno-plt=" can be specified and
> all call-sites corresponding to these named functions will be done
> indirectly using the mechanism described above without the use of a
> PLT stub.

The argument seems awkward. The command line may get very long.
Better an attribute?

Longer term it would be probably better to support it properly
in the linker.

-Andi

-- 
ak@linux.intel.com -- Speaking for myself only


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