[PATCH][MIPS] Add linker emulation for N64 ABI with forced 32-bit symbols

David Daney ddaney.cavm@gmail.com
Thu Nov 15 22:19:00 GMT 2012


On 11/15/2012 02:08 PM, Richard Sandiford wrote:
> Maxim Kuvyrkov <maxim@codesourcery.com> writes:
>> This patch fixes linking of applications compiled with -mabi=64 -msym32.
>>
>> The -msym32 GCC options forces MIPS backend to emit 32-bit symbols even
>> when compiling for 64-bit ABI, which is most useful for building
>> applications with PLTs to call shared library functions (PLTs are
>> currently supported only for 32-bit symbols for MIPS).  Certain
>> applications get a significant performance edge from using PLTs, and
>> such applications should be compiled with "-mabi=64 -mplt -msym32".
>>
>> Compiling with the above options works just fine until linker tries to
>> put the .text section above 32-bit address space at
>> TEXT_START_ADDR="0x120000000".  This patch sets TEXT_START_ADDR to
>> 0x10000000 (which is same as for N32 ABI) when compiling with -msym32
>> GCC option.
>>
>> I will post the corresponding GCC patch that tells the linker to use new
>> emulations shortly.
>>
>> Tested by building complete MIPS64 toolchain (including glibc) with -msym32.
>>
>> OK to apply?
>
> Huh.  I'm very much against the original change to the n64 TEXT_START_ADDR.
> See this previous discussion on the topic:
>
>      http://sourceware.org/ml/binutils/2008-06/msg00285.html
>
> The justification for changing TEXT_START_ADDR from a 32-bit value to a
> larger value was that it would show users if their code was non-portable,
> because anything that assumed 32-bit addresses would now fault.  But that
> seems to me like the tools lecturing to the user.  As I said in that thread,
> I think users who want to smoke out such portability problems (by making
> sure that the lower 4GB aren't mapped) should be the ones who need to do
> something special.
>
> I think we should simply revert to a 32-bit TEXT_START_ADDR for all
> n64 emulations.  This time I'm even in a position to approve it :-)
>

How about if we move it really low instead of just back to 0x10000000?

At 0x10000000 we are wasting 1/8 of the available -msym32 address space.

David Daney


> That has the advantage that no GCC patches are needed.  Users who
> want this functionality can just upgrade their binutils.
>
> Richard
>
>



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