GNU build sets GP to _f_sdata instead of __SDATA_BEGIN__

[iguryanov]

Issue Summary

• Type: Bug
• Category: Library
• Priority: Low
• Release Version: 2019.06

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Bug

Development Environment

• HOST OS
  All OS

• TOOLCHAIN
  ARC GNU 2020.03=9.

• BOARD
  Customer's board, irrelevant.

• ARC CORE
  EM9D.

Bug Description

gcc builds use linker scripts with both _f_sdata and __SDATA_BEGIN__.
Normally they are the same in our scripts.
Native GNU toolchain startup uses __SDATA_BEGIN__ to configure %GP register.
But embARC startup sticks to MWDT agreement and uses _f_sdata.

This works ok until user decides to put __SDATA_BEGIN__ in the middle of the section to extend it's range.
(GP-based offset is signed). And linker uses __SDATA_BEGIN__ to calculate relocations and actual offsets.

So when user changes related line in the script to something like this - __SDATA_BEGIN__ = . + 0x100;
small data access becomes wrong. Runtime offsets will be 0x100 less (easily driving into negative zone),
while startup still sets %GP to the beginning of small data section.

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Question

Is it possible to change arc_startup.s in a way that for GCC builds it will choose __SDATA_BEGIN__ for GP?