[RFA/ARM][Patch 00/05]: Introduction - Generate LDRD/STRD in prologue/epilogue instead of PUSH/POP.

[Sameera Deshpande <sameera dot deshpande at arm dot com>]

This series of 5 patches generate LDRD/STRD instead of POP/PUSH in
epilogue/prologue for ARM and Thumb-2 mode of A15.

Patch [1/5] introduces new field in tune which can be used to indicate
whether LDRD/STRD are preferred over POP/PUSH by the specific core.

Patches [2-5/5] use this field to determine if LDRD/STRD can be
generated instead of PUSH/POP in ARM and Thumb-2 mode.

Patch [2/5] generates LDRD instead of POP for Thumb-2 epilogue in A15.
This patch depends on patch [1/5].

Patch [3/5] generates STRD instead of PUSH for Thumb-2 prologue in A15.
This patch depends for variables, functions and patterns defined in
[1/5] and [2/5].

Patch [4/5] generates STRD instead of PUSH for ARM prologue in A15. This
patch depends on [1/5].

Patch [5/5] generates LDRD instead of POP for ARM epilogue in A15. This
patch depends for variables, functions and patterns defined in [1/5] and
[4/5].

All these patches depend upon the Thumb2/ARM RTL epilogue patches
http://gcc.gnu.org/ml/gcc-patches/2011-09/msg01854.html,
http://gcc.gnu.org/ml/gcc-patches/2011-09/msg01855.html submitted for
review.

All these patches are applied in given order and tested with check-gcc,
check-gdb and bootstrap without regression. 

In case of ARM mode, significant performance improvement can be seen on
some parts of a popular embedded consumer benchmark (~26%). 
However, in most of the cases, not much effect is seen on performance.
(~ 3% improvement) 

In case of thumb2, the performance improvement observed on same parts
the benchmark is ~11% (2.5% improvement). 

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