[PATCH,ARM] Peephole individual LDR/STD into LDRD/STRD

[Greta Yorsh]

This patch defines peephole2 patterns that merge two individual LDR
instructions into LDRD instruction (resp. STR into STRD) whenever possible
using the following transformations:
* reorder two memory accesses,
* rename registers when storing two constants, and
* reorder target registers of a load when they are used by a commutative
operation.

In ARM mode only, the pair of registers IP and SP is allowed as operands in
LDRD/STRD. To handle it, this patch defines a new constraint "q" to be
CORE_REGS in ARM mode and GENERAL_REGS (i.e., equivalent to "r") otherwise.
Note that in ARM mode "q" is not equivalent to "rk" because of the way
constraints are matched. The new constraint "q" is used in place of "r" for
DImode move between register and memory.

This is a new version of the patch posted for review a long time ago:
http://gcc.gnu.org/ml/gcc-patches/2011-11/msg00914.html
All the dependencies mentioned in the previous patch have already been
upstreamed.
Compared to the previous version, the new patch
* handles both ARM and Thumb modes in the same peephole pattern,
* does not attempt to generate LDRD/STRD when optimizing for size and non of
the LDM/STM patterns match (but it would be easy to add),
* does not include scan-assembly tests specific for cortex-a15 and
cortex-a9, because they are not stable and highly sensitive to other
optimizations.

No regression on qemu for arm-none-eabi with cpu cortex-a15.

Bootstrap successful on Cortex-A15 TC2.

Spec2k results:
Performance: slight improvement in overall scores (less than 1%) in both
CINT2000 and CFP2000. 
For individual benchmarks, there is a slight variation in performance,
within less than 1%, which I consider to be just noise.
Object size: there is a slight reduction in size in all the benchmarks -
overall 0.2% and at most 0.5% for individual benchmarks.
Baseline compiler is gcc r194473 from December 2012.
Compiled in thumb mode with hardfp.
Run on Cortex-A15 hardware.

Ok for gcc4.9 stage 1?

Thanks,
Greta

gcc/

2013-02-13  Greta Yorsh  <Greta.Yorsh@arm.com>

        * config/arm/constraints.md (q): New constraint.
        * config/arm/ldrdstrd.md: New file.
        * config/arm/arm.md (ldrdstrd.md) New include.
        (arm_movdi): Use "q" instead of "r" constraint
        for double-word memory access.
        (movdf_soft_insn): Likewise.
        * config/arm/vfp.md (movdi_vfp): Likewise.
        * config/arm/t-arm (MD_INCLUDES): Add ldrdstrd.md.
        * config/arm/arm-protos.h (gen_operands_ldrd_strd): New declaration.
        * config/arm/arm.c (gen_operands_ldrd_strd): New function.
        (mem_ok_for_ldrd_strd): Likewise.
        (output_move_double): Update assertion.

gcc/testsuite

2013-02-13  Greta Yorsh  <Greta.Yorsh@arm.com>

        * gcc.target/arm/peep-ldrd-1.c: New test.
        * gcc.target/arm/peep-strd-1.c: Likewise.
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