[PATCH, ARM] Suppress Redundant Flag Setting for Cortex-A15

[Christophe Lyon]

Hi,

On 28 January 2014 13:10, Ramana Radhakrishnan
<ramana.gcc@googlemail.com> wrote:
> On Fri, Jan 24, 2014 at 5:16 PM, Ian Bolton <ian.bolton@arm.com> wrote:
>> Hi there!
>>
>> An existing optimisation for Thumb-2 converts t32 encodings to
>> t16 encodings to reduce codesize, at the expense of causing
>> redundant flag setting for ADD, AND, etc.  This redundant flag
>> setting can have negative performance impact on cortex-a15.
>>
>> This patch introduces two new tuning options so that the conversion
>> from t32 to t16, which takes place in thumb2_reorg, can be suppressed
>> for cortex-a15.
>>
>> To maintain some of the original benefit (reduced codesize), the
>> suppression is only done where the enclosing basic block is deemed
>> worthy of optimising for speed.
>>
>> This tested with no regressions and performance has improved for
>> the workloads tested on cortex-a15.  (It might be beneficial to
>> other processors too, but that has not been investigated yet.)
>>
>> OK for stage 1?
>
> This is OK for stage1.
>
> Ramana
>
>>
>> Cheers,
>> Ian
>>
>>
>> 2014-01-24  Ian Bolton  <ian.bolton@arm.com>
>>
>> gcc/
>>         * config/arm/arm-protos.h (tune_params): New struct members.
>>         * config/arm/arm.c: Initialise tune_params per processor.
>>         (thumb2_reorg): Suppress conversion from t32 to t16 when
>>         optimizing for speed, based on new tune_params.

This causes
gcc.target/arm/negdi-1.c
gcc.target/arm/negdi-2.c
to FAIL when GCC is configured as:
--with-mode=ar
--with-cpu=cortex-a15
--with-fpu=neon-vfpv4

both tests used to PASS.
(see http://cbuild.validation.linaro.org/build/cross-validation/gcc/209561/report-build-info.html)

Christophe.