[PATCH, ARM] stop changing signedness in PROMOTE_MODE

[Christophe Lyon]

On 17 February 2016 at 11:05, Kyrill Tkachov
<kyrylo.tkachov@foss.arm.com> wrote:
>
> On 17/02/16 10:03, Christophe Lyon wrote:
>>
>> On 15 February 2016 at 12:32, Kyrill Tkachov
>> <kyrylo.tkachov@foss.arm.com> wrote:
>>>
>>> On 04/02/16 08:58, Ramana Radhakrishnan wrote:
>>>>
>>>> On Tue, Jun 30, 2015 at 2:15 AM, Jim Wilson <jim.wilson@linaro.org>
>>>> wrote:
>>>>>
>>>>> This is my suggested fix for PR 65932, which is a linux kernel
>>>>> miscompile with gcc-5.1.
>>>>>
>>>>> The problem here is caused by a chain of events.  The first is that
>>>>> the relatively new eipa_sra pass creates fake parameters that behave
>>>>> slightly differently than normal parameters.  The second is that the
>>>>> optimizer creates phi nodes that copy local variables to fake
>>>>> parameters and/or vice versa.  The third is that the ouf-of-ssa pass
>>>>> assumes that it can emit simple move instructions for these phi nodes.
>>>>> And the fourth is that the ARM port has a PROMOTE_MODE macro that
>>>>> forces QImode and HImode to unsigned, but a
>>>>> TARGET_PROMOTE_FUNCTION_MODE hook that does not.  So signed char and
>>>>> short parameters have different in register representations than local
>>>>> variables, and require a conversion when copying between them, a
>>>>> conversion that the out-of-ssa pass can't easily emit.
>>>>>
>>>>> Ultimately, I think this is a problem in the arm backend.  It should
>>>>> not have a PROMOTE_MODE macro that is changing the sign of char and
>>>>> short local variables.  I also think that we should merge the
>>>>> PROMOTE_MODE macro with the TARGET_PROMOTE_FUNCTION_MODE hook to
>>>>> prevent this from happening again.
>>>>>
>>>>> I see four general problems with the current ARM PROMOTE_MODE
>>>>> definition.
>>>>> 1) Unsigned char is only faster for armv5 and earlier, before the sxtb
>>>>> instruction was added.  It is a lose for armv6 and later.
>>>>> 2) Unsigned short was only faster for targets that don't support
>>>>> unaligned accesses.  Support for these targets was removed a while
>>>>> ago, and this PROMODE_MODE hunk should have been removed at the same
>>>>> time.  It was accidentally left behind.
>>>>> 3) TARGET_PROMOTE_FUNCTION_MODE used to be a boolean hook, when it was
>>>>> converted to a function, the PROMOTE_MODE code was copied without the
>>>>> UNSIGNEDP changes.  Thus it is only an accident that
>>>>> TARGET_PROMOTE_FUNCTION_MODE and PROMOTE_MODE disagree.  Changing
>>>>> TARGET_PROMOTE_FUNCTION_MODE is an ABI change, so only PROMOTE_MODE
>>>>> changes to resolve the difference are safe.
>>>>> 4) There is a general principle that you should only change signedness
>>>>> in PROMOTE_MODE if the hardware forces it, as otherwise this results
>>>>> in extra conversion instructions that make code slower.  The mips64
>>>>> hardware for instance requires that 32-bit values be sign-extended
>>>>> regardless of type, and instructions may trap if this is not true.
>>>>> However, it has a set of 32-bit instructions that operate on these
>>>>> values, and hence no conversions are required.  There is no similar
>>>>> case on ARM. Thus the conversions are unnecessary and unwise.  This
>>>>> can be seen in the testcases where gcc emits both a zero-extend and a
>>>>> sign-extend inside a loop, as the sign-extend is required for a
>>>>> compare, and the zero-extend is required by PROMOTE_MODE.
>>>>
>>>> Given Kyrill's testing with the patch and the reasonably detailed
>>>> check of the effects of code generation changes - The arm.h hunk is ok
>>>> - I do think we should make this explicit in the documentation that
>>>> TARGET_PROMOTE_MODE and TARGET_PROMOTE_FUNCTION_MODE should agree and
>>>> better still maybe put in a checking assert for the same in the
>>>> mid-end but that could be the subject of a follow-up patch.
>>>>
>>>> Ok to apply just the arm.h hunk as I think Kyrill has taken care of
>>>> the testsuite fallout separately.
>>>
>>> Hi all,
>>>
>>> I'd like to backport the arm.h from this ( r233130) to the GCC 5
>>> branch. As the CSE patch from my series had some fallout on x86_64
>>> due to a deficiency in the AVX patterns that is too invasive to fix
>>> at this stage (and presumably backport), I'd like to just backport
>>> this arm.h fix and adjust the tests to XFAIL the fallout that comes
>>> with not applying the CSE patch. The attached patch does that.
>>>
>>> The code quality fallout on code outside the testsuite is not
>>> that gread. The SPEC benchmarks are not affected by not applying
>>> the CSE change, and only a single sequence in a popular embedded
>>> benchmark
>>> shows some degradation for -mtune=cortex-a9 in the same way as the
>>> wmul-1.c and wmul-2.c tests.
>>>
>>> I think that's a fair tradeoff for fixing the wrong code bug on that
>>> branch.
>>>
>>> Ok to backport r233130 and the attached testsuite patch to the GCC 5
>>> branch?
>>>
>>> Thanks,
>>> Kyrill
>>>
>>> 2016-02-15  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>
>>>
>>>      PR target/65932
>>>      * gcc.target/arm/wmul-1.c: Add -mtune=cortex-a9 to dg-options.
>>>      xfail the scan-assembler test.
>>>      * gcc.target/arm/wmul-2.c: Likewise.
>>>      * gcc.target/arm/wmul-3.c: Simplify test to generate a single
>>> smulbb.
>>>
>>>
>> Hi Kyrill,
>>
>> I've noticed that wmul-3 still fails on the gcc-5 branch when forcing GCC
>> configuration to:
>> --with-cpu cortex-a5 --with-fpu vfpv3-d16-fp16
>> (target arm-none-linux-gnueabihf)
>>
>> The generated code is:
>>          sxth    r0, r0
>>          sxth    r1, r1
>>          mul     r0, r1, r0
>> instead of
>>          smulbb  r0, r1, r0
>> on trunk.
>>
>> I guess we don't worry?
>
>
> Hi Christophe,
> Hmmm, I suspect we might want to backport
> https://gcc.gnu.org/ml/gcc-patches/2016-01/msg01714.html
> to fix backend the costing logic of smulbb.
> Could you please try that patch to see if it helps?
>

Ha indeed, with the attached patch, we now generate smulbb.
I didn't run a full make check though.

OK with a suitable ChangeLog entry?

Christophe.

> Thanks,
> Kyrill
>
>
>>>
>>>> regards
>>>> Ramana
>>>>
>>>>
>>>>
>>>>
>>>>> My change was tested with an arm bootstrap, make check, and SPEC
>>>>> CPU2000 run.  The original poster verified that this gives a linux
>>>>> kernel that boots correctly.
>>>>>
>>>>> The PRMOTE_MODE change causes 3 testsuite testcases to fail.  These
>>>>> are tests to verify that smulbb and/or smlabb are generated.
>>>>> Eliminating the unnecessary sign conversions causes us to get better
>>>>> code that doesn't include the smulbb and smlabb instructions.  I had
>>>>> to modify the testcases to get them to emit the desired instructions.
>>>>> With the testcase changes there are no additional testsuite failures,
>>>>> though I'm concerned that these testcases with the changes may be
>>>>> fragile, and future changes may break them again.
>>>>
>>>>
>>>>
>>>>> If there are ARM parts where smulbb/smlabb are faster than mul/mla,
>>>>> then maybe we should try to add new patterns to get the instructions
>>>>> emitted again for the unmodified testcases.
>>>>>
>>>>> Jim
>>>
>>>
>
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