Re: [PATCH, ARM] stop changing signedness in PROMOTE_MODE

[Kyrill Tkachov <kyrylo.tkachov@foss.arm.com>]

On 17/02/16 10:20, Christophe Lyon wrote:
> 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.

Thanks for checking.

> OK with a suitable ChangeLog entry?

Can you please do a full test run when you get the chance?
Since the patch is mine we'll need an ok from another arm maintainer ;)

Thanks,
Kyrill

>
> 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
>>>>