[PATCH, ARM] stop changing signedness in PROMOTE_MODE

Ramana Radhakrishnan ramana.gcc@googlemail.com
Tue Feb 16 10:44:00 GMT 2016

On Mon, Feb 15, 2016 at 11:32 AM, 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
>>> 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
>>> 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
>> 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.

I would hope that someone looks to fix the AVX port for GCC 7  - as
the CSE patch is something that is useful on ARM for performance in
real terms and could have benefits elsewhere.

OK to apply if no regressions - thanks for pushing this through.


> 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.
>> 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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