[PING] [PATCH, ARM] Reduce stack usage in sha512 (PR target/77308)

[Bernd Edlinger <bernd dot edlinger at hotmail dot de>]

Hi,

I'd like to ping for this patch:
https://gcc.gnu.org/ml/gcc-patches/2016-09/msg02362.html

Thanks,
Bernd.


On 09/30/16 15:33, Bernd Edlinger wrote:
> On 09/30/16 12:14, Bernd Edlinger wrote:
>> Eric Botcazou wrote:
>>>> A comment before the SETs and a testcase would be nice.  IIRC
>>>> we do have stack size testcases via using -fstack-usage.
>>>
>>> Or -Wstack-usage, which might be more appropriate here.
>>
>> Yes.  good idea.  I was not aware that we already have that kind of
>> tests.
>>
>> When trying to write this test. I noticed, that I did not try -Os so far.
>> But for -Os the stack is still the unchanged 3500 bytes.
>>
>> However for embedded targets I am often inclined to use -Os, and
>> would certainly not expect the stack to explode...
>>
>> I see in arm.md there are places like
>>
>>        /* If we're optimizing for size, we prefer the libgcc calls.  */
>>        if (optimize_function_for_size_p (cfun))
>>          FAIL;
>>
>
> Oh, yeah.  The comment is completely misleading.
>
> If this pattern fails, expmed.c simply expands some
> less efficient rtl, which also results in two shifts
> and one or-op.  No libgcc calls at all.
>
> So in simple cases without spilling the resulting
> assembler is the same, regardless if this pattern
> fails or not.  But the half-defined out registers
> make a big difference when it has to be spilled.
>
>>        /* Expand operation using core-registers.
>>           'FAIL' would achieve the same thing, but this is a bit
>> smarter.  */
>>        scratch1 = gen_reg_rtx (SImode);
>>        scratch2 = gen_reg_rtx (SImode);
>>        arm_emit_coreregs_64bit_shift (LSHIFTRT, operands[0], operands[1],
>>                                       operands[2], scratch1, scratch2);
>>
>>
>> .. that explains why this happens.  I think it would be better to
>> use the emit_coreregs for shift count >= 32, because these are
>> effectively 32-bit shifts.
>>
>> Will try if that can be improved, and come back with the
>> results.
>>
>
> The test case with -Os has 3520 bytes stack usage.
> When only shift count >= 32 are handled we
> have still 3000 bytes stack usage.
> And when arm_emit_coreregs_64bit_shift is always
> allowed to run, we have 2360 bytes stack usage.
>
> Also for the code size it is better not to fail this
> pattern.  So I propose to remove this exception in all
> three expansions.
>
> Here is an improved patch with the test case from the PR.
> And a comment on the redundant SET why it is better to clear
> the out register first.
>
>
> Bootstrap and reg-testing on arm-linux-gnueabihf.
> Is it OK for trunk?
>
>
> Thanks
> Bernd.