[LLVMdev] Memset/memcpy: user control of loop-idiom recognizer

[ehostunreach]

On Fri, Dec 5, 2014 at 6:08 PM, Philip Reames <listmail at philipreames.com> wrote:
>
> On 12/04/2014 11:46 PM, David Chisnall wrote:
>>
>> On 3 Dec 2014, at 23:36, Robert Lougher <rob.lougher at gmail.com> wrote:
>>
>>> On 2 December 2014 at 22:18, Alex Rosenberg <alexr at leftfield.org> wrote:
>>>>
>>>> Our C library amplifies this problem by being in a dynamic library, so
>>>> the
>>>> call has additional overhead, which for small trip counts swamps the
>>>> copy/set.
>>>>
>>> I can't imagine we're the only platform (now or in the future) that
>>> has comparatively slow library calls.  We had discussed some sort of
>>> platform flag (has slow library calls) but this would be too late to
>>> affect the loop-idiom.  However, it could affect lowering.  Following
>>> on from Reid's earlier idea to lower short memcpys to an inlined,
>>> slightly widened loop, we could expand into a guarded loop for small
>>> values and a call?
>>
>> I think the bug is not that we are recognising that the loop is memcpy,
>> it's that we're then generating an inefficient memcpy.  We do this for a
>> variety of reasons, some of which apply elsewhere.  One issue I hit a few
>> months ago was that the vectoriser doesn't notice whether unaligned loads
>> and stores are supported, so will happily replace two adjacent i32 align 4
>> loads followed by two adjacent i64 align 4 stores with an i64 align 4 load
>> followed by an i64 align 4 store, which more than doubles the number of
>> instructions that the back end emits.
>>
>> We expand memcpy and friends in several different places (in the IR in at
>> least one place, then in SelectionDAG, and then again in the back end, as I
>> recall - I remember playing whack-a-bug with this for a while as the
>> lowering was differently broken for our target in each place).  In
>> SelectionDAG, we're dealing with a single basic block, so we can't construct
>> the loop.  In the back end we've already lost a lot of high-level type
>> information that would make this easier.
>>
>> I'd be in favour of consolidating the memcpy / memset / memmove expansion
>> into an IR pass that would take a cost model from the target.
>
> +1
>
> It sounds like we might also be loosing information about alignment in the
> loop-idiom recognizer.  Or at least not using it when we lower.
>
>

The LoadCombine pass suffers from the same problem too. It's producing unaligned
loads and that's why it's not enabled by default.

I'm currently working on that problem (trying to combine stores too)
and I suppose
that once it's solved we will be able to draw important conclusions about how to
make _alignment-aware_ the rest of the LLVM places mentioned in this thread.

I don't know a lot about the vectorizer yet, but I don't think that
discovering the
load/store alignment of adjacent loads/stores should be one of its main tasks.
By enabling a properly working LoadStoreCombine pass we could probably eliminate
altogether the alignment problem in the vectorizer.

Regards,
Vasileios Kalintiris