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

[Smith, Kevin B]

There are a large number of ways to lose information in translating loops into memset/memcpy calls, alignment is one of them.
As previously mentioned, loop-trip-count is another.  Another is size of accesses.  For example, the loop may have originally been using
int64_t sized copies.  This has definite impact on what the best memset/memcpy expansion is, because effectively, the loop knows that
it is always writing a multiple of 8 bytes, and does so in 8 bytes chunks.  So, that the number of bytes has some specific value property (like the lower 3 bits
are always 0, another reason for having known bits and known bit values :-)) all (should) affect the lowering of such loops/calls, but probably doesn't.

Database folks often write their own copy routines for use in specific instances, as do OSes, such as when they know they are clearing or copying exact
page size on exact page-size boundaries, and have very special implementations of these, including some that will use non-temporal hints, so as not to
pollute cache.

It is also worth pointing out that most loops have a very specific behavior in the case of overlaps that is well-defined, and that memcpy does not.

There are definitely good reasons why various knowledgeable users would not want a compiler to perform such a transform on at least some of their loops.

Kevin Smith 

-----Original Message-----
From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On Behalf Of Philip Reames
Sent: Friday, December 05, 2014 10:08 AM
To: David Chisnall; Robert Lougher
Cc: LLVM Developers Mailing List
Subject: Re: [LLVMdev] Memset/memcpy: user control of loop-idiom recognizer


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.
>
> David
>
>
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