[llvm-dev] RFC: Inline expansion of memcmp vs call to standard library

[Philip Reames via llvm-dev]

I'd definitely support having a memcmp intrinsic for the reasons 
previously specified.  However, this is somewhat orthogonal from the 
original direction of the patch.  We can easily improve the lowering of 
the existing target function and then introduce the intrinsic.  Porting 
the existing lowering code over should be straight forward.  I'm only 
point this out so that we don't get blocked on the eventual end goal and 
fail to make progress.

Philip


On 12/30/2016 02:27 AM, Martin J. O'Riordan wrote:
>
> With the intrinsic support for ‘memcpy’ and ‘memset’ the operands also 
> have associated alignment operands.  I think that ‘memcmp’ should also 
> provide the alignment information for each of the source operands 
> (when statically known).  In some cases this will lead to more optimal 
> alignment aware lowering, and for targets for which unaligned access 
> is costly or fatal, it can be lowered safely.
>
> MartinO
>
> *From:*llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] *On Behalf Of 
> *David Jones via llvm-dev
> *Sent:* 30 December 2016 00:28
> *To:* Philip Reames <listmail at philipreames.com>
> *Cc:* llvm-dev <llvm-dev at lists.llvm.org>; Zaara Syeda <syzaara at ca.ibm.com>
> *Subject:* Re: [llvm-dev] RFC: Inline expansion of memcmp vs call to 
> standard library
>
> Can I make another suggestion: create an intrinsic for memory 
> equality, e.g. llvm.memcmp_eq.p0i8.p0i8.i64(i8*a, i8*b, i64 len).  
> This intrinsic would return zero if the memory regions are equal, and 
> nonzero otherwise. However, it does NOT return any notion of "greater" 
> or "less".
>
> Many applications require only determining equality, rather than a 
> total ordering. Given that "greater" and "less" also require some 
> knowledge of endianness, even a fancy lowered version of memcmp can be 
> slower than an equality-only compare.
>
> On Thu, Dec 29, 2016 at 4:14 PM, Philip Reames via llvm-dev 
> <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>
>     Improving lowering for memcmp is definitely something we should do
>     for all targets. Doing it in a target specific way is decidedly
>     non-ideal.
>
>     It looks like we already have some code in SelectionDAGBuilder
>     which tries to optimize the lowering for the memcpy library call. 
>     I am a bit confused by the problem you are trying to solve. Are
>     you specifically interested in lowering for constant lengths
>     greater than a legal size?  (i.e. do you need the loop?)
>
>     If so, there are two approaches you might consider:
>     - Expand the memcmp call into the loop form in CodeGenPrep (or a
>     similar timed pass) where working with multiple basic blocks is
>     much easier.  Long term, the "right place" for this type of thing
>     is clearly GlobalISEL, but we have a number of other such hacks in
>     lowering today and continuing to build off of that seems reasonable.
>     - Emit the non-early exit form for small constant values (a[0] ==
>     b[0] && a[1] == b[1] ...). Assuming your backend has handling for
>     efficiently lowering and chains using branches, you may very well
>     get the code you want.
>
>     Using the psuedo instruction here feels messy.  In particular, I
>     don't like the fact it basically opts out of all of the combines
>     which might further improve the lowering.
>
>     Philip
>
>
>     On 12/29/2016 11:35 AM, Zaara Syeda via llvm-dev wrote:
>
>         Currently on PowerPC, calls to memcmp are not expanded and are
>         left as library calls. In certain conditions, expansion can
>         improve performance rather than calling the library function
>         as done for functions like memcpy, memmove, etc. This patch
>         *(**https://reviews.llvm.org/D28163**)*is an initial
>         implementation for PowerPC to expand memcmp when the size is
>         an 8 byte multiple.
>
>         The approach currently added for this expansion tries to use
>         the existing infrastructure by overriding the virtual function
>         EmitTargetCodeForMemcmp. This function works on the
>         SelectionDAG, but the expansion requires control flow for
>         early exit. So, instead of implementing the expansion within
>         EmitTargetCodeForMemcmp, a new pseudo instruction is added for
>         memcmp and a SelectionDAG node for this new pseudo is created
>         in EmitTargetCodeForMemcmp. This pseudo instruction is then
>         expanded during lowering in EmitInstrWithCustomInserter.
>
>         The advantage of this approach is that it uses the existing
>         infrastructure and does not impact other targets. If other
>         targets would like to expand memcmp, they can also override
>         EmitTargetCodeForMemcmp and create their own expansion.
>
>         Another option to consider is adding a new optimization pass
>         for this expansion that isn’t target specific if other targets
>         would benefit from a more general infrastructure.
>
>         Please provide feedback if this approach should be continued
>         to implement the PowerPC specific memcmp expansions or whether
>         the community is interested in devising a more general approach.
>
>         Thanks,
>
>         Zaara Syeda
>
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