[llvm-dev] Change memcpy/memmove/memset to have dest and source alignment attributes

Chris Lattner via llvm-dev llvm-dev at lists.llvm.org
Thu Jan 18 20:48:23 PST 2018

> On Jan 18, 2018, at 7:45 AM, Daniel Neilson via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> Hi all,
>  This change has been reviewed, and appears to be ready to land (review available here if anyone still wants to chime in: https://reviews.llvm.org/D41675 <https://reviews.llvm.org/D41675> ). The process that we’re going to use for landing this will take a few steps. To wit:
> Step 1) Remove align argument, and add align attribute to pointer args.

What does this mean? Are you suggesting that alignment become part of PointerType?  That would introduce lots of problems, because unaligned and aligned pointers would require casts, which then interfere with optimizations.


> Require that src & dest have the same alignment via verifier rule.  Also update Clang & Polly tests to pattern-match the new form.
> Step 2) Expand the IRBuilder API to allow creation of memcpy/memmove with differing source and dest alignments.
> Step 3) Update Clang to use the new IRBuilder API.
> Step 4) Update Polly to use the new IRBuilder API.
> Step 5) Update LLVM passes that create memcpy/memmove calls to use the new IRBuilder API, and those that use use MemIntrinsicInst::[get|set]Alignment() to use  getDestAlignment() and getSourceAlignment() instead.
> Step 6) Remove the single-alignment IRBuilder API for memcpy/memmove, and the MemIntrinsicInst::[get|set]Alignment() methods.
>  Unless there are objections, I would like to land step 1 tomorrow (Friday Jan 19) morning. My hope is to be completing up to step 4 by the middle of next week, and then steps 5 & 6 in the coming weeks.
>  Landing step 1 will potentially have a large impact on downstream consumer’s tests. For example, a front-end that checks IR generation of constructs implemented using memcpy/memmove/memset; the Clang tests for OpenMP firstprivate/lastprivate clauses, for instance, required a lot of manual updating. These sorts of tests will need to be updated to match the new pattern printed for the memory intrinsics. I’ve included an extended sed script in the commit log that can update many patterns, but definitely does not catch them all so some manual updating will be required.
> -Daniel
> ---
> Daniel Neilson, Ph.D.
> Azul Systems
>> On Jan 2, 2018, at 2:11 PM, Daniel Neilson via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>> Good day all,
>>  I’ve spent a few days resurrecting the circa-2015 work on removing the explicit alignment argument (4th arg) from the @llvm.memcpy/memmove/memset intrinsics in favour of using the alignment attribute on the pointer args of calls to the intrinsic. This work was first proposed back in August 2015 by Lang Hames:
>> http://lists.llvm.org/pipermail/llvm-dev/2015-August/089384.html <http://lists.llvm.org/pipermail/llvm-dev/2015-August/089384.html>   (item 2)
>>  and an attempt at landing the work was made by Pete Cooper in November 2015, but then backed out due to unspecified bot failures:
>> http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20151109/312083.html <http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20151109/312083.html>
>>  I’ve prepared changes for LLVM, Clang, and Polly that are now up for review:
>> * https://reviews.llvm.org/D41675 <https://reviews.llvm.org/D41675>   (LLVM part)
>> * https://reviews.llvm.org/D41676 <https://reviews.llvm.org/D41676>   (polly part)
>> * https://reviews.llvm.org/D41677 <https://reviews.llvm.org/D41677>   (Clang part)
>>  Importantly for those maintaining downstream users of the LLVM API, this changes the prototypes for the @llvm.memcpy/memmove/memset intrinsics and changes the IRBuilder API for creating memcpy and memmove calls.
>> For example, IR which used to read:
>>      call void @llvm.memcpy.p0i8.p0i8.i32(i8* %dest, i8* %src, i32 100, i32 4, i1 false)
>> will now read
>>      call void @llvm.memcpy.p0i8.p0i8.i32(i8* align 4 %dest, i8* align 4 %src, i32 100, i1 false)
>>  The LLVM change includes auto upgrade of the old IR. However, match expressions in IR tests and calls to IRBuilder’s CreateMemCpy & CreateMemMove will need to be updated.
>>  My plan is to post another note to the list when the change is landed, and stable.
>>  Comments? Concerns?
>> -Daniel
>> ---
>> Daniel Neilson, Ph.D.
>> Azul Systems
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