[cfe-dev] Clang generates calls to llvm.memcpy with overlapping arguments, but LangRef requires the arguments to not overlap

Florian Hahn via cfe-dev cfe-dev at lists.llvm.org
Wed Aug 26 08:40:00 PDT 2020

> On Aug 25, 2020, at 21:16, Richard Smith <richard at metafoo.co.uk> wrote:
> To be clear: the concern here is that the source and destination could be equal, not that they might partially overlap (which I'm pretty sure the relevant language rules disallow)?

Yes after going through your response and the discussion at  https://bugs.llvm.org/show_bug.cgi?id=11763 <https://bugs.llvm.org/show_bug.cgi?id=11763> it seems like the only issue is that source & destination could be equal. 

> On Aug 25, 2020, at 22:45, David Rector <davrecthreads at gmail.com> wrote:
> This was also discussed in https://bugs.llvm.org/show_bug.cgi?id=11763 <https://bugs.llvm.org/show_bug.cgi?id=11763>, I’ll copy and paste James’ comment at the end, which seems reasonable and also confirms the partial-overlap case is not an issue:
> ***
> James Y Knight 2018-07-19 11:19:11 PDT
> In light of the last updates, it sounds like what's required here is just a documentation update.
> 1. LLVM requires that the platform provide a memcpy function which works when the two pointers are equal. That is not a requirement of the C standard for the user-visible memcpy function, but it is a requirement that LLVM has on platforms it compiles code for.
> If there are any platform which provides a memcpy which does not satisfy that requirement, I believe that platform needs to be modified, not LLVM. I'd be surprised if there were any such platforms, since GCC has the same requirement.
> 2. Likewise, the same requirement exists for the llvm.memcpy IR intrinsic. The LangRef should be updated to state that.
> 3. Partially-overlapping calls from struct assignment are UB, so we're fine there, such issues correspond to actual bugs in user-code.
> ***
> Unless anyone disagrees with that approach, or anything has changed since, Florian perhaps you could update the relevant documentation via the noalias patch I believe you were working on, since this is in that same family of issues?

Thanks for pointing out the existing, longstanding issue! I am not sure if the change is as simple as updating the LangRef for `llvm.memcpy` I am afraid, as we would have to also track down all the places in the LLVM codebase that use the assumption. But there are likely not too many (mostly alias analysis).

> On Aug 25, 2020, at 23:06, John McCall <rjmccall at apple.com> wrote:
> C allows overlapping assignments only when the overlap is exact, i.e. the addresses are exactly the same. I agree that just emitting memcpy isn’t strictly legal, because neither C nor LLVM allows even exact overlap. On the other hand, Clang would really like to avoid emitting extra control flow here, because exact overlap is uncommon enough that emitting a no-op memcpy (if it were indeed a no-op) is definitely the right trade-off in practice; and LLVM probably won’t reliably remove branches around a memcpy call if we add them. And it’s very hard to imagine a memcpy implementation that actually wouldn’t work with exact overlap (maybe something that tried to just remap pages?).
> I think we have are four options:
> We can relax llvm.memcpy to allow exact overlap. Practically, this would depend on memcpy being a no-op on exact overlap; we definitely wouldn’t want LLVM to have to start inserting control flow around memcpy calls.
> We can make Clang emit assignments with llvm.memmove. This would make assignment work even with non-exact overlap, which is unnecessary; I’m not sure the cost is that high.
> We can make Clang emit an explicit check and control flow around memcpy when there might be overlap. Clang IRGen should already maintain the right information to avoid doing this when e.g. initializing a variable.
> We can add a new intrinsic — or some sort of decoration of the existing one — that does a memcpy but allows exact overlap.

Thanks for summarizing the options! I think 4. would probably the safest option, because we won’t have to track down the places that use the non-overlapping assumption. There might also be such cases somewhere downstream, which might be broken silently. If Clang already has the information when the operands cannot overlap (as mentioned in 3.), we only have to emit the non-overlapping version for those cases, causing the least amount of change for the generated IR.

1. Also seems possible (there probably are not too many places in LLVM that use the non-overlapping assumption), but I think in some cases the non-overlapping info can be a useful property to convey for frontends.


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