[llvm-dev] Infinite loops with no side effects

[Reid Kleckner via llvm-dev]

Personally, I don't like the side effect intrinsic. It will pollute all the
IR generated by non-C frontends. What most of these frontends really want
is just a switch to disable a targeted set of optimizations.

One thing I like about the function attribute idea is that it's
conservatively correct to discard it when doing cross-language inlining. It
just becomes something that C-family frontends need to remember to add to
enable their special-case language rules, rather than something that non-C
languages need to think about. Similar to the 'access', builtin vs
nonbuiltin discussion happening in parallel, the attribute enables the
optimization, rather than inhibiting it.

On Fri, Oct 27, 2017 at 12:37 PM, Hal Finkel via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

>
> On 10/27/2017 12:08 AM, Dan Gohman via llvm-dev wrote:
>
> Hello,
>
> This email picks up the thread that to my knowledge was last discussed
> here:
>
> http://lists.llvm.org/pipermail/llvm-dev/2015-July/088103.html
>
> In brief, infinite loops containing no side effects produce undefined
> behavior in C++ (and C in some cases), however in other languages, they
> have fully defined behavior. LLVM's optimizer currently assumes that
> infinite loops eventually terminate in a few places, and will sometimes
> delete them in practice. There is currently no clean way to opt out of this
> behavior from languages where it's not valid.
>
> This is the subject of a long-standing LLVM bug:
>
> https://bugs.llvm.org/show_bug.cgi?id=965
>
> I wrote a patch implementing Chandler's idea from the above thread,
> @llvm.sideeffect, a new intrinsic which is a no-op except that it tells the
> optimizer to behave as if there were side effects present:
>
> https://reviews.llvm.org/D38336
>
> Similar results can be achieved with empty inline asms, however they tend
> to pessimize optimizations. The patch above allows all of the major
> optimizations to work in the presence of @llvm.sideeffect.
>
>
> I think that we should move forward with this approach (as may be obvious
> given that I've okay'd the patch). It's a lightweight solution, at least on
> LLVM's side of things, and does not prevent other solutions later.
>
>
> One of the concerns raised is that front-ends would have to emit a lot of
> these intrinsics, potentially one in every loop, one in every function (due
> to opportunistic tail-call optimization), and one in front of every label
> reachable by goto or similar, if a front-end can't determine when they
> aren't needed.
>
>
> This is a valid concern, however, I expect that most programs from
> higher-level languages will have well-structured loops, and it will be
> straightforward to emit the intrinsics.
>
> This is indeed a downside. It's mitigated in this patch by making sure
> that the major optimization passes aren't pessimized.
>
> From the alternatives I've read, the most promising alternative is Reid's
> proposal here:
>
> https://bugs.llvm.org/show_bug.cgi?id=965#c25
>
> to make infinite loops defined by default, and add a "known to be
> productive" attribute to functions. It would be a more complex change, and
> could potentially require changes in out-of-tree codebases. And it would be
> suboptimal in some cases when cross-language inlining. However, it would
> solve the problem in a much less cluttered way. I'm willing to implement
> the LLVM portion of this if there's consensus that it's a better approach.
>
>
> The problem is that it is not a function-level property, it is a per-loop
> property. This is even true in C. In C, we would need to mark loops that
> have source-level-constant controlling conditions, and only those loops,
> and allowed to be infinite. And, so, maybe we could use loop-level
> metadata, but that seems hard to place/preserve for unstructured loops
> (and, arguably, that's more important in C/C++ than in other languages).
>
>  -Hal
>
>
> Thoughts?
>
> Dan
>
>
>
> _______________________________________________
> LLVM Developers mailing listllvm-dev at lists.llvm.orghttp://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
> --
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
>
>
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>
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