[LLVMdev] Proposal: add intrinsics for safe division

[Filip Pizlo]

On April 25, 2014 at 9:52:35 AM, Eric Christopher (echristo at gmail.com) wrote:

Hi Michael,

> I’d like to propose to extend LLVM IR intrinsics set, adding new ones for
> safe-division. There are intrinsics for detecting overflow errors, like
> sadd.with.overflow, and the intrinsics I’m proposing will augment this set.
>
> The new intrinsics will return a structure with two elements according to
> the following rules:
>
> safe.[us]div(x,0) = safe.[us]rem(x,0) = {0, 1}
> safe.sdiv(min<T>, -1) = safe.srem(min<T>, -1) = {min<T>, 1}
> In other cases: safe.op(x,y) = {x op y, 0}, where op is sdiv, udiv, srem, or
> urem
>
>
> The use of these intrinsics would be quite the same as it was for
> arith.with.overflow intrinsics. For instance:
> %res = call {i32, i1} @llvm.safe.sdiv.i32(i32 %a, i32 %b)
> %div = extractvalue {i32, i1} %res, 0
> %bit = extractvalue {i32, i1} %res, 1
> br i1 %bit, label %trap, label %normal
>

I notice that the patch is still in ToT even though we're now having a
discussion on whether it should go in. It should be reverted.

That said, I don't see how this is anything except for an optimization
intrinsic and not necessary for correct behavior for any language.
I.e. You can do what the PNaCl people are doing and emit branches
instead. Since this will only happen on a single target and not on all
targets why not just make this a target intrinisic? I don't buy the
argument that it biases the target independent IR since any pass that
uses TTI in the IR level does the same.
The sdiv operation in LLVM IR only makes sense for C and its very direct relatives.  The amount of control flow necessary to represent a safe division for any other language is ghastly.  (a/b) becomes something like (b != 0 ? ((a != INT_MIN || b != -1) ? a / b : INT_MIN) : 0).  It's more useful to the optimizer to see this as the single operation that it really is.

Also, this isn't about just a single target.  Any target that has to emulate integer division (like a lot of embedded systems would do) would benefit from this since the division emulation can most naturally be written to provide the "safe" semantics rather than LLVM's default semantics (i.e. undefined behavior on x/0 and INT_MIN/-1).

I think adding these intrinics to the IR would be a great help for non-C clients of LLVM.

-Filip




-eric


> Now a few words about their implementation in LLVM. Though the new
> intrinsics look quite similar to the ones with overflow, there are
> significant differences. One of them is that during lowering we need to
> create control-flow for the new ones, while for the existing ones it was
> sufficient to simply compute the overflow flag. The control flow is needed
> to guard the division operation, which otherwise can cause an undefined
> behaviour.
>
> The existing intrinsics are lowered in a back-end, during legalization
> steps. To do the same for the new ones, we’d need a more complicated
> implementation because of the need to create a new control flow. Also, that
> would be needed to be done in every backend.
>
> Another alternative here is to lower the new intrinsics in CodeGenPrepare
> pass. That approach looks more convenient to me, because it allows us to
> have a single implementation for all targets in one place, and it’s easier
> to introduce control-flow at this point.
>
> The patch below implements the second alternative. Along with a
> straight-forward lowering (which is valid and could be used as a base on all
> platforms), during the lowering some simple optimizations are performed
> (which I think is also easier to implement in CodeGenPrepare, than on DAGs):
>
> We don’t to generate code for unused part of the result structure.
> If div-instruction on the given platform behaves exactly as needed for the
> intrinsic (e.g. it takes place for ARM64), we don’t guard the div
> instruction. As a result, we could avoid branches at all if the second part
> of the result structure is not used.
> The most expected users of the result structure are extractvalue
> instructions. Having that in mind, we try to propagate the results - in most
> cases that allows to get rid of all corresponding extractvalues.
>
>
> Attached are two patches: the first one with the described implementation
> and tests, and the second one with the corresponding documentation changes.
>
> The first patch happened to already get to the trunk, but the topic is open,
> and any suggestions are welcome.
>
>
>
>
> Best regards,
> Michael
>

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