[llvm-dev] An issue with new PM's requirements on call graph changes

[Sean Silva via llvm-dev]

On Fri, Jun 30, 2017 at 1:02 AM, Chandler Carruth via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> I have hit a fairly isolated practical issue deploying the new PM, but it
> does point to a latent theoretical issues as well. I see various ways to
> address it, but want feedback from others before moving forward.
>
> The issue is that we can introduce out-of-thin-air calls to known library
> functions (`SimplifyLibCalls`, etc). These can be introduced in function
> passes (`InstCombine` in particular) and that seems highly desirable.
>
> These all look like one of these cases:
> 1a) Introducing a new call to an LLVM intrinsic
> 1b) Replacing an existing call with a call to an LLVM intrinsic
> 2a) Introducing a new call to a declared library function (but not defined)
> 2b) Replacing an existing call with a call to a declared library function
> 3a) Introducing a new call to a defined library function
> 3b) Replacing an existing call with a call to a defined library function
>
> Both #1 and #2 are easy to handle in reality. Intrinsics and declared
> functions don't impact the PM's call graph because there is no need to
> order the walk over them. But #3 is a real issue.
>
> The only case I have found that actually hits #3 at all hits #3b when
> building FORTIFY code with the new pass manager because after inlining we
> do a lot of (really nice) optimizations on library calls to remove
> unnecessary FORTIFY checks. But this is in *theory* a problem when LTO-ing
> with libc. More likely it could be a problem when LTO-ing with a vector
> math library.
>
> So what do we do?
>
> My initial idea: find all *defined* library functions in the module, and
> every time we create a ref edge to one of them, synthesize a ref edge to
> all of them. This should completely solve #3b above. But it doesn't really
> address #3a at all.
>
> Is that OK? It would be very convenient to say that if we want to
> introduce truly novel and new calls to library functions, we should have an
> LLVM intrinsic to model those routines.
>

That seems reasonable, though it would in theory block inlining of, say,
memset_pattern if we have a definition available when LTO'ing but that
seems acceptable (and we already live with similar restrictions with other
memory intrinsics anyway and I don't hear anyone complaining).


>
> But we actually have an example (I think) of #3a, introducing a call to a
> library function out of the blue: memset_pattern. =/
>
> The only way I see to reasonably handle #3a is to have *every* function
> implicitly contain a reference edge to every defined library function in
> the module. This is, needless to say, amazingly wasteful. Hence my email.
> How important is this?
>

This seems like the simplest and most natural solution overall. Can you get
some numbers on how wasteful it actually is? (and for the alternative
approach)

Even if it ends up not being that wasteful and worth doing just for
simplicity's sake, I do like the idea of "out-of-thin-air calls must be
intrinsics".

-- Sean Silva


>
> If we need to correctly handle this, I think I would probably implement
> this by actually changing the *iteration* of reference edges in the graph
> to just implicitly walk the list of defined library functions so that we
> didn't burn any space on this. But it will make iteration of reference
> edges slower and add a reasonable amount of complexity. So I'd like to hear
> some other opinions before going down either of these roads.
>
>
> Thanks,
> -Chandler
>
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