[PATCH] D71435: [WIP] [Attributor] Function level undefined behavior attribute

[Stefanos Baziotis via Phabricator via llvm-commits]

baziotis marked an inline comment as done.
baziotis added inline comments.


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Comment at: llvm/lib/Transforms/IPO/Attributor.cpp:2040
+  }
+};
+
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jdoerfert wrote:
> baziotis wrote:
> > jdoerfert wrote:
> > > baziotis wrote:
> > > > jdoerfert wrote:
> > > > > Let's also add a manifest method to replace the loads that are still considered UB with an undef (for now). That should allow us to test this.
> > > > The whole instruction? Like replace `%a = load i32, i32* null` with `undef`. I don't think this can happen. Maybe I misunderstood and you meant replace it with this: `%a = load i32, i32* undef` (i.e. replace the pointer operand value).
> > > Your first interpretation was what I meant. Replace the load (instruction) with undef. We actually want to be more aggressive but we will need that step either way.
> > Sorry but I couldn't find a way to do that. I've seen in the language reference that one can do this: `%some_name = undef` and so I guess this is what we want, but I couldn't produce it. The closest thing I found is `deleteAfterManifest` which replaces _its uses_ with `undef` but will also delete the instruction. I guess we actually need the instruction (or, its replaced counterpart) so that the info of undefined behavior stays.
> First, no worries, ask if you need to find an API. The code below was obviously not tested but should do what I described earlier:
> 
> `I.replaceAllUsesWith(UndefValue::get(I.getType()))`
> 
> You can also use `deleteAfterManifest`, actually you probably should use that instead. Or we directly go one step further and use `llvm::changeToUnreachable` (as AAIsDead does).
> 
> P.S.
> I don't think we can do `%some_name = undef` though, the lang ref might need an update there.
Well, I was assuming initially that `%c = undef` can't appear anywhere, but I changed my mind because of the ref: https://llvm.org/docs/LangRef.html#undefined-values. However, it seems that one can't actually write this (only maybe LLVM from one pass to another ? I've no idea, it's obscure).


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