[PATCH] D117754: [clang][dataflow] Intersect ExprToLoc when joining environments

[Gábor Horváth via Phabricator via cfe-commits]

xazax.hun added inline comments.


================
Comment at: clang/lib/Analysis/FlowSensitive/TypeErasedDataflowAnalysis.cpp:49
+  // in tests.
+  std::set<const CFGBlock *> Preds;
   Preds.insert(Block.pred_begin(), Block.pred_end());
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sgatev wrote:
> xazax.hun wrote:
> > xazax.hun wrote:
> > > Are we sure that the memory addresses of CFGBlocks are stable enough for a deterministic order? Alternatively, we could use the block ids for the ordering.
> > Also, could you describe where the flakiness is originated from? Naively, I'd expect that the order in which we process the predecessors should not change the results of the analysis.
> You're right, using block ids for ordering is better. I updated the code.
> 
> > Also, could you describe where the flakiness is originated from?
> 
> Say we have a block `B1` with predecessors `B2` and `B3`. Let the environment of `B2` after evaluating all of its statements is `B2Env = { Expr1 -> Loc1 }` and the environment of `B3` after evaluating all of its statement is `B3Env = { Expr2 -> Loc2 }` where `ExprX -> LocX` refers to a particular mapping of storage locations to expressions.
> 
> What we want for the input environment of `B1` is `{}` because `B2Env` and `B3Env` do not contain common assignments of storage locations to expressions. What we got before this patch is either `B2Env.join(B3Env) = { Expr1 -> Loc1 }` or `B3Env.join(B2Env) = { Expr2 -> Loc2 }`.
> 
> Without deterministic ordering of predecessors the test that I'm introducing in this patch is flaky.
> What we got before this patch is either B2Env.join(B3Env) = { Expr1 -> Loc1 } or B3Env.join(B2Env) = { Expr2 -> Loc2 }.

I think I'm still missing something. With this patch, wouldn't both B2Env.join(B3Env) and B3Env.join(B2Env) produce the empty environment? If that is the case, do we still care about a deterministic order?





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