xazax.hun accepted this revision.
xazax.hun added inline comments.
This revision is now accepted and ready to land.
================
Comment at: clang/lib/Analysis/FlowSensitive/TypeErasedDataflowAnalysis.cpp:78
+/// and integers in the framework.
+static const Expr *ignoreParenImpCastsExceptToBool(const Expr *E) {
+ const Expr *LastE = nullptr;
----------------
li.zhe.hua wrote:
> sgatev wrote:
> > xazax.hun wrote:
> > > li.zhe.hua wrote:
> > > > sgatev wrote:
> > > > > I don't recall why we need to ignore implicit casts here. Can't we
> > > > > ignore parens and rely on the built-in transfer function, possibly
> > > > > adding handling of some missing casts there?
> > > > > https://github.com/llvm/llvm-project/blob/main/clang/lib/Analysis/FlowSensitive/Transfer.cpp#L192
> > > > If we only ignore parens, a test in the optional checker tests begins
> > > > to fail, specifically `UncheckedOptionalAccessTest.ValueOrComparison`.
> > > > The missing "cast" is an `ExprWithCleanups`. I didn't know how to deal
> > > > with that, so this patch just working around the assert.
> > > In general, I prefer to handle as much as possible with transfer
> > > functions and skip as little as possible in the AST. We might skip
> > > `ExprWithCleanups` nodes today, but we will need them tomorrow to
> > > properly model where certain destructors are being invoked.
> > We already have `skipExprWithCleanups` [1]. I wonder if it makes sense to
> > replace that with a simple transfer function like the one for the `CK_NoOp`
> > implicit cast. Would that solve the problem and remove the need for
> > `ignoreParenImpCastsExceptToBool`? In the future we might replace that
> > transfer function with a proper one, as Gábor suggested.
> >
> > [1]
> > https://github.com/llvm/llvm-project/blob/main/clang/lib/Analysis/FlowSensitive/Transfer.cpp#L36
> I sat down with @ymandel to get a better understanding of what was happening
> here, which was really helpful in understanding why my random flailing was
> making tests pass, and what I believe to be a better way forward.
>
> So, the CFG does not emit `ParenExpr` nodes. As such, the transfer function
> never sees them, and so when we do any kind of manual traversal (e.g. to look
> at a sub-expression), we use `ignoreParens()` because they effectively don't
> exist. The same thing happens with `ExprWithCleanups`. The CFG doesn't appear
> to emit them explicitly, and so we should similarly avoid them when manually
> traversing the AST.
>
> Note: Their position in the AST is slightly more constrained, likely as the
> immediate children of `*Stmt` nodes. This means we don't need to sprinkle
> these skips as widely as `ignoreParens()`.
>
> In terms of why adding `VisitFullExpr` "fixed" the failing test:
> `extendFlowCondition()` [[
> https://github.com/llvm/llvm-project/blob/ed1b32791dbb4c02cd761742a63cdf1e9d644ae6/clang/lib/Analysis/FlowSensitive/TypeErasedDataflowAnalysis.cpp#L108
> | manually calls ]] `transfer` on the provided `Cond` expression, which is a
> `ExprWithCleanups` if the caller uses `ignoreParens()` instead of
> `ignoreParenImpCasts()`.
>
> ---
>
> @xazax.hun:
>
> > In general, I prefer to handle as much as possible with transfer functions
> > and skip as little as possible in the AST. We might skip `ExprWithCleanups`
> > nodes today, but we will need them tomorrow to properly model where certain
> > destructors are being invoked.
>
> The CFG already emits calls to destructors as a result of `ExprWithCleanups`
> ([[ https://godbolt.org/z/fo846dcEa | godbolt ]]), so skipping them will not
> cause us to miss calls to destructors.
>
> ---
>
> @sgatev:
>
> > We already have `skipExprWithCleanups` [1]. I wonder if it makes sense to
> > replace that with a simple transfer function like the one for the `CK_NoOp`
> > implicit cast. Would that solve the problem and remove the need for
> > `ignoreParenImpCastsExceptToBool`? In the future we might replace that
> > transfer function with a proper one, as Gábor suggested.
> >
> > [1]
> > https://github.com/llvm/llvm-project/blob/main/clang/lib/Analysis/FlowSensitive/Transfer.cpp#L36
>
> I believe that `ParenExpr` and `ExprWithCleanups` are categorically the same
> within the framework (roughly, nodes that are not emitted by the CFG) and so
> we should handle them in the same way. The strategy right now for `ParenExpr`
> is to skip them, so I am inclined to do so as well with `ExprWithCleanups`
> for now. (I'm not necessarily opposed to having `ParenExpr` and
> `ExprWithCleanups` nodes being handled in the transfer function, but there's
> a question of how to get them in there if the CFG isn't going to emit them.)
>
> That would mean //more// usage of `skipExprWithCleanups`, and is reflected in
> this most recent revision.
Thanks for the detailed explanation, this sounds good to me! Could you add a
short summary as a comment to `ignoreExprWithCleanups`?
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D124807/new/
https://reviews.llvm.org/D124807
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