sgatev added inline comments.
================
Comment at: clang/lib/Analysis/FlowSensitive/Transfer.cpp:516
+ // assigned to it.
+ Visit(&SubExpr);
+ if (auto *Val = dyn_cast_or_null<BoolValue>(
----------------
xazax.hun wrote:
> sgatev wrote:
> > xazax.hun wrote:
> > > Could you elaborate on when would this happen? I'd expect the traversal
> > > to always visit the predecessor basic blocks first and within a basic
> > > block always visit subexpressions first. So I'd be quite surprised if
> > > there is a subexpression we did not visit.
> > From what I've seen, logic operators influence the structure of the CFG
> > through additional basic blocks and terminators, but their sub-expression
> > operators are not added directly in the basic blocks.
> >
> > For example:
> > ```
> > void test(bool a, bool b, bool c) {
> > bool d = a && (b || c);
> > }
> > ```
> >
> > results in:
> > ```
> >
> > void test(bool a, bool b, bool c)
> > [B5 (ENTRY)]
> > Succs (1): B4
> >
> > [B1]
> > 1: [B4.2] && ([B3.2] || [B2.2])
> > 2: bool d = a && (b || c);
> > Preds (3): B2 B3 B4
> > Succs (1): B0
> >
> > [B2]
> > 1: c
> > 2: [B2.1] (ImplicitCastExpr, LValueToRValue, _Bool)
> > Preds (1): B3
> > Succs (1): B1
> >
> > [B3]
> > 1: b
> > 2: [B3.1] (ImplicitCastExpr, LValueToRValue, _Bool)
> > T: [B3.2] || ...
> > Preds (1): B4
> > Succs (2): B1 B2
> >
> > [B4]
> > 1: a
> > 2: [B4.1] (ImplicitCastExpr, LValueToRValue, _Bool)
> > T: [B4.2] && ...
> > Preds (1): B5
> > Succs (2): B3 B1
> >
> > [B0 (EXIT)]
> > Preds (1): B1
> > ```
> >
> > So, when we evaluate `a && (b || c)` in `B1`, the sub-expression `b || c`
> > has not been evaluated yet. I updated the comment in the code to make that
> > more clear.
> I understand the structure of the CFG and also understand that certain
> subexpressions are in different basic blocks. But I still don't understand
> why would `b || c` be not evaluated when we evaluate `a && (b || c)`.
>
> The operator `&&` in your example is evaluated in `B1`. The operator `||` is
> evaluated in `B3`. `B3` is a predecessor of `B1`, so if we process the CFG in
> reverse-post order, we should visit `B3` before `B1`.
>
> I am pretty sure if the traversal is well-written we should have the `||`
> evaluated before we visit `B1`.
>
> I suspect that something different might be going on. Is it possible that you
> want to evaluate the `&&` in `B4`?
>
> Note that `&&` is a terminator there because of the short-circuiting. So at
> that point we should NOT ask for the value of `||`.
> The operator `||` is evaluated in `B3`.
I don't think that's the case. Similar to your observation about `&&` being a
terminator in `B4`, I believe that `||` is a terminator in `B3`.
I interpret `B3` as the "`a` is true" branch. It still doesn't contain
information about `b` and `c` which might be subject to short-circuiting.
Does that make sense?
Repository:
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CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D121455/new/
https://reviews.llvm.org/D121455
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