xazax.hun 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>(
----------------
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 `||`.
Repository:
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https://reviews.llvm.org/D121455/new/
https://reviews.llvm.org/D121455
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