[PATCH] D121455: [clang][dataflow] Add support for nested composite bool expressions

[Gábor Horváth via Phabricator via cfe-commits]

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:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D121455/new/

https://reviews.llvm.org/D121455