[clang] [clang][dataflow] Process terminator condition within `transferCFGBlock()`. (PR #77750)
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Thu Jan 11 02:43:00 PST 2024
https://github.com/martinboehme created https://github.com/llvm/llvm-project/pull/77750
In particular, it's important that we create the "fallback" atomic at this point
(which we produce if the transfer function didn't produce a value for the
expression) so that it is placed in the correct environment.
Previously, we processed the terminator condition in the `TerminatorVisitor`,
which put the fallback atomic in a copy of the environment that is produced as
input for the _successor_ block, rather than the environment for the block
containing the expression for which we produce the fallback atomic.
As a result, we produce different fallback atomics every time we process the
successor block, and hence we don't have a consistent representation of the
terminator condition in the flow condition.
This patch includes a test (authored by ymand@) that fails without the fix.
>From 732a0b343b24eee4bb5f17e4c2edbe7268aa8955 Mon Sep 17 00:00:00 2001
From: Martin Braenne <mboehme at google.com>
Date: Thu, 11 Jan 2024 10:42:23 +0000
Subject: [PATCH] [clang][dataflow] Process terminator condition within
`transferCFGBlock()`.
In particular, it's important that we create the "fallback" atomic at this point
(which we produce if the transfer function didn't produce a value for the
expression) so that it is placed in the correct environment.
Previously, we processed the terminator condition in the `TerminatorVisitor`,
which put the fallback atomic in a copy of the environment that is produced as
input for the _successor_ block, rather than the environment for the block
containing the expression for which we produce the fallback atomic.
As a result, we produce different fallback atomics every time we process the
successor block, and hence we don't have a consistent representation of the
terminator condition in the flow condition.
This patch includes a test (authored by ymand@) that fails without the fix.
---
.../TypeErasedDataflowAnalysis.cpp | 42 +++++++++++++------
.../Analysis/FlowSensitive/LoggerTest.cpp | 1 +
.../Analysis/FlowSensitive/TransferTest.cpp | 31 ++++++++++++++
3 files changed, 62 insertions(+), 12 deletions(-)
diff --git a/clang/lib/Analysis/FlowSensitive/TypeErasedDataflowAnalysis.cpp b/clang/lib/Analysis/FlowSensitive/TypeErasedDataflowAnalysis.cpp
index faf83a8920d4ea..b2b1acd288bd9f 100644
--- a/clang/lib/Analysis/FlowSensitive/TypeErasedDataflowAnalysis.cpp
+++ b/clang/lib/Analysis/FlowSensitive/TypeErasedDataflowAnalysis.cpp
@@ -126,19 +126,12 @@ class TerminatorVisitor
private:
TerminatorVisitorRetTy extendFlowCondition(const Expr &Cond) {
- // The terminator sub-expression might not be evaluated.
- if (Env.getValue(Cond) == nullptr)
- transfer(StmtToEnv, Cond, Env);
-
auto *Val = Env.get<BoolValue>(Cond);
- // Value merging depends on flow conditions from different environments
- // being mutually exclusive -- that is, they cannot both be true in their
- // entirety (even if they may share some clauses). So, we need *some* value
- // for the condition expression, even if just an atom.
- if (Val == nullptr) {
- Val = &Env.makeAtomicBoolValue();
- Env.setValue(Cond, *Val);
- }
+ // In transferCFGBlock(), we ensure that we always have a `Value` for the
+ // terminator condition, so assert this.
+ // We consciously assert ourselves instead of asserting via `cast()` so
+ // that we get a more meaningful line number if the assertion fails.
+ assert(Val != nullptr);
bool ConditionValue = true;
// The condition must be inverted for the successor that encompasses the
@@ -489,6 +482,31 @@ transferCFGBlock(const CFGBlock &Block, AnalysisContext &AC,
}
AC.Log.recordState(State);
}
+
+ // If we have a terminator, evaluate its condition.
+ // This `Expr` may not appear as a `CFGElement` anywhere else, and it's
+ // important that we evaluate it here (rather than while processing the
+ // terminator) so that we put the corresponding value in the right
+ // environment.
+ if (const Expr *TerminatorCond = dyn_cast_or_null<Expr>(
+ Block.getTerminatorCondition())) {
+ if (State.Env.getValue(*TerminatorCond) == nullptr)
+ // FIXME: This only runs the builtin transfer, not the analysis-specific
+ // transfer. Fixing this isn't trivial, as the analysis-specific transfer
+ // takes a `CFGElement` as input, but some expressions only show up as a
+ // terminator condition, but not as a `CFGElement`. The condition of an if
+ // statement is one such example.
+ transfer(StmtToEnvMap(AC.CFCtx, AC.BlockStates), *TerminatorCond,
+ State.Env);
+
+ // If the transfer function didn't produce a value, create an atom so that
+ // we have *some* value for the condition expression. This ensures that
+ // when we extend the flow condition, it actually changes.
+ if (State.Env.getValue(*TerminatorCond) == nullptr)
+ State.Env.setValue(*TerminatorCond, State.Env.makeAtomicBoolValue());
+ AC.Log.recordState(State);
+ }
+
return State;
}
diff --git a/clang/unittests/Analysis/FlowSensitive/LoggerTest.cpp b/clang/unittests/Analysis/FlowSensitive/LoggerTest.cpp
index a60dbe1f61f6d6..c5594aa3fe9d1f 100644
--- a/clang/unittests/Analysis/FlowSensitive/LoggerTest.cpp
+++ b/clang/unittests/Analysis/FlowSensitive/LoggerTest.cpp
@@ -123,6 +123,7 @@ recordState(Elements=1, Branches=0, Joins=0)
enterElement(b (ImplicitCastExpr, LValueToRValue, _Bool))
transfer()
recordState(Elements=2, Branches=0, Joins=0)
+recordState(Elements=2, Branches=0, Joins=0)
enterBlock(3, false)
transferBranch(0)
diff --git a/clang/unittests/Analysis/FlowSensitive/TransferTest.cpp b/clang/unittests/Analysis/FlowSensitive/TransferTest.cpp
index 056c4f3383d832..6d88e25f77c807 100644
--- a/clang/unittests/Analysis/FlowSensitive/TransferTest.cpp
+++ b/clang/unittests/Analysis/FlowSensitive/TransferTest.cpp
@@ -6408,4 +6408,35 @@ TEST(TransferTest, DifferentReferenceLocInJoin) {
});
}
+// This test verifies correct modeling of a relational dependency that goes
+// through unmodeled functions (the simple `cond()` in this case).
+TEST(TransferTest, ConditionalRelation) {
+ std::string Code = R"(
+ bool cond();
+ void target() {
+ bool a = true;
+ bool b = true;
+ if (cond()) {
+ a = false;
+ if (cond()) {
+ b = false;
+ }
+ }
+ (void)0;
+ // [[p]]
+ }
+ )";
+ runDataflow(
+ Code,
+ [](const llvm::StringMap<DataflowAnalysisState<NoopLattice>> &Results,
+ ASTContext &ASTCtx) {
+ const Environment &Env = getEnvironmentAtAnnotation(Results, "p");
+ auto &A = Env.arena();
+ auto &VarA = getValueForDecl<BoolValue>(ASTCtx, Env, "a").formula();
+ auto &VarB = getValueForDecl<BoolValue>(ASTCtx, Env, "b").formula();
+
+ EXPECT_FALSE(Env.allows(A.makeAnd(VarA, A.makeNot(VarB))));
+ });
+}
+
} // namespace
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