[clang][dataflow] Drop block-relative cap on worklist iterations. #80033
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@llvm/pr-subscribers-clang-analysis @llvm/pr-subscribers-clang Author: Yitzhak Mandelbaum (ymand) ChangesAs per the FIXME, this cap never really served its purpose. This patch Full diff: https://github.com/llvm/llvm-project/pull/80033.diff 1 Files Affected:
diff --git a/clang/lib/Analysis/FlowSensitive/TypeErasedDataflowAnalysis.cpp b/clang/lib/Analysis/FlowSensitive/TypeErasedDataflowAnalysis.cpp
index 49e425bde66aa..4c88c46142d64 100644
--- a/clang/lib/Analysis/FlowSensitive/TypeErasedDataflowAnalysis.cpp
+++ b/clang/lib/Analysis/FlowSensitive/TypeErasedDataflowAnalysis.cpp
@@ -540,14 +540,6 @@ runTypeErasedDataflowAnalysis(
Worklist.enqueueSuccessors(&Entry);
AnalysisContext AC(CFCtx, Analysis, StartingEnv, BlockStates);
-
- // FIXME: remove relative cap. There isn't really any good setting for
- // `MaxAverageVisitsPerBlock`, so it has no clear value over using
- // `MaxBlockVisits` directly.
- static constexpr std::int32_t MaxAverageVisitsPerBlock = 4;
- const std::int32_t RelativeMaxBlockVisits =
- MaxAverageVisitsPerBlock * BlockStates.size();
- MaxBlockVisits = std::min(RelativeMaxBlockVisits, MaxBlockVisits);
std::int32_t BlockVisits = 0;
while (const CFGBlock *Block = Worklist.dequeue()) {
LLVM_DEBUG(llvm::dbgs()
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Xazax-hun
approved these changes
Jan 30, 2024
martinboehme
approved these changes
Jan 30, 2024
As per the FIXME, this cap never really served its purpose. This patch simplifies to a single, caller-specified, absolute cap.
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This expands to three nested for loops and led to "max blocks visited" errors when ymand@ initially tried to change the order in which block successors are visited in the dataflow framework ([#80030](llvm/llvm-project#80030)). We now no longer see these "max blocks visited" errors with this test because of [other](3dfd35e) [fixes](llvm/llvm-project#80033) that have landed in the meantime. Nevertheless, it seems worthwhile to put this test in place to prevent regressions. PiperOrigin-RevId: 603020024 Change-Id: I7a8428a77a2f4252d012eec30e42ccb0c1b96bc5
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Our intent is to signal "perform the default behavior", i.e. keep the `MergedVal` that has been produced by the framework. This would be done by returning true (which is also what the [default implementation](https://github.com/llvm/llvm-project/blob/d5c9d402f07e7448cd46870a59d981f582682548/clang/include/clang/Analysis/FlowSensitive/DataflowEnvironment.h#L105) does). However, we were returning false, which means that the framework should drop the `MergedVal` and remove the corresponding entry from `LocToVal`. This was initially causing "max blocks visited" errors on the `TriplyNestedForLoopSingleIteration` test when ymand@ tried to change the order in which the dataflow framework visits block successors ([#80030](llvm/llvm-project#80030)). We now no longer see these errors because of [other](3dfd35e) [fixes](llvm/llvm-project#80033) that have landed in the meantime. Nevertheless, it seems worthwhile to fix `merge()`, as this reduces the number of block visits required to achieve convergence. On the `TriplyNestedForLoopSingleIteration` test, we require 66 block visits before this patch, and 53 block visits after. The issue was that when performing the join at the loop head, our custom merge logic was causing the merged value for the variable `int c` to be discarded from `LocToVal`. This in turn was blocking convergence because widening [considers](https://github.com/llvm/llvm-project/blob/d5c9d402f07e7448cd46870a59d981f582682548/clang/lib/Analysis/FlowSensitive/DataflowEnvironment.cpp#L626) two environments to be different if the sizes of their `LocToVal` maps differ. As an aside, widening is inconsistent in this respect with the convergence check that we perform on blocks that don't get widened. [`Environment::equivalentTo()`](https://github.com/llvm/llvm-project/blob/d5c9d402f07e7448cd46870a59d981f582682548/clang/lib/Analysis/FlowSensitive/DataflowEnvironment.cpp#L570), which performs this check, uses `compareKeyToValueMaps()` to compare the `LocToVal` maps of the two environments, and `compareKeyToValueMaps()` only compares values for locations that are in both `LocToVal` maps and ignores locations that only occur in one of the maps. We should probably fix this inconsistency. PiperOrigin-RevId: 603025629 Change-Id: Id02993e576023c44a335c8991020f7e233a75320
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As per the FIXME, this cap never really served its purpose. This patch
simplifies to a single, caller-specified, absolute cap.