[CycleInfo] skip unreachable predecessors #101316
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If an unreachable block B branches to a block S inside a cycle, it may cause S to be incorrectly treated as an entry to the cycle. We avoid that by skipping unreachable predecessors when locating entries.
llvmbot
added
llvm:analysis
|
@llvm/pr-subscribers-llvm-analysis @llvm/pr-subscribers-llvm-adt Author: Sameer Sahasrabuddhe (ssahasra) ChangesIf an unreachable block B branches to a block S inside a cycle, it may cause S to be incorrectly treated as an entry to the cycle. We avoid that by skipping unreachable predecessors when locating entries. Full diff: https://github.com/llvm/llvm-project/pull/101316.diff 2 Files Affected:
diff --git a/llvm/include/llvm/ADT/GenericCycleImpl.h b/llvm/include/llvm/ADT/GenericCycleImpl.h
index ab9c421a44693..56d5ba6e06077 100644
--- a/llvm/include/llvm/ADT/GenericCycleImpl.h
+++ b/llvm/include/llvm/ADT/GenericCycleImpl.h
@@ -134,6 +134,8 @@ template <typename ContextT> class GenericCycleInfoCompute {
DFSInfo() = default;
explicit DFSInfo(unsigned Start) : Start(Start) {}
+ explicit operator bool() const { return Start; }
+
/// Whether this node is an ancestor (or equal to) the node \p Other
/// in the DFS tree.
bool isAncestorOf(const DFSInfo &Other) const {
@@ -231,6 +233,8 @@ void GenericCycleInfoCompute<ContextT>::run(BlockT *EntryBlock) {
for (BlockT *Pred : predecessors(HeaderCandidate)) {
const DFSInfo PredDFSInfo = BlockDFSInfo.lookup(Pred);
+ // This automatically ignores unreachable predecessors since they have
+ // zeros in their DFSInfo.
if (CandidateInfo.isAncestorOf(PredDFSInfo))
Worklist.push_back(Pred);
}
@@ -257,6 +261,10 @@ void GenericCycleInfoCompute<ContextT>::run(BlockT *EntryBlock) {
const DFSInfo PredDFSInfo = BlockDFSInfo.lookup(Pred);
if (CandidateInfo.isAncestorOf(PredDFSInfo)) {
Worklist.push_back(Pred);
+ } else if (!PredDFSInfo) {
+ // Ignore an unreachable predecessor. It will will incorrectly cause
+ // Block to be treated as a cycle entry.
+ LLVM_DEBUG(errs() << " skipped unreachable predecessor.\n");
} else {
IsEntry = true;
}
diff --git a/llvm/test/Analysis/CycleInfo/unreachable-predecessor.ll b/llvm/test/Analysis/CycleInfo/unreachable-predecessor.ll
new file mode 100644
index 0000000000000..36a2115bd7a94
--- /dev/null
+++ b/llvm/test/Analysis/CycleInfo/unreachable-predecessor.ll
@@ -0,0 +1,23 @@
+; RUN: opt < %s -disable-output -passes='print<cycles>' 2>&1 | FileCheck %s
+; CHECK-LABEL: CycleInfo for function: unreachable
+; CHECK: depth=1: entries(loop.body) loop.latch inner.block
+define void @unreachable(i32 %n) {
+entry:
+ br label %loop.body
+
+loop.body:
+ br label %inner.block
+
+; This branch should not cause %inner.block to appear as an entry.
+unreachable.block:
+ br label %inner.block
+
+inner.block:
+ br i1 undef, label %loop.exit, label %loop.latch
+
+loop.latch:
+ br label %loop.body
+
+loop.exit:
+ ret void
+}
|
arsenm
reviewed
Jul 31, 2024
| @@ -0,0 +1,23 @@ | |||
| ; RUN: opt < %s -disable-output -passes='print<cycles>' 2>&1 | FileCheck %s | |||
There was a problem hiding this comment.
Does this need an implicit-check-not to make sure this doesn't appear in the output?
There was a problem hiding this comment.
The CHECK is an exact one. The incorrect entry will show up in the entries() parentheses below and the CHECK will fail.
arsenm
approved these changes
Jul 31, 2024
|
LLVM Buildbot has detected a new failure on builder Full details are available at: https://lab.llvm.org/buildbot/#/builders/73/builds/2968 Here is the relevant piece of the build log for the reference: |
|
LLVM Buildbot has detected a new failure on builder Full details are available at: https://lab.llvm.org/buildbot/#/builders/51/builds/2004 Here is the relevant piece of the build log for the reference: |
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If an unreachable block B branches to a block S inside a cycle, it may cause S to be incorrectly treated as an entry to the cycle. We avoid that by skipping unreachable predecessors when locating entries.