[llvm] [BasicBlockUtils] Fixed LoopInfo update in UpdateAnalysisInformation() (PR #177147)
Yevgeny Rouban via llvm-commits
llvm-commits at lists.llvm.org
Wed Jan 28 21:49:19 PST 2026
https://github.com/yrouban updated https://github.com/llvm/llvm-project/pull/177147
>From b866fa92b1cbe1d6a7298acd58aa787585093fcc Mon Sep 17 00:00:00 2001
From: Yevgeny Rouban <yrouban at gmail.com>
Date: Wed, 21 Jan 2026 13:13:56 +0300
Subject: [PATCH 1/4] [BasicBlockUtils] Test with broken LoopInfo after
SplitLandingPadPredecessors(). NFC
SplitLandingPadPredecessors() results in an irreducible loop
and makes LoopInfo invalid. Verification results in a crash:
Assertion `CB != OutsideLoopPreds[i] && "Loop has multiple entry points!"' failed.
---
.../Transforms/Utils/BasicBlockUtilsTest.cpp | 43 +++++++++++++++++++
1 file changed, 43 insertions(+)
diff --git a/llvm/unittests/Transforms/Utils/BasicBlockUtilsTest.cpp b/llvm/unittests/Transforms/Utils/BasicBlockUtilsTest.cpp
index 00d9e9ff81e05..e8b16424d3cd3 100644
--- a/llvm/unittests/Transforms/Utils/BasicBlockUtilsTest.cpp
+++ b/llvm/unittests/Transforms/Utils/BasicBlockUtilsTest.cpp
@@ -26,6 +26,8 @@
#include "llvm/Support/SourceMgr.h"
#include "gtest/gtest.h"
+#define DEBUG_TYPE "basic-block-utils-tests"
+
using namespace llvm;
static std::unique_ptr<Module> parseIR(LLVMContext &C, const char *IR) {
@@ -400,6 +402,47 @@ define i32 @basic_func(i1 %cond) {
EXPECT_TRUE(DT.verify());
}
+TEST(BasicBlockUtils, SplitLandingPadPredecessors) {
+ LLVMContext C;
+ std::unique_ptr<Module> M = parseIR(C, R"IR(
+declare void @foo()
+
+define void @split-lp-predecessors-test() personality ptr null {
+entry:
+ invoke void @foo()
+ to label %loop unwind label %catch_dest
+
+loop:
+ invoke void @foo()
+ to label %latch unwind label %catch_dest
+
+latch:
+ br label %loop
+
+catch_dest:
+ %lp = landingpad i32
+ cleanup
+ invoke void @foo()
+ to label %exit unwind label %catch_dest
+
+exit:
+ ret void
+}
+)IR");
+ Function *F = M->getFunction("split-lp-predecessors-test");
+ DominatorTree DT(*F);
+ LoopInfo LI(DT);
+
+ EXPECT_TRUE(DT.verify());
+ LI.verify(DT);
+ SplitBlockPredecessors(getBasicBlockByName(*F, "catch_dest"),
+ {getBasicBlockByName(*F, "loop")}, "", &DT, &LI);
+
+ EXPECT_TRUE(DT.verify());
+ LLVM_DEBUG(F->dump(); LI.print(dbgs()));
+ EXPECT_DEATH({LI.verify(DT);}, "Assertion.*Loop has multiple entry points!");
+}
+
TEST(BasicBlockUtils, SplitCriticalEdge) {
LLVMContext C;
std::unique_ptr<Module> M = parseIR(C, R"IR(
>From ed5422f1016a5c4f6a63fbcfb64da55d6d0e9fea Mon Sep 17 00:00:00 2001
From: Yevgeny Rouban <yrouban at gmail.com>
Date: Wed, 21 Jan 2026 13:15:50 +0300
Subject: [PATCH 2/4] [BasicBlockUtils] Fixed LoopInfo update in
UpdateAnalysisInformation()
When UpdateAnalysisInformation() detects an irreducible loop created
then the loop is removed from LoopInfo.
---
llvm/lib/Transforms/Utils/BasicBlockUtils.cpp | 32 +++++++++++++++++--
.../Transforms/Utils/BasicBlockUtilsTest.cpp | 3 +-
2 files changed, 32 insertions(+), 3 deletions(-)
diff --git a/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp b/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
index b0c04086f5e84..b9d817ade443d 100644
--- a/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
+++ b/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
@@ -1109,6 +1109,28 @@ BasicBlock *llvm::splitBlockBefore(BasicBlock *Old, BasicBlock::iterator SplitPt
return New;
}
+static bool HasReachableLoopEntry(const Loop &L, const DominatorTree &DT) {
+ for (const auto Pred : inverse_children<const BasicBlock *>(L.getHeader()))
+ if (!L.contains(Pred) && DT.isReachableFromEntry(Pred))
+ return true;
+
+ return false;
+}
+
+// Destroy the loop.
+static void Destroy(Loop *L, LoopInfo &LI) {
+ auto *PerentL = L->getParentLoop();
+ for (auto *BB : L->getBlocks())
+ LI.changeLoopFor(BB, PerentL);
+
+ if (PerentL)
+ PerentL->removeChildLoop(L);
+ else
+ LI.removeLoop(llvm::find(LI, L));
+
+ LI.destroy(L);
+}
+
/// Update DominatorTree, LoopInfo, and LCCSA analysis information.
/// Invalidates DFS Numbering when DTU or DT is provided.
static void UpdateAnalysisInformation(BasicBlock *OldBB, BasicBlock *NewBB,
@@ -1218,8 +1240,14 @@ static void UpdateAnalysisInformation(BasicBlock *OldBB, BasicBlock *NewBB,
InnermostPredLoop->addBasicBlockToLoop(NewBB, *LI);
} else {
L->addBasicBlockToLoop(NewBB, *LI);
- if (SplitMakesNewLoopHeader)
- L->moveToHeader(NewBB);
+ if (SplitMakesNewLoopHeader) {
+ if (!HasReachableLoopEntry(*L, *DT))
+ // Old header lost all entries.
+ L->moveToHeader(NewBB);
+ else
+ // Both OldBB and NewBB have loop entries.
+ Destroy(L, *LI);
+ }
}
}
diff --git a/llvm/unittests/Transforms/Utils/BasicBlockUtilsTest.cpp b/llvm/unittests/Transforms/Utils/BasicBlockUtilsTest.cpp
index e8b16424d3cd3..d905d31184a4d 100644
--- a/llvm/unittests/Transforms/Utils/BasicBlockUtilsTest.cpp
+++ b/llvm/unittests/Transforms/Utils/BasicBlockUtilsTest.cpp
@@ -440,7 +440,8 @@ define void @split-lp-predecessors-test() personality ptr null {
EXPECT_TRUE(DT.verify());
LLVM_DEBUG(F->dump(); LI.print(dbgs()));
- EXPECT_DEATH({LI.verify(DT);}, "Assertion.*Loop has multiple entry points!");
+ LI.verify(DT);
+ EXPECT_EQ(LI.getLoopFor(getBasicBlockByName(*F, "catch_dest")), nullptr);
}
TEST(BasicBlockUtils, SplitCriticalEdge) {
>From 390a112d590360b09a93a54725db3195a05d7ecd Mon Sep 17 00:00:00 2001
From: Yevgeny Rouban <yrouban at gmail.com>
Date: Thu, 29 Jan 2026 08:41:49 +0300
Subject: [PATCH 3/4] fixup! [BasicBlockUtils] Fixed LoopInfo update in
UpdateAnalysisInformation()
---
llvm/lib/Transforms/Utils/BasicBlockUtils.cpp | 12 ++++++------
1 file changed, 6 insertions(+), 6 deletions(-)
diff --git a/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp b/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
index b9d817ade443d..1a82f77ab4d86 100644
--- a/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
+++ b/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
@@ -1109,7 +1109,7 @@ BasicBlock *llvm::splitBlockBefore(BasicBlock *Old, BasicBlock::iterator SplitPt
return New;
}
-static bool HasReachableLoopEntry(const Loop &L, const DominatorTree &DT) {
+static bool hasReachableLoopEntry(const Loop &L, const DominatorTree &DT) {
for (const auto Pred : inverse_children<const BasicBlock *>(L.getHeader()))
if (!L.contains(Pred) && DT.isReachableFromEntry(Pred))
return true;
@@ -1119,12 +1119,12 @@ static bool HasReachableLoopEntry(const Loop &L, const DominatorTree &DT) {
// Destroy the loop.
static void Destroy(Loop *L, LoopInfo &LI) {
- auto *PerentL = L->getParentLoop();
+ auto *ParentL = L->getParentLoop();
for (auto *BB : L->getBlocks())
- LI.changeLoopFor(BB, PerentL);
+ LI.changeLoopFor(BB, ParentL);
- if (PerentL)
- PerentL->removeChildLoop(L);
+ if (ParentL)
+ ParentL->removeChildLoop(L);
else
LI.removeLoop(llvm::find(LI, L));
@@ -1241,7 +1241,7 @@ static void UpdateAnalysisInformation(BasicBlock *OldBB, BasicBlock *NewBB,
} else {
L->addBasicBlockToLoop(NewBB, *LI);
if (SplitMakesNewLoopHeader) {
- if (!HasReachableLoopEntry(*L, *DT))
+ if (!hasReachableLoopEntry(*L, *DT))
// Old header lost all entries.
L->moveToHeader(NewBB);
else
>From b7b01ecc745f5db6b0b466ec550ca71dda32b7a8 Mon Sep 17 00:00:00 2001
From: Yevgeny Rouban <yrouban at gmail.com>
Date: Thu, 29 Jan 2026 08:42:51 +0300
Subject: [PATCH 4/4] amend! [BasicBlockUtils] Test with broken LoopInfo after
SplitLandingPadPredecessors(). NFC
SplitLandingPadPredecessors() results in an irreducible loop
and makes LoopInfo invalid. Verification results in a crash:
Assertion `CB != OutsideLoopPreds[i] && "Loop has multiple entry points!"' failed.
The test @split-lp-predecessors-test() after
SplitBlockPredecessors(catch_dest, { loop }, "", DT, LI) changes to
the following IR where the loop {%catch_dest} gets into irreducible
loop {%catch_dest.split-lp, %catch_dest}:
==========
define void @split-lp-predecessors-test() personality ptr null {
entry:
invoke void @foo()
to label %loop unwind label %catch_dest.split-lp
loop: ; preds = %latch, %entry
invoke void @foo()
to label %latch unwind label %catch_dest1
latch: ; preds = %loop
br label %loop
catch_dest1: ; preds = %loop
%lpad = landingpad i32
cleanup
br label %catch_dest
catch_dest.split-lp: ; preds = %entry, %catch_dest
%lpad.split-lp = landingpad i32
cleanup
br label %catch_dest
catch_dest: ; preds = %catch_dest.split-lp, %catch_dest1
invoke void @foo()
to label %exit unwind label %catch_dest.split-lp
exit: ; preds = %catch_dest
ret void
}
==========
Irreducible loops must be removed from LoopInfo.
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