[llvm] [LoopSimplifyCFG] Add check for missing loop preheader (PR #149743)
Justus Klausecker via llvm-commits
llvm-commits at lists.llvm.org
Tue Jul 22 04:53:13 PDT 2025
https://github.com/Justus2308 updated https://github.com/llvm/llvm-project/pull/149743
>From 5ab6c7e575ceda59a6a4226158d03f0e6b10b708 Mon Sep 17 00:00:00 2001
From: Justus Klausecker <justus at klausecker.de>
Date: Sun, 20 Jul 2025 23:54:47 +0200
Subject: [PATCH 1/3] [LoopSimplifyCFG] Add check for missing loop preheader
Adds a check for a missing loop preheader during analysis. This fixes a
nullptr dereference that happened whenever LoopSimplify was unable to
generate a preheader because the loop was entered by an indirectbr
instruction (as stated in the LoopSimplify.cpp doc comment).
---
.../lib/Transforms/Scalar/LoopSimplifyCFG.cpp | 19 +++++++++++++++++++
.../enter-through-indirectbr.ll | 16 ++++++++++++++++
2 files changed, 35 insertions(+)
create mode 100644 llvm/test/Transforms/LoopSimplifyCFG/enter-through-indirectbr.ll
diff --git a/llvm/lib/Transforms/Scalar/LoopSimplifyCFG.cpp b/llvm/lib/Transforms/Scalar/LoopSimplifyCFG.cpp
index 221094f170ac7..c63578c63e8be 100644
--- a/llvm/lib/Transforms/Scalar/LoopSimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopSimplifyCFG.cpp
@@ -128,6 +128,8 @@ class ConstantTerminatorFoldingImpl {
// from any other block. So this variable set to true means that loop's latch
// has become unreachable from loop header.
bool DeleteCurrentLoop = false;
+ // Whether or not we enter the loop through an indirectbr.
+ bool HasIndirectEntry = false;
// The blocks of the original loop that will still be reachable from entry
// after the constant folding.
@@ -216,6 +218,17 @@ class ConstantTerminatorFoldingImpl {
return;
}
+ // We need a loop preheader to split in handleDeadExits(). If LoopSimplify
+ // wasn't able to form one because the loop can be entered through an
+ // indirectbr we cannot continue.
+ if (!L.getLoopPreheader()) {
+ assert(any_of(predecessors(L.getHeader()), [&](BasicBlock *Pred) {
+ return isa<IndirectBrInst>(Pred->getTerminator());
+ }) && "Loop should have preheader if it is not entered indirectly");
+ HasIndirectEntry = true;
+ return;
+ }
+
// Collect live and dead loop blocks and exits.
LiveLoopBlocks.insert(L.getHeader());
for (auto I = DFS.beginRPO(), E = DFS.endRPO(); I != E; ++I) {
@@ -546,6 +559,12 @@ class ConstantTerminatorFoldingImpl {
return false;
}
+ if (HasIndirectEntry) {
+ LLVM_DEBUG(dbgs() << "Loops which can be entered indirectly are not"
+ " supported!\n");
+ return false;
+ }
+
// Nothing to constant-fold.
if (FoldCandidates.empty()) {
LLVM_DEBUG(
diff --git a/llvm/test/Transforms/LoopSimplifyCFG/enter-through-indirectbr.ll b/llvm/test/Transforms/LoopSimplifyCFG/enter-through-indirectbr.ll
new file mode 100644
index 0000000000000..bf1b6a70e6c35
--- /dev/null
+++ b/llvm/test/Transforms/LoopSimplifyCFG/enter-through-indirectbr.ll
@@ -0,0 +1,16 @@
+; RUN: opt -S -enable-loop-simplifycfg-term-folding=true -passes='require<domtree>,loop(loop-simplifycfg)' -verify-loop-info -verify-dom-info -verify-loop-lcssa < %s | FileCheck %s
+
+define void @test() {
+; CHECK-LABEL: @test(
+
+ indirectbr ptr null, [label %A, label %C]
+
+A:
+ br i1 true, label %B, label %C
+
+B:
+ br i1 true, label %A, label %C
+
+C:
+ unreachable
+}
>From 35c44836fec21a74bc4d3104657aa13c24e7481c Mon Sep 17 00:00:00 2001
From: Justus Klausecker <justus at klausecker.de>
Date: Mon, 21 Jul 2025 18:14:40 +0200
Subject: [PATCH 2/3] address comments
---
.../enter-through-indirectbr.ll | 20 +++++++++++++++----
1 file changed, 16 insertions(+), 4 deletions(-)
diff --git a/llvm/test/Transforms/LoopSimplifyCFG/enter-through-indirectbr.ll b/llvm/test/Transforms/LoopSimplifyCFG/enter-through-indirectbr.ll
index bf1b6a70e6c35..dd524ab7d140d 100644
--- a/llvm/test/Transforms/LoopSimplifyCFG/enter-through-indirectbr.ll
+++ b/llvm/test/Transforms/LoopSimplifyCFG/enter-through-indirectbr.ll
@@ -1,9 +1,21 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
; RUN: opt -S -enable-loop-simplifycfg-term-folding=true -passes='require<domtree>,loop(loop-simplifycfg)' -verify-loop-info -verify-dom-info -verify-loop-lcssa < %s | FileCheck %s
-
-define void @test() {
-; CHECK-LABEL: @test(
- indirectbr ptr null, [label %A, label %C]
+define void @test(ptr %addr) {
+; CHECK-LABEL: define void @test(
+; CHECK-SAME: ptr [[ADDR:%.*]]) {
+; CHECK-NEXT: indirectbr ptr [[ADDR]], [label %[[A:.*]], label %C]
+; CHECK: [[A]]:
+; CHECK-NEXT: br i1 true, label %[[B:.*]], label %[[C_LOOPEXIT:.*]]
+; CHECK: [[B]]:
+; CHECK-NEXT: br i1 true, label %[[A]], label %[[C_LOOPEXIT]]
+; CHECK: [[C_LOOPEXIT]]:
+; CHECK-NEXT: br label %[[C:.*]]
+; CHECK: [[C]]:
+; CHECK-NEXT: unreachable
+;
+
+ indirectbr ptr %addr, [label %A, label %C]
A:
br i1 true, label %B, label %C
>From 53a43f8c5ac8f8b2e79e686d9a1a77d3cd748f25 Mon Sep 17 00:00:00 2001
From: Justus Klausecker <justus at klausecker.de>
Date: Mon, 21 Jul 2025 18:29:06 +0200
Subject: [PATCH 3/3] fix formatting
---
llvm/lib/Transforms/Scalar/LoopSimplifyCFG.cpp | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)
diff --git a/llvm/lib/Transforms/Scalar/LoopSimplifyCFG.cpp b/llvm/lib/Transforms/Scalar/LoopSimplifyCFG.cpp
index c63578c63e8be..b9546c5fa236b 100644
--- a/llvm/lib/Transforms/Scalar/LoopSimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopSimplifyCFG.cpp
@@ -222,9 +222,11 @@ class ConstantTerminatorFoldingImpl {
// wasn't able to form one because the loop can be entered through an
// indirectbr we cannot continue.
if (!L.getLoopPreheader()) {
- assert(any_of(predecessors(L.getHeader()), [&](BasicBlock *Pred) {
- return isa<IndirectBrInst>(Pred->getTerminator());
- }) && "Loop should have preheader if it is not entered indirectly");
+ assert(any_of(predecessors(L.getHeader()),
+ [&](BasicBlock *Pred) {
+ return isa<IndirectBrInst>(Pred->getTerminator());
+ }) &&
+ "Loop should have preheader if it is not entered indirectly");
HasIndirectEntry = true;
return;
}
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