[SimplifyCFG] Avoid threading for loop headers #151142

ArneStenkrona2 · 2025-07-29T13:00:29Z

Updates SimplifyCFG to avoid jump threading through loop headers if -keep-loops is requested. Canonical loop form requires a loop header that dominates all blocks in the loop. If we thread through a header, we risk breaking its domination of the loop. This change avoids this issue by conservatively avoiding threading through headers entirely.

Fixes: #151144

Updates SimplifyCFG to avoid jump threading through loop headers if -keep-loops is requested. Canonical loop form requires a loop header that dominates all blocks in the loop. If we thread through a header, we risk breaking its domination of the loop. This change avoids this issue by conservatively avoiding threading through headers entirely.

github-actions · 2025-07-29T13:00:51Z

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llvmbot · 2025-07-29T13:01:21Z

@llvm/pr-subscribers-llvm-transforms

@llvm/pr-subscribers-backend-arm

Author: Arne Stenkrona (ArneStenkrona2)

Changes

Updates SimplifyCFG to avoid jump threading through loop headers if -keep-loops is requested. Canonical loop form requires a loop header that dominates all blocks in the loop. If we thread through a header, we risk breaking its domination of the loop. This change avoids this issue by conservatively avoiding threading through headers entirely.

Full diff: https://github.com/llvm/llvm-project/pull/151142.diff

7 Files Affected:

(modified) llvm/lib/Transforms/Utils/SimplifyCFG.cpp (+8-2)
(modified) llvm/test/CodeGen/ARM/2013-05-05-IfConvertBug.ll (+3-3)
(modified) llvm/test/Transforms/SimplifyCFG/2008-07-13-InfLoopMiscompile.ll (+1-2)
(added) llvm/test/Transforms/SimplifyCFG/2025-07-29-non-canoncial-loop.ll (+37)
(modified) llvm/test/Transforms/SimplifyCFG/branch-phi-thread.ll (+1-1)
(modified) llvm/test/Transforms/SimplifyCFG/jump-threading.ll (+1-1)
(modified) llvm/test/Transforms/SimplifyCFG/two-entry-phi-return.ll (+1-1)

diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 94b0ab892f2dd..d8385beb349d6 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -8030,8 +8030,14 @@ bool SimplifyCFGOpt::simplifyCondBranch(BranchInst *BI, IRBuilder<> &Builder) {
   // If this is a branch on something for which we know the constant value in
   // predecessors (e.g. a phi node in the current block), thread control
   // through this block.
-  if (foldCondBranchOnValueKnownInPredecessor(BI, DTU, DL, Options.AC))
-    return requestResimplify();
+  // Note: If BB is a loop header then there is a risk that threading introduces
+  // a non-canonical loop by moving a back edge. So we avoid this optimization
+  // for loop headers if NeedCanonicalLoop is set.
+  bool InHeader = !LoopHeaders.empty() && is_contained(LoopHeaders, BB);
+  bool AvoidThreading = Options.NeedCanonicalLoop && InHeader;
+  if (!AvoidThreading)
+    if (foldCondBranchOnValueKnownInPredecessor(BI, DTU, DL, Options.AC))
+      return requestResimplify();
 
   // Scan predecessor blocks for conditional branches.
   for (BasicBlock *Pred : predecessors(BB))
diff --git a/llvm/test/CodeGen/ARM/2013-05-05-IfConvertBug.ll b/llvm/test/CodeGen/ARM/2013-05-05-IfConvertBug.ll
index 344bb15d2a8b8..8f798fac06f54 100644
--- a/llvm/test/CodeGen/ARM/2013-05-05-IfConvertBug.ll
+++ b/llvm/test/CodeGen/ARM/2013-05-05-IfConvertBug.ll
@@ -1,7 +1,7 @@
 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
-; RUN: llc < %s -mtriple=thumbv7-apple-ios -mcpu=cortex-a8 | FileCheck %s
-; RUN: llc < %s -mtriple=thumbv8 | FileCheck -check-prefix=CHECK-V8 %s
-; RUN: llc < %s -mtriple=thumbv7 -arm-restrict-it | FileCheck -check-prefix=CHECK-RESTRICT-IT %s
+; RUN: llc -keep-loops="false" < %s -mtriple=thumbv7-apple-ios -mcpu=cortex-a8 | FileCheck %s
+; RUN: llc -keep-loops="false" < %s -mtriple=thumbv8 | FileCheck -check-prefix=CHECK-V8 %s
+; RUN: llc -keep-loops="false" < %s -mtriple=thumbv7 -arm-restrict-it | FileCheck -check-prefix=CHECK-RESTRICT-IT %s
 
 define i32 @t1(i32 %a, i32 %b, ptr %retaddr) {
 ; CHECK-LABEL: t1:
diff --git a/llvm/test/Transforms/SimplifyCFG/2008-07-13-InfLoopMiscompile.ll b/llvm/test/Transforms/SimplifyCFG/2008-07-13-InfLoopMiscompile.ll
index 2e9e7b19c73e2..44d92e1a1c210 100644
--- a/llvm/test/Transforms/SimplifyCFG/2008-07-13-InfLoopMiscompile.ll
+++ b/llvm/test/Transforms/SimplifyCFG/2008-07-13-InfLoopMiscompile.ll
@@ -1,5 +1,5 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
-; RUN: opt < %s -passes=simplifycfg -simplifycfg-require-and-preserve-domtree=1 -S | FileCheck %s
+; RUN: opt < %s -passes=simplifycfg -simplifycfg-require-and-preserve-domtree=1 -keep-loops="false" -S | FileCheck %s
 ; PR2540
 ; Outval should end up with a select from 0/2, not all constants.
 
@@ -52,4 +52,3 @@ func_1.exit:		; preds = %cowblock, %entry
 }
 
 declare i32 @printf(ptr, ...) nounwind
-
diff --git a/llvm/test/Transforms/SimplifyCFG/2025-07-29-non-canoncial-loop.ll b/llvm/test/Transforms/SimplifyCFG/2025-07-29-non-canoncial-loop.ll
new file mode 100644
index 0000000000000..a64108f6346b9
--- /dev/null
+++ b/llvm/test/Transforms/SimplifyCFG/2025-07-29-non-canoncial-loop.ll
@@ -0,0 +1,37 @@
+; RUN: opt < %s -passes=simplifycfg -simplifycfg-require-and-preserve-domtree=1 -S | FileCheck --check-prefix=NO-THREADING %s
+; Checks that we do not thread the control flow through the loop header bb1 as
+; that will introduce a non-canonical loop
+
+; NO-THREADING-LABEL: define void @__start
+; NO-THREADING: bb3:
+; NO-THREADING-NEXT: br i1 %cond, label %bb1, label %bb5
+
+; RUN: opt < %s -passes=simplifycfg -simplifycfg-require-and-preserve-domtree=1 --keep-loops="false" -S | FileCheck --check-prefix=THREADING %s
+; Checks that we thread the control flow through the loop header bb1 since we
+; do not request --keep-loops
+
+; THREADING-LABEL: define void @__start
+; THREADING: bb3:
+; THREADING-NEXT: br i1 %cond, label %bb4, label %bb5
+
+define void @__start(i1 %cond) {
+entry:
+  br label %bb1
+
+bb1:                                            ; preds = %bb3, %entry
+  br i1 %cond, label %bb4, label %bb2
+
+bb2:                                            ; preds = %bb1
+  %_0_ = add i16 0, 0
+  br label %bb3
+
+bb3:                                            ; preds = %bb4, %bb2
+  br i1 %cond, label %bb1, label %bb5
+
+bb4:                                            ; preds = %bb1
+  %_1_ = add i32 0, 1
+  br label %bb3
+
+bb5:                                            ; preds = %bb3
+  ret void
+}
diff --git a/llvm/test/Transforms/SimplifyCFG/branch-phi-thread.ll b/llvm/test/Transforms/SimplifyCFG/branch-phi-thread.ll
index 0afec05ecbd6a..ec9423bd81675 100644
--- a/llvm/test/Transforms/SimplifyCFG/branch-phi-thread.ll
+++ b/llvm/test/Transforms/SimplifyCFG/branch-phi-thread.ll
@@ -1,5 +1,5 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
-; RUN: opt < %s -passes=simplifycfg,adce -simplifycfg-require-and-preserve-domtree=1 -S | FileCheck %s
+; RUN: opt < %s -passes=simplifycfg,adce -simplifycfg-require-and-preserve-domtree=1 -keep-loops="false" -S | FileCheck %s
 
 declare void @f1()
 
diff --git a/llvm/test/Transforms/SimplifyCFG/jump-threading.ll b/llvm/test/Transforms/SimplifyCFG/jump-threading.ll
index 50a32413a0551..a4073ae6eb0b4 100644
--- a/llvm/test/Transforms/SimplifyCFG/jump-threading.ll
+++ b/llvm/test/Transforms/SimplifyCFG/jump-threading.ll
@@ -1,5 +1,5 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
-; RUN: opt -S -passes=simplifycfg < %s | FileCheck %s
+; RUN: opt -S -passes=simplifycfg -keep-loops="false" < %s | FileCheck %s
 
 declare void @foo()
 declare void @bar()
diff --git a/llvm/test/Transforms/SimplifyCFG/two-entry-phi-return.ll b/llvm/test/Transforms/SimplifyCFG/two-entry-phi-return.ll
index 57930c91b9796..f6d71ddda74fe 100644
--- a/llvm/test/Transforms/SimplifyCFG/two-entry-phi-return.ll
+++ b/llvm/test/Transforms/SimplifyCFG/two-entry-phi-return.ll
@@ -1,5 +1,5 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
-; RUN: opt < %s -passes=simplifycfg -simplifycfg-require-and-preserve-domtree=1 -S | FileCheck %s
+; RUN: opt < %s -passes=simplifycfg -simplifycfg-require-and-preserve-domtree=1 -keep-loops="false" -S | FileCheck %s
 
 define i1 @qux(ptr %m, ptr %n, ptr %o, ptr %p) nounwind  {
 ; CHECK-LABEL: @qux(

ArneStenkrona2 · 2025-07-29T13:10:02Z

@nikic I didn't see a specific secction for SimplifyCFG in Maintainers.md. Perhaps either you, or someone you think is appropriate, can take a look at this.
I've opened a corresponding issue as well here #151144
Thanks :)

nikic · 2025-07-29T13:31:28Z

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

+  bool InHeader = !LoopHeaders.empty() && is_contained(LoopHeaders, BB);
+  bool AvoidThreading = Options.NeedCanonicalLoop && InHeader;
+  if (!AvoidThreading)
+    if (foldCondBranchOnValueKnownInPredecessor(BI, DTU, DL, Options.AC))


I think it would be cleaner to move these checks in foldCondBranchOnValueKnownInPredecessor.

Agreed. I changed foldCondBranchOnValueKnownInPredecessor to a method of SimplifyCFGOpt and moved the checks inside it.

nikic · 2025-07-29T13:32:11Z

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

+  // Note: If BB is a loop header then there is a risk that threading introduces
+  // a non-canonical loop by moving a back edge. So we avoid this optimization
+  // for loop headers if NeedCanonicalLoop is set.
+  bool InHeader = !LoopHeaders.empty() && is_contained(LoopHeaders, BB);


The empty check is redundant?

Noted. Removed.

nikic · 2025-07-29T13:33:11Z

llvm/test/Transforms/SimplifyCFG/2025-07-29-non-canoncial-loop.ll

@@ -0,0 +1,37 @@
+; RUN: opt < %s -passes=simplifycfg -simplifycfg-require-and-preserve-domtree=1 -S | FileCheck --check-prefix=NO-THREADING %s


Please use update_test_checks.py.

nikic

This generally sounds like a sensible thing to do. The dedicated JumpThreading pass checks this as well.

nikic · 2025-07-29T15:25:49Z

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

-                                                    DomTreeUpdater *DTU,
-                                                    const DataLayout &DL,
-                                                    AssumptionCache *AC) {
+#include <iostream>


Stray iostream?

Yes, sorry about that. Local debugging. Will remove right away.

nikic · 2025-07-29T15:25:58Z

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

+  // for loop headers if NeedCanonicalLoop is set.
+  bool InHeader = is_contained(LoopHeaders, BI->getParent());
+  bool AvoidThreading = Options.NeedCanonicalLoop && InHeader;
+  if (AvoidThreading) {


Omit braces for one-line if.

nikic · 2025-07-29T15:26:57Z

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

+  // Note: If BB is a loop header then there is a risk that threading introduces
+  // a non-canonical loop by moving a back edge. So we avoid this optimization
+  // for loop headers if NeedCanonicalLoop is set.
+  bool InHeader = is_contained(LoopHeaders, BI->getParent());


Avoid the variable and move this behind the && for short-circuiting. No need to do this linear scan if !NeedCanonicalLoop.

github-actions · 2025-07-29T15:27:07Z

✅ With the latest revision this PR passed the C/C++ code formatter.

ArneStenkrona2 · 2025-08-04T08:33:39Z

@nikic Let me know if there is anything else you'd like to see changed with the PR.

llvmbot added backend:ARM llvm:transforms labels Jul 29, 2025

ArneStenkrona2 mentioned this pull request Jul 29, 2025

SimplifyCFG breaks canonical loops even if --keep-loops is requested #151144

Open

nikic reviewed Jul 29, 2025

View reviewed changes

nikic mentioned this pull request Jul 29, 2025

Task submission dtcxzyw/llvm-opt-benchmark#1312

Open

zyw-bot mentioned this pull request Jul 29, 2025

pre-commit: PR151142 dtcxzyw/llvm-opt-benchmark#2618

Closed

nikic reviewed Jul 29, 2025

View reviewed changes

Move threading check, run update_test_checks.py

e6c9227

nikic reviewed Jul 29, 2025

View reviewed changes

Formatting, removing stray <iostream> include

b082c08

		@@ -0,0 +1,37 @@
		; RUN: opt < %s -passes=simplifycfg -simplifycfg-require-and-preserve-domtree=1 -S \| FileCheck --check-prefix=NO-THREADING %s

[SimplifyCFG] Avoid threading for loop headers #151142

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