[llvm] [SimplifyCFG] Avoid threading for loop headers (PR #151142)
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Tue Jul 29 06:01:27 PDT 2025
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-llvm-transforms
@llvm/pr-subscribers-backend-arm
Author: Arne Stenkrona (ArneStenkrona2)
<details>
<summary>Changes</summary>
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
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(
``````````
</details>
https://github.com/llvm/llvm-project/pull/151142
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