Revert "[SimplifyCFG] Avoid threading for loop headers (#151142)"

This reverts commit ea2f539.

Author: alexfh

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

@@ -291,7 +291,6 @@ class SimplifyCFGOpt {
   bool simplifyBranch(BranchInst *Branch, IRBuilder<> &Builder);
   bool simplifyUncondBranch(BranchInst *BI, IRBuilder<> &Builder);
   bool simplifyCondBranch(BranchInst *BI, IRBuilder<> &Builder);
-  bool foldCondBranchOnValueKnownInPredecessor(BranchInst *BI);
 
   bool tryToSimplifyUncondBranchWithICmpInIt(ICmpInst *ICI,
                                              IRBuilder<> &Builder);
@@ -3690,19 +3689,15 @@ foldCondBranchOnValueKnownInPredecessorImpl(BranchInst *BI, DomTreeUpdater *DTU,
   return false;
 }
 
-bool SimplifyCFGOpt::foldCondBranchOnValueKnownInPredecessor(BranchInst *BI) {
-  // 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.
-  if (Options.NeedCanonicalLoop && is_contained(LoopHeaders, BI->getParent()))
-    return false;
-
+static bool foldCondBranchOnValueKnownInPredecessor(BranchInst *BI,
+                                                    DomTreeUpdater *DTU,
+                                                    const DataLayout &DL,
+                                                    AssumptionCache *AC) {
   std::optional<bool> Result;
   bool EverChanged = false;
   do {
     // Note that None means "we changed things, but recurse further."
-    Result =
-        foldCondBranchOnValueKnownInPredecessorImpl(BI, DTU, DL, Options.AC);
+    Result = foldCondBranchOnValueKnownInPredecessorImpl(BI, DTU, DL, AC);
     EverChanged |= Result == std::nullopt || *Result;
   } while (Result == std::nullopt);
   return EverChanged;
@@ -8123,7 +8118,7 @@ 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))
+  if (foldCondBranchOnValueKnownInPredecessor(BI, DTU, DL, Options.AC))
     return requestResimplify();
 
   // Scan predecessor blocks for conditional branches.

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 -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
+; 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
 
 define i32 @t1(i32 %a, i32 %b, ptr %retaddr) {
 ; CHECK-LABEL: t1:

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 -keep-loops="false" -S | FileCheck %s
+; RUN: opt < %s -passes=simplifycfg -simplifycfg-require-and-preserve-domtree=1 -S | FileCheck %s
 ; PR2540
 ; Outval should end up with a select from 0/2, not all constants.
 
@@ -52,3 +52,4 @@ func_1.exit:		; preds = %cowblock, %entry
 }
 
 declare i32 @printf(ptr, ...) nounwind
+

llvm/test/Transforms/SimplifyCFG/2025-07-29-non-canoncial-loop.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 -keep-loops="false" -S | FileCheck %s
+; RUN: opt < %s -passes=simplifycfg,adce -simplifycfg-require-and-preserve-domtree=1 -S | FileCheck %s
 
 declare void @f1()
 

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 -keep-loops="false" < %s | FileCheck %s
+; RUN: opt -S -passes=simplifycfg < %s | FileCheck %s
 
 declare void @foo()
 declare void @bar()

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 -simplifycfg-require-and-preserve-domtree=1 -keep-loops="false" -S | FileCheck %s
+; RUN: opt < %s -passes=simplifycfg -simplifycfg-require-and-preserve-domtree=1 -S | FileCheck %s
 
 define i1 @qux(ptr %m, ptr %n, ptr %o, ptr %p) nounwind  {
 ; CHECK-LABEL: @qux(