[ARM] Allow findLoopPreheader to return headers with multiple loop successors

The findLoopPreheader function will currently not find a preheader if it
branches to multiple different loop headers. This patch adds an option
to relax that, allowing ARMLowOverheadLoops to process more loops
successfully. This helps with WhileLoopStart setup instructions that can
branch/fallthrough to the low overhead loop and to branch to a separate
loop from the same preheader (but I don't believe it is possible for
both loops to be low overhead loops).

Differential Revision: https://reviews.llvm.org/D102747

1 files changed, 10 insertions, 8 deletions

diff --git a/llvm/lib/CodeGen/MachineLoopInfo.cpp b/llvm/lib/CodeGen/MachineLoopInfo.cpp
index 78480d0..8f91a5b 100644
--- a/llvm/lib/CodeGen/MachineLoopInfo.cpp
+++ b/llvm/lib/CodeGen/MachineLoopInfo.cpp
@@ -115,8 +115,8 @@ DebugLoc MachineLoop::getStartLoc() const {
 }
 
 MachineBasicBlock *
-MachineLoopInfo::findLoopPreheader(MachineLoop *L,
-                                   bool SpeculativePreheader) const {
+MachineLoopInfo::findLoopPreheader(MachineLoop *L, bool SpeculativePreheader,
+                                   bool FindMultiLoopPreheader) const {
   if (MachineBasicBlock *PB = L->getLoopPreheader())
     return PB;
 
@@ -139,12 +139,14 @@ MachineLoopInfo::findLoopPreheader(MachineLoop *L,
 
   // Check if the preheader candidate is a successor of any other loop
   // headers. We want to avoid having two loop setups in the same block.
-  for (MachineBasicBlock *S : Preheader->successors()) {
-    if (S == HB)
-      continue;
-    MachineLoop *T = getLoopFor(S);
-    if (T && T->getHeader() == S)
-      return nullptr;
+  if (!FindMultiLoopPreheader) {
+    for (MachineBasicBlock *S : Preheader->successors()) {
+      if (S == HB)
+        continue;
+      MachineLoop *T = getLoopFor(S);
+      if (T && T->getHeader() == S)
+        return nullptr;
+    }
   }
   return Preheader;
 }