[llvm] Only guard loop metadata that has non-debug info in it (PR #118825)

[Dominik Steenken via llvm-commits]

https://github.com/dominik-steenken updated https://github.com/llvm/llvm-project/pull/118825

>From db64757fc1c83b2273488ad8a0e5adbe49d4a620 Mon Sep 17 00:00:00 2001
From: Dominik Steenken <dost at de.ibm.com>
Date: Mon, 25 Nov 2024 14:30:01 +0100
Subject: [PATCH] Only guard loop metadata that has non-debug info in it

---
 llvm/include/llvm/IR/Instruction.h  |  4 ++++
 llvm/lib/IR/Instruction.cpp         | 25 +++++++++++++++++++++++++
 llvm/lib/Transforms/Utils/Local.cpp |  2 +-
 3 files changed, 30 insertions(+), 1 deletion(-)

diff --git a/llvm/include/llvm/IR/Instruction.h b/llvm/include/llvm/IR/Instruction.h
index 730baa8cc00520..ea3d93d856efb7 100644
--- a/llvm/include/llvm/IR/Instruction.h
+++ b/llvm/include/llvm/IR/Instruction.h
@@ -367,6 +367,10 @@ class Instruction : public User,
   /// Return true if this instruction has any metadata attached to it.
   bool hasMetadata() const { return DbgLoc || Value::hasMetadata(); }
 
+  // Return true if this instruction contains loop metadata other than
+  // a debug location
+  bool hasLoopMetadataOtherThanDebugLoc() const;
+
   /// Return true if this instruction has metadata attached to it other than a
   /// debug location.
   bool hasMetadataOtherThanDebugLoc() const { return Value::hasMetadata(); }
diff --git a/llvm/lib/IR/Instruction.cpp b/llvm/lib/IR/Instruction.cpp
index 0137bb281e7ec5..6c19712481ca9a 100644
--- a/llvm/lib/IR/Instruction.cpp
+++ b/llvm/lib/IR/Instruction.cpp
@@ -12,6 +12,7 @@
 
 #include "llvm/IR/Instruction.h"
 #include "llvm/ADT/DenseSet.h"
+#include "llvm/ADT/STLExtras.h"
 #include "llvm/IR/AttributeMask.h"
 #include "llvm/IR/Attributes.h"
 #include "llvm/IR/Constants.h"
@@ -19,6 +20,7 @@
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/IntrinsicInst.h"
 #include "llvm/IR/Intrinsics.h"
+#include "llvm/IR/LLVMContext.h"
 #include "llvm/IR/MemoryModelRelaxationAnnotations.h"
 #include "llvm/IR/Module.h"
 #include "llvm/IR/Operator.h"
@@ -461,6 +463,29 @@ bool Instruction::hasPoisonGeneratingMetadata() const {
          hasMetadata(LLVMContext::MD_align);
 }
 
+bool Instruction::hasLoopMetadataOtherThanDebugLoc() const {
+  // If there is no loop metadata at all, we also don't have
+  // non-debug loop metadata, obviously.
+  if  (!hasMetadata(LLVMContext::MD_loop))
+    return false;
+
+  // If we do have loop metadata, retrieve it.
+  MDNode * LoopMD = getMetadata(LLVMContext::MD_loop);
+
+  // Check if the existing operands are debug locations. This loop
+  // should terminate after at most three iterations. Skip
+  // the first item because it is a self-reference.
+  for (const MDOperand& Op : llvm::drop_begin(LoopMD->operands())) {
+    // check for debug location type by attempting a cast.
+    if (!dyn_cast<DILocation>(Op)) {
+      return true;
+    }
+  }
+
+  // If we get here, then all we have is debug locations in the loop metadata.
+  return false;
+}
+
 void Instruction::dropPoisonGeneratingMetadata() {
   eraseMetadata(LLVMContext::MD_range);
   eraseMetadata(LLVMContext::MD_nonnull);
diff --git a/llvm/lib/Transforms/Utils/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index cdc3f0308fe59c..876017e9051067 100644
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/llvm/lib/Transforms/Utils/Local.cpp
@@ -1279,7 +1279,7 @@ bool llvm::TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB,
   // |    for.body <---- (md2)
   // |_______|  |______|
   if (Instruction *TI = BB->getTerminator())
-    if (TI->hasMetadata(LLVMContext::MD_loop))
+    if (TI->hasLoopMetadataOtherThanDebugLoc())
       for (BasicBlock *Pred : predecessors(BB))
         if (Instruction *PredTI = Pred->getTerminator())
           if (PredTI->hasMetadata(LLVMContext::MD_loop))