Only guard loop metadata that has non-debug info in it

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 hasNonDebugLocLoopMetadata() const;
+
   /// Return true if this instruction has metadata attached to it other than a
   /// debug location.
   bool hasMetadataOtherThanDebugLoc() const { return Value::hasMetadata(); }

llvm/lib/IR/Instruction.cpp

@@ -12,13 +12,15 @@
 
 #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"
 #include "llvm/IR/InstrTypes.h"
 #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::hasNonDebugLocLoopMetadata() 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);

llvm/lib/Transforms/Utils/Local.cpp

@@ -1279,10 +1279,10 @@ bool llvm::TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB,
   // |    for.body <---- (md2)
   // |_______|  |______|
   if (Instruction *TI = BB->getTerminator())
-    if (TI->hasMetadata(LLVMContext::MD_loop))
+    if (TI->hasNonDebugLocLoopMetadata())
       for (BasicBlock *Pred : predecessors(BB))
         if (Instruction *PredTI = Pred->getTerminator())
-          if (PredTI->hasMetadata(LLVMContext::MD_loop))
+          if (PredTI->hasNonDebugLocLoopMetadata())
             return false;
 
   if (BBKillable)
@@ -1345,12 +1345,15 @@ bool llvm::TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB,
     }
   }
 
-  // If the unconditional branch we replaced contains llvm.loop metadata, we
-  // add the metadata to the branch instructions in the predecessors.
+  // If the unconditional branch we replaced contains non-debug llvm.loop
+  // metadata, we add the metadata to the branch instructions in the
+  // predecessors.
   if (Instruction *TI = BB->getTerminator())
-    if (MDNode *LoopMD = TI->getMetadata(LLVMContext::MD_loop))
+    if (TI->hasNonDebugLocLoopMetadata()) {
+      MDNode *LoopMD = TI->getMetadata(LLVMContext::MD_loop);
       for (BasicBlock *Pred : predecessors(BB))
         Pred->getTerminator()->setMetadata(LLVMContext::MD_loop, LoopMD);
+    }
 
   if (BBKillable) {
     // Everything that jumped to BB now goes to Succ.

llvm/test/Transforms/SimplifyCFG/preserve-llvm-loop-metadata.ll

@@ -28,7 +28,7 @@ if.then:                                          ; preds = %while.body
   br label %if.end
 
 ; CHECK: if.then:
-; CHECK:  br label %while.cond, !llvm.loop !0
+; CHECK:  br label %while.cond, !llvm.loop !1
 
 if.else:                                          ; preds = %while.body
   %4 = load i32, ptr %count, align 4
@@ -37,10 +37,10 @@ if.else:                                          ; preds = %while.body
   br label %if.end
 
 ; CHECK: if.else:
-; CHECK:  br label %while.cond, !llvm.loop !0
+; CHECK:  br label %while.cond, !llvm.loop !1
 
 if.end:                                           ; preds = %if.else, %if.then
-  br label %while.cond, !llvm.loop !0
+  br label %while.cond, !llvm.loop !1
 
 while.end:                                        ; preds = %while.cond
   ret void
@@ -74,7 +74,7 @@ entry:
   br label %while.cond
 
 while.cond.loopexit:                              ; preds = %for.body
-  br label %while.cond, !llvm.loop !2
+  br label %while.cond, !llvm.loop !3
 
 while.cond:                                       ; preds = %while.cond.loopexit, %entry
   %i.0 = phi i32 [ %a, %entry ], [ %add, %while.cond.loopexit ]
@@ -96,22 +96,74 @@ for.body:                                         ; preds = %while.body, %for.bo
   %1 = tail call i32 asm sideeffect "add ${0:w}, ${1:w}\0A", "=r,r,~{cc}"(i32 %0)
   %inc = add nuw nsw i32 %k.07, 1
   %cmp1 = icmp ult i32 %inc, 5
-  br i1 %cmp1, label %for.body, label %while.cond.loopexit, !llvm.loop !4
+  br i1 %cmp1, label %for.body, label %while.cond.loopexit, !llvm.loop !5
 
 while.end:                                        ; preds = %while.cond
   %sum.0.lcssa = phi i32 [ %sum.0, %while.cond ]
   ret i32 %sum.0.lcssa
 }
 
-!0 = distinct !{!0, !1}
-!1 = !{!"llvm.loop.distribute.enable", i1 true}
-!2 = distinct !{!2, !3}
-!3 = !{!"llvm.loop.mustprogress"}
-!4 = distinct !{!4, !3, !5}
-!5 = !{!"llvm.loop.unroll.enable"}
-; CHECK: !0 = distinct !{!0, !1}
-; CHECK: !1 = !{!"llvm.loop.distribute.enable", i1 true}
-; CHECK: !2 = distinct !{!2, !3}
-; CHECK: !3 = !{!"llvm.loop.mustprogress"}
-; CHECK: !4 = distinct !{!4, !3, !5}
-; CHECK: !5 = !{!"llvm.loop.unroll.enable"}
+; Test that the condition tested above does not trigger when the loop metadata consists only of debug locations,
+; i.e.the empty loop latch `while.cond.loopexit` *will* be folded into its successor if its
+; predecessor blocks are also loop latches and any loop metadata attached to it consists of debug information.
+;
+define i32 @test3(i32 %a, i32 %b, i32 %step, i32 %remainder, ptr %input) !dbg !7 {
+entry:
+  br label %while.cond
+
+;CHECK-LABEL: @test3( 
+;CHECK-NOT: while.cond.loopexit
+while.cond.loopexit:                              ; preds = %for.body
+  br label %while.cond, !llvm.loop !10
+
+while.cond:                                       ; preds = %while.cond.loopexit, %entry
+  %i.0 = phi i32 [ %a, %entry ], [ %add, %while.cond.loopexit ]
+  %sum.0 = phi i32 [ 0, %entry ], [ %1, %while.cond.loopexit ]
+  %sub = sub nsw i32 %b, %i.0
+  %cmp = icmp sgt i32 %sub, %remainder
+  br i1 %cmp, label %while.body, label %while.end
+
+while.body:                                       ; preds = %while.cond
+  %add = add nsw i32 %i.0, %step
+  br label %for.body
+
+for.body:                                         ; preds = %while.body, %for.body
+  %k.07 = phi i32 [ 0, %while.body ], [ %inc, %for.body ]
+  %add2 = add nsw i32 %k.07, %add
+  %idxprom = sext i32 %add2 to i64
+  %arrayidx = getelementptr inbounds i32, ptr %input, i64 %idxprom
+  %0 = load i32, ptr %arrayidx, align 4
+  %1 = tail call i32 asm sideeffect "add ${0:w}, ${1:w}\0A", "=r,r,~{cc}"(i32 %0)
+  %inc = add nuw nsw i32 %k.07, 1
+  %cmp1 = icmp ult i32 %inc, 5
+  br i1 %cmp1, label %for.body, label %while.cond.loopexit, !llvm.loop !5
+
+while.end:                                        ; preds = %while.cond
+  %sum.0.lcssa = phi i32 [ %sum.0, %while.cond ]
+  ret i32 %sum.0.lcssa
+}
+
+!llvm.module.flags = !{!0}
+
+!0 = !{i32 2, !"Debug Info Version", i32 3}
+!1 = distinct !{!1, !2}
+!2 = !{!"llvm.loop.distribute.enable", i1 true}
+!3 = distinct !{!3, !4}
+!4 = !{!"llvm.loop.mustprogress"}
+!5 = distinct !{!5, !4, !6}
+!6 = !{!"llvm.loop.unroll.enable"}
+!7 = distinct !DISubprogram(name: "test3", scope: !8, file: !8, spFlags: DISPFlagDefinition, unit: !9)
+!8 = !DIFile(filename: "preserve-llvm-loop-metadata.ll", directory: "/")
+!9 = distinct !DICompileUnit(language: DW_LANG_C99, file: !8, isOptimized: false, runtimeVersion: 0, emissionKind: NoDebug)
+!10 = distinct !{!10, !11, !13}
+!11 = !DILocation(line: 8, column: 4, scope: !12)
+!12 = distinct !DILexicalBlock(scope: !7, file: !8, line: 8, column: 2)
+!13 = !DILocation(line: 9, column: 23, scope: !12)
+
+; CHECK: !1 = distinct !{!1, !2}
+; CHECK: !2 = !{!"llvm.loop.distribute.enable", i1 true}
+; CHECK: !3 = distinct !{!3, !4}
+; CHECK: !4 = !{!"llvm.loop.mustprogress"}
+; CHECK: !5 = distinct !{!5, !4, !6}
+; CHECK: !6 = !{!"llvm.loop.unroll.enable"}
+; CHECK-NOT: !10 = distinct !{!10, !11, !13}