[Transforms] Allow SimplifyCFG to fold loop latches with debug-only metadata

The transform TryToSimplifyUncondBranchFromEmptyBlock in SimplifyCFG contains
a guard that is intended to prevent loop latches of nested loops that might
have relevant loop metadata attached to them from being folded away, thus
potentially destroying the metadata of the predecessor loop latch.

This guard overprotects loop latches though, as in the case of compiling
with debug information, loop metadata can be created that only contains
debug location information, and doesn't carry any design intent.

This commit changes the guard in such a way that metadata like this no longer
protects against the loop latch from being folded away.

The debug-only loop metadata is dropped in such a case.

Author: dominik-steenken

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}