[DebugInfo][LCSSA] Rewrite pre-existing debug values outside loop

Summary:
Extend LCSSA so that debug values outside loops are rewritten to
use the PHI nodes that the pass creates.

This fixes PR39019. In that case, we ran LCSSA on a loop that
was later on vectorized, which left us with something like this:

  for.cond.cleanup:
    %add.lcssa = phi i32 [ %add, %for.body ], [ %34, %middle.block ]
    call void @llvm.dbg.value(metadata i32 %add,
    ret i32 %add.lcssa

  for.body:
    %add =
    [...]
    br i1 %exitcond, label %for.cond.cleanup, label %for.body

which later resulted in the debug.value becoming undef when
removing the scalar loop (and the location would have probably
been wrong for the vectorized case otherwise).

As we now may need to query the AvailableVals cache more than
once for a basic block, FindAvailableVals() in SSAUpdaterImpl is
changed so that it updates the cache for blocks that we do not
create a PHI node for, regardless of the block's number of
predecessors. The debug value in the attached IR reproducer
would not be properly rewritten without this.

Debug values residing in blocks where we have not inserted any
PHI nodes are currently left as-is by this patch. I'm not sure
what should be done with those uses.

Reviewers: mattd, aprantl, vsk, probinson

Reviewed By: mattd, aprantl

Subscribers: jmorse, gbedwell, JDevlieghere, llvm-commits

Differential Revision: https://reviews.llvm.org/D53130

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@344589 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/include/llvm/Transforms/Utils/SSAUpdater.h b/include/llvm/Transforms/Utils/SSAUpdater.h
index 4a79116..d02607a 100644 (file)
--- a/include/llvm/Transforms/Utils/SSAUpdater.h
+++ b/include/llvm/Transforms/Utils/SSAUpdater.h
@@ -76,6 +76,10 @@ public:
   /// block.
   bool HasValueForBlock(BasicBlock *BB) const;
 
+  /// Return the value for the specified block if the SSAUpdater has one,
+  /// otherwise return nullptr.
+  Value *FindValueForBlock(BasicBlock *BB) const;
+
   /// Construct SSA form, materializing a value that is live at the end
   /// of the specified block.
   Value *GetValueAtEndOfBlock(BasicBlock *BB);

diff --git a/include/llvm/Transforms/Utils/SSAUpdaterImpl.h b/include/llvm/Transforms/Utils/SSAUpdaterImpl.h
index b7649ba..cab0f3e 100644 (file)
--- a/include/llvm/Transforms/Utils/SSAUpdaterImpl.h
+++ b/include/llvm/Transforms/Utils/SSAUpdaterImpl.h
@@ -357,10 +357,9 @@ public:
       BBInfo *Info = *I;
 
       if (Info->DefBB != Info) {
-        // Record the available value at join nodes to speed up subsequent
-        // uses of this SSAUpdater for the same value.
-        if (Info->NumPreds > 1)
-          (*AvailableVals)[Info->BB] = Info->DefBB->AvailableVal;
+        // Record the available value to speed up subsequent uses of this
+        // SSAUpdater for the same value.
+        (*AvailableVals)[Info->BB] = Info->DefBB->AvailableVal;
         continue;
       }
 

diff --git a/lib/Transforms/Utils/LCSSA.cpp b/lib/Transforms/Utils/LCSSA.cpp
index a1f8e74..53d444b 100644 (file)
--- a/lib/Transforms/Utils/LCSSA.cpp
+++ b/lib/Transforms/Utils/LCSSA.cpp
@@ -41,6 +41,7 @@
 #include "llvm/IR/Dominators.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/Instructions.h"
+#include "llvm/IR/IntrinsicInst.h"
 #include "llvm/IR/PredIteratorCache.h"
 #include "llvm/Pass.h"
 #include "llvm/Transforms/Utils.h"
@@ -201,6 +202,21 @@ bool llvm::formLCSSAForInstructions(SmallVectorImpl<Instruction *> &Worklist,
       SSAUpdate.RewriteUse(*UseToRewrite);
     }
 
+    SmallVector<DbgValueInst *, 4> DbgValues;
+    llvm::findDbgValues(DbgValues, I);
+
+    // Update pre-existing debug value uses that reside outside the loop.
+    auto &Ctx = I->getContext();
+    for (auto DVI : DbgValues) {
+      BasicBlock *UserBB = DVI->getParent();
+      if (InstBB == UserBB || L->contains(UserBB))
+        continue;
+      // We currently only handle debug values residing in blocks where we have
+      // inserted a PHI instruction.
+      if (Value *V = SSAUpdate.FindValueForBlock(UserBB))
+        DVI->setOperand(0, MetadataAsValue::get(Ctx, ValueAsMetadata::get(V)));
+    }
+
     // SSAUpdater might have inserted phi-nodes inside other loops. We'll need
     // to post-process them to keep LCSSA form.
     for (PHINode *InsertedPN : InsertedPHIs) {

diff --git a/lib/Transforms/Utils/SSAUpdater.cpp b/lib/Transforms/Utils/SSAUpdater.cpp
index 4a1fd8d..9e5fb0e 100644 (file)
--- a/lib/Transforms/Utils/SSAUpdater.cpp
+++ b/lib/Transforms/Utils/SSAUpdater.cpp
@@ -64,6 +64,11 @@ bool SSAUpdater::HasValueForBlock(BasicBlock *BB) const {
   return getAvailableVals(AV).count(BB);
 }
 
+Value *SSAUpdater::FindValueForBlock(BasicBlock *BB) const {
+  AvailableValsTy::iterator AVI = getAvailableVals(AV).find(BB);
+  return (AVI != getAvailableVals(AV).end()) ? AVI->second : nullptr;
+}
+
 void SSAUpdater::AddAvailableValue(BasicBlock *BB, Value *V) {
   assert(ProtoType && "Need to initialize SSAUpdater");
   assert(ProtoType == V->getType() &&

diff --git a/test/Transforms/LCSSA/rewrite-existing-dbg-values.ll b/test/Transforms/LCSSA/rewrite-existing-dbg-values.ll
new file mode 100644 (file)
index 0000000..563a75f
--- /dev/null
+++ b/test/Transforms/LCSSA/rewrite-existing-dbg-values.ll
@@ -0,0 +1,69 @@
+; RUN: opt -S -lcssa < %s | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; Reproducer for PR39019.
+;
+; Verify that the llvm.dbg.value in the %for.cond.cleanup2 block is rewritten
+; to use the PHI node for %add that is created by LCSSA.
+
+; CHECK-LABEL: for.cond.cleanup2:
+; CHECK-NEXT: [[PN:%[^ ]*]] = phi i32 [ %add.lcssa, %for.cond.cleanup1 ]
+; CHECK-NEXT: call void @llvm.dbg.value(metadata i32 [[PN]], metadata [[VAR:![0-9]+]], metadata !DIExpression())
+; CHECK-NEXT: call void @bar(i32 [[PN]])
+
+; CHECK-LABEL: for.body:
+; CHECK: %add = add nsw i32 0, 2
+; CHECK: call void @llvm.dbg.value(metadata i32 %add, metadata [[VAR]], metadata !DIExpression())
+
+; CHECK: [[VAR]] = !DILocalVariable(name: "sum",
+
+; Function Attrs: nounwind
+define void @foo() #0 !dbg !6 {
+entry:
+  br label %for.cond.preheader, !dbg !12
+
+for.cond.preheader:                               ; preds = %for.cond.cleanup1, %entry
+  br label %for.body, !dbg !12
+
+for.cond.cleanup2:                                ; preds = %for.cond.cleanup1
+  call void @llvm.dbg.value(metadata i32 %add, metadata !9, metadata !DIExpression()), !dbg !12
+  tail call void @bar(i32 %add) #0, !dbg !12
+  ret void, !dbg !12
+
+for.cond.cleanup1:                                ; preds = %for.body
+  br i1 false, label %for.cond.preheader, label %for.cond.cleanup2, !dbg !12
+
+for.body:                                         ; preds = %for.body, %for.cond.preheader
+  %add = add nsw i32 0, 2, !dbg !12
+  call void @llvm.dbg.value(metadata i32 %add, metadata !9, metadata !DIExpression()), !dbg !12
+  br i1 false, label %for.body, label %for.cond.cleanup1, !dbg !12
+}
+
+; Function Attrs: nounwind
+declare void @bar(i32) #0
+
+; Function Attrs: nounwind readnone speculatable
+declare void @llvm.dbg.value(metadata, metadata, metadata) #1
+
+attributes #0 = { nounwind }
+attributes #1 = { nounwind readnone speculatable }
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4}
+!llvm.ident = !{!5}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 8.0.0", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, globals: !2, nameTableKind: None)
+!1 = !DIFile(filename: "foo.c", directory: "/")
+!2 = !{}
+!3 = !{i32 2, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{!"clang version 8.0.0"}
+!6 = distinct !DISubprogram(name: "foo", scope: !1, file: !1, line: 10, type: !7, isLocal: false, isDefinition: true, scopeLine: 10, isOptimized: true, unit: !0, retainedNodes: !8)
+!7 = !DISubroutineType(types: !2)
+!8 = !{!9}
+!9 = !DILocalVariable(name: "sum", scope: !10, file: !1, line: 11, type: !11)
+!10 = !DILexicalBlockFile(scope: !6, file: !1, discriminator: 0)
+!11 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!12 = !DILocation(line: 0, scope: !10)