[DebugInfo] Nerf placeDbgValues, with prejudice

CodeGenPrepare::placeDebugValues moves variable location intrinsics to be
immediately after the Value they refer to. This makes tracking of locations
very easy; but it changes the order in which assignments appear to the
debugger, from the source programs order to the order in which the
optimised program computes values. This then leads to PR43986 and PR38754,
where variable locations that were in a conditional block are made
unconditional, which is highly misleading.

This patch adjusts placeDbgValues to only re-order variable location
intrinsics if they use a Value before it is defined, significantly reducing
the damage that it does. This is still not 100% safe, but the rest of
CodeGenPrepare needs polishing to correctly update debug info when
optimisations are performed to fully fix this.

This will probably break downstream debuginfo tests -- if the
instruction-stream position of variable location changes isn't the focus of
the test, an easy fix should be to manually apply placeDbgValues' behaviour
to the failing tests, moving dbg.value intrinsics next to SSA variable
definitions thus:

  %foo = inst1
  %bar = ...
  %baz = ...
  void call @llvm.dbg.value(metadata i32 %foo, ...

to

  %foo = inst1
  void call @llvm.dbg.value(metadata i32 %foo, ...
  %bar = ...
  %baz = ...

This should return your test to exercising whatever it was testing before.

Differential Revision: https://reviews.llvm.org/D58453

1 files changed, 31 insertions, 27 deletions

diff --git a/llvm/lib/CodeGen/CodeGenPrepare.cpp b/llvm/lib/CodeGen/CodeGenPrepare.cpp
index a041808199d..a683fcf939d 100644
--- a/llvm/lib/CodeGen/CodeGenPrepare.cpp
+++ b/llvm/lib/CodeGen/CodeGenPrepare.cpp
@@ -7230,42 +7230,46 @@ bool CodeGenPrepare::fixupDbgValue(Instruction *I) {
   return false;
 }
 
-// llvm.dbg.value is far away from the value then iSel may not be able
-// handle it properly. iSel will drop llvm.dbg.value if it can not
-// find a node corresponding to the value.
+// A llvm.dbg.value may be using a value before its definition, due to
+// optimizations in this pass and others. Scan for such dbg.values, and rescue
+// them by moving the dbg.value to immediately after the value definition.
+// FIXME: Ideally this should never be necessary, and this has the potential
+// to re-order dbg.value intrinsics.
 bool CodeGenPrepare::placeDbgValues(Function &F) {
   bool MadeChange = false;
+  DominatorTree DT(F);
+
   for (BasicBlock &BB : F) {
-    Instruction *PrevNonDbgInst = nullptr;
     for (BasicBlock::iterator BI = BB.begin(), BE = BB.end(); BI != BE;) {
       Instruction *Insn = &*BI++;
       DbgValueInst *DVI = dyn_cast<DbgValueInst>(Insn);
-      // Leave dbg.values that refer to an alloca alone. These
-      // intrinsics describe the address of a variable (= the alloca)
-      // being taken.  They should not be moved next to the alloca
-      // (and to the beginning of the scope), but rather stay close to
-      // where said address is used.
-      if (!DVI || (DVI->getValue() && isa<AllocaInst>(DVI->getValue()))) {
-        PrevNonDbgInst = Insn;
+      if (!DVI)
         continue;
-      }
 
       Instruction *VI = dyn_cast_or_null<Instruction>(DVI->getValue());
-      if (VI && VI != PrevNonDbgInst && !VI->isTerminator()) {
-        // If VI is a phi in a block with an EHPad terminator, we can't insert
-        // after it.
-        if (isa<PHINode>(VI) && VI->getParent()->getTerminator()->isEHPad())
-          continue;
-        LLVM_DEBUG(dbgs() << "Moving Debug Value before :\n"
-                          << *DVI << ' ' << *VI);
-        DVI->removeFromParent();
-        if (isa<PHINode>(VI))
-          DVI->insertBefore(&*VI->getParent()->getFirstInsertionPt());
-        else
-          DVI->insertAfter(VI);
-        MadeChange = true;
-        ++NumDbgValueMoved;
-      }
+
+      if (!VI || VI->isTerminator())
+        continue;
+
+      // If VI is a phi in a block with an EHPad terminator, we can't insert
+      // after it.
+      if (isa<PHINode>(VI) && VI->getParent()->getTerminator()->isEHPad())
+        continue;
+
+      // If the defining instruction dominates the dbg.value, we do not need
+      // to move the dbg.value.
+      if (DT.dominates(VI, DVI))
+        continue;
+
+      LLVM_DEBUG(dbgs() << "Moving Debug Value before :\n"
+                        << *DVI << ' ' << *VI);
+      DVI->removeFromParent();
+      if (isa<PHINode>(VI))
+        DVI->insertBefore(&*VI->getParent()->getFirstInsertionPt());
+      else
+        DVI->insertAfter(VI);
+      MadeChange = true;
+      ++NumDbgValueMoved;
     }
   }
   return MadeChange;