[DebugInfo] Don't drop dbg.value's of nullptr

Currently, dbg.value's of "nullptr" are dropped when entering a SelectionDAG --
apparently just because of an oversight when recognising Values that are
constant (see PR39787). This patch adds ConstantPointerNull to the list of
constants that can be turned into DBG_VALUEs.

The matter of what bit-value a null pointer constant in LLVM has was raised
in this mailing list thread:

  http://lists.llvm.org/pipermail/llvm-dev/2018-December/128234.html

Where it transpires LLVM relies on (IR) null pointers being zero valued,
thus I've baked this assumption into the patch.

Differential Revision: https://reviews.llvm.org/D55227

llvm-svn: 348753

2 files changed, 5 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/InstrEmitter.cpp b/llvm/lib/CodeGen/SelectionDAG/InstrEmitter.cpp
index e4baa3322ba..b51c23c6a74 100644
--- a/llvm/lib/CodeGen/SelectionDAG/InstrEmitter.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/InstrEmitter.cpp
@@ -749,6 +749,9 @@ InstrEmitter::EmitDbgValue(SDDbgValue *SD,
         MIB.addImm(CI->getSExtValue());
     } else if (const ConstantFP *CF = dyn_cast<ConstantFP>(V)) {
       MIB.addFPImm(CF);
+    } else if (isa<ConstantPointerNull>(V)) {
+      // Note: This assumes that all nullptr constants are zero-valued.
+      MIB.addImm(0);
     } else {
       // Could be an Undef.  In any case insert an Undef so we can see what we
       // dropped.
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
index 8ca29625626..0de149a4368 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -5312,7 +5312,8 @@ SelectionDAGBuilder::visitIntrinsicCall(const CallInst &I, unsigned Intrinsic) {
       return nullptr;
 
     SDDbgValue *SDV;
-    if (isa<ConstantInt>(V) || isa<ConstantFP>(V) || isa<UndefValue>(V)) {
+    if (isa<ConstantInt>(V) || isa<ConstantFP>(V) || isa<UndefValue>(V) ||
+        isa<ConstantPointerNull>(V)) {
       SDV = DAG.getConstantDbgValue(Variable, Expression, V, dl, SDNodeOrder);
       DAG.AddDbgValue(SDV, nullptr, false);
       return nullptr;