PR32288: More efficient encoding for DWARF expr subregister access.

Citing http://bugs.llvm.org/show_bug.cgi?id=32288

  The DWARF generated by LLVM includes this location:

  0x55 0x93 0x04 DW_OP_reg5 DW_OP_piece(4) When GCC's DWARF is simply
  0x55 (DW_OP_reg5) without the DW_OP_piece. I believe it's reasonable
  to assume the DWARF consumer knows which part of a register
  logically holds the value (low bytes, high bytes, how many bytes,
  etc) for a primitive value like an integer.

This patch gets rid of the redundant DW_OP_piece when a subregister is
at offset 0. It also adds previously missing subregister masking when
a subregister is followed by another operation.

(This reapplies r297960 with two additional testcase updates).

rdar://problem/31069390
https://reviews.llvm.org/D31010

llvm-svn: 297965

2 files changed, 35 insertions, 3 deletions

diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp
index 43c98442d8b..caf0bc0f258 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp
@@ -66,6 +66,12 @@ void DwarfExpression::AddShr(unsigned ShiftBy) {
   EmitOp(dwarf::DW_OP_shr);
 }
 
+void DwarfExpression::AddAnd(unsigned Mask) {
+  EmitOp(dwarf::DW_OP_constu);
+  EmitUnsigned(Mask);
+  EmitOp(dwarf::DW_OP_and);
+}
+
 bool DwarfExpression::AddMachineRegIndirect(const TargetRegisterInfo &TRI,
                                             unsigned MachineReg, int Offset) {
   if (isFrameRegister(TRI, MachineReg)) {
@@ -230,6 +236,12 @@ void DwarfExpression::AddExpression(DIExpressionCursor &&ExprCursor,
                                     unsigned FragmentOffsetInBits) {
   while (ExprCursor) {
     auto Op = ExprCursor.take();
+
+    // If we need to mask out a subregister, do it now, unless the next
+    // operation would emit an OpPiece anyway.
+    if (SubRegisterSizeInBits && Op->getOp() != dwarf::DW_OP_LLVM_fragment)
+      maskSubRegister();
+
     switch (Op->getOp()) {
     case dwarf::DW_OP_LLVM_fragment: {
       unsigned SizeInBits = Op->getArg(1);
@@ -285,9 +297,24 @@ void DwarfExpression::AddExpression(DIExpressionCursor &&ExprCursor,
   }
 }
 
+/// Add masking operations to stencil out a subregister.
+void DwarfExpression::maskSubRegister() {
+  assert(SubRegisterSizeInBits && "no subregister was registered");
+  if (SubRegisterOffsetInBits > 0)
+    AddShr(SubRegisterOffsetInBits);
+  uint64_t Mask = (1UL << SubRegisterSizeInBits) - 1;
+  AddAnd(Mask);
+}
+
+
 void DwarfExpression::finalize() {
-  if (SubRegisterSizeInBits)
-    AddOpPiece(SubRegisterSizeInBits, SubRegisterOffsetInBits);
+  // Emit any outstanding DW_OP_piece operations to mask out subregisters.
+  if (SubRegisterSizeInBits == 0)
+    return;
+  // Don't emit a DW_OP_piece for a subregister at offset 0.
+  if (SubRegisterOffsetInBits == 0)
+    return;
+  AddOpPiece(SubRegisterSizeInBits, SubRegisterOffsetInBits);
 }
 
 void DwarfExpression::addFragmentOffset(const DIExpression *Expr) {
diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfExpression.h b/llvm/lib/CodeGen/AsmPrinter/DwarfExpression.h
index 01f66fa7eac..2f5a701499d 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfExpression.h
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfExpression.h
@@ -99,6 +99,9 @@ protected:
     SubRegisterOffsetInBits = OffsetInBits;
   }
 
+  /// Add masking operations to stencil out a subregister.
+  void maskSubRegister();
+
 public:
   DwarfExpression(unsigned DwarfVersion) : DwarfVersion(DwarfVersion) {}
   virtual ~DwarfExpression() {};
@@ -126,8 +129,10 @@ public:
   /// is at the top of the DWARF stack.
   void AddOpPiece(unsigned SizeInBits, unsigned OffsetInBits = 0);
 
-  /// Emit a shift-right dwarf expression.
+  /// Emit a shift-right dwarf operation.
   void AddShr(unsigned ShiftBy);
+  /// Emit a bitwise and dwarf operation.
+  void AddAnd(unsigned Mask);
 
   /// Emit a DW_OP_stack_value, if supported.
   ///