Document the shortcomings of DwarfExpression::addMachineReg().

Also make a drive-by-fix of a bug in the subregister scan code that
only triggers with an incomplete or otherwise very irregular machine
description.

rdar://problem/37404493

This re-applies r324972 with an early exit in the case of a complete
failure to make this commit NFC again as intended.

llvm-svn: 325041

1 files changed, 12 insertions, 4 deletions

diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp
index 06095fe9337..f626c3711dc 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp
@@ -123,7 +123,10 @@ bool DwarfExpression::addMachineReg(const TargetRegisterInfo &TRI,
   const TargetRegisterClass *RC = TRI.getMinimalPhysRegClass(MachineReg);
   unsigned RegSize = TRI.getRegSizeInBits(*RC);
   // Keep track of the bits in the register we already emitted, so we
-  // can avoid emitting redundant aliasing subregs.
+  // can avoid emitting redundant aliasing subregs. Because this is
+  // just doing a greedy scan of all subregisters, it is possible that
+  // this doesn't find a combination of subregisters that fully cover
+  // the register (even though one may exist).
   SmallBitVector Coverage(RegSize, false);
   for (MCSubRegIterator SR(MachineReg, &TRI); SR.isValid(); ++SR) {
     unsigned Idx = TRI.getSubRegIndex(MachineReg, *SR);
@@ -143,7 +146,7 @@ bool DwarfExpression::addMachineReg(const TargetRegisterInfo &TRI,
     if (CurSubReg.test(Coverage)) {
       // Emit a piece for any gap in the coverage.
       if (Offset > CurPos)
-        DwarfRegs.push_back({-1, Offset - CurPos, nullptr});
+        DwarfRegs.push_back({-1, Offset - CurPos, "no DWARF register encoding"});
       DwarfRegs.push_back(
           {Reg, std::min<unsigned>(Size, MaxSize - Offset), "sub-register"});
       if (Offset >= MaxSize)
@@ -154,8 +157,13 @@ bool DwarfExpression::addMachineReg(const TargetRegisterInfo &TRI,
       CurPos = Offset + Size;
     }
   }
-
-  return CurPos;
+  // Failed to find any DWARF encoding.
+  if (CurPos == 0)
+    return false;
+  // Found a partial or complete DWARF encoding.
+  if (CurPos < RegSize)
+    DwarfRegs.push_back({-1, RegSize - CurPos, "no DWARF register encoding"});
+  return true;
 }
 
 void DwarfExpression::addStackValue() {