[DebugInfo] Fix a misleading usage of DWARF forms with DIEExpr. NFCI.

For now, DIEExpr is used only in two places:

 1) in the debug info library unit test suite to emit
    a DW_AT_str_offsets_base attribute with the DW_FORM_sec_offset
    form, see dwarfgen::DIE::addStrOffsetsBaseAttribute();

 2) in DwarfCompileUnit::addLocationAttribute() to generate the location
    attribute for a TLS variable.

The later case used an incorrect DWARF form of DW_FORM_udata, which
implies storing an uleb128 value, not a 4/8 byte constant. The generated
result was as expected because DIEExpr::SizeOf() did not handle the used
form, but returned the size of the code pointer by default.

The patch fixes the issue by using more appropriate DWARF forms for
the problematic case and making DIEExpr::SizeOf() more straightforward.

Differential Revision: https://reviews.llvm.org/D83958

llvm/lib/CodeGen/AsmPrinter/DIE.cpp

@@ -472,10 +472,17 @@ void DIEExpr::emitValue(const AsmPrinter *AP, dwarf::Form Form) const {
 /// SizeOf - Determine size of expression value in bytes.
 ///
 unsigned DIEExpr::SizeOf(const AsmPrinter *AP, dwarf::Form Form) const {
-  if (Form == dwarf::DW_FORM_data4) return 4;
-  if (Form == dwarf::DW_FORM_sec_offset) return 4;
-  if (Form == dwarf::DW_FORM_strp) return 4;
-  return AP->getPointerSize();
+  switch (Form) {
+  case dwarf::DW_FORM_data4:
+    return 4;
+  case dwarf::DW_FORM_data8:
+    return 8;
+  case dwarf::DW_FORM_sec_offset:
+    // FIXME: add support for DWARF64
+    return 4;
+  default:
+    llvm_unreachable("DIE Value form not supported yet");
+  }
 }
 
 LLVM_DUMP_METHOD

llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp

@@ -260,7 +260,9 @@ void DwarfCompileUnit::addLocationAttribute(
                                      : dwarf::DW_OP_const8u);
             // 2) containing the (relocated) offset of the TLS variable
             //    within the module's TLS block.
-            addExpr(*Loc, dwarf::DW_FORM_udata,
+            addExpr(*Loc,
+                    PointerSize == 4 ? dwarf::DW_FORM_data4
+                                     : dwarf::DW_FORM_data8,
                     Asm->getObjFileLowering().getDebugThreadLocalSymbol(Sym));
           } else {
             addUInt(*Loc, dwarf::DW_FORM_data1, dwarf::DW_OP_GNU_const_index);