[llvm][DebugInfo] Add support for DW_OP_GNU_implicit_pointer (llvm#142913)

This patch introduces support for the DWARF operation
`DW_OP_GNU_implicit_pointer `(value 0xf3) within LLVM's DWARF parsing
and expression evaluation infrastructure. This GNU extension is used to
describe the location of a variable that is itself a pointer, where the
value of this pointer is stored at an address derived from another DWARF
location expression plus a constant offset.
Motivation:
Compilers like GCC use `DW_OP_GNU_implicit_pointer `to represent the
location of certain variables.Without support for this opcode, debuggers
like LLDB (and other tools relying on LLVM's DWARF libraries) cannot
correctly resolve the location of such variables, leading to an
inability to inspect their values or an incorrect debugging experience.

Author: Thrrreeee

llvm/include/llvm/BinaryFormat/Dwarf.def

@@ -887,6 +887,7 @@ HANDLE_DW_OP(0xed, WASM_location, -1, -1, 0, WASM)
 HANDLE_DW_OP(0xee, WASM_location_int, -1, -1, 0, WASM)
 // Historic and not implemented in LLVM.
 HANDLE_DW_OP(0xf0, APPLE_uninit, -1, -1, 0, APPLE)
+HANDLE_DW_OP(0xf2, GNU_implicit_pointer, 2, 0, 4, GNU)
 // The GNU entry value extension.
 HANDLE_DW_OP(0xf3, GNU_entry_value, 2, 0, 0, GNU)
 HANDLE_DW_OP(0xf8, PGI_omp_thread_num, -1, -1, 0, PGI)

llvm/lib/DebugInfo/DWARF/DWARFExpression.cpp

@@ -104,6 +104,8 @@ static std::vector<Desc> getOpDescriptions() {
   Descriptions[DW_OP_GNU_addr_index] = Desc(Op::Dwarf4, Op::SizeLEB);
   Descriptions[DW_OP_GNU_const_index] = Desc(Op::Dwarf4, Op::SizeLEB);
   Descriptions[DW_OP_GNU_entry_value] = Desc(Op::Dwarf4, Op::SizeLEB);
+  Descriptions[DW_OP_GNU_implicit_pointer] =
+      Desc(Op::Dwarf4, Op::SizeRefAddr, Op::SignedSizeLEB);
   // This Description acts as a marker that getSubOpDesc must be called
   // to fetch the final Description for the operation. Each such final
   // Description must share the same first SizeSubOpLEB operand.

llvm/test/tools/llvm-dwarfdump/X86/DW_OP_GNU_implicit_pointer.yaml

@@ -0,0 +1,87 @@
+# Test that we can decode `DW_OP_GNU_implicit_pointer` (0xf2)
+# RUN: yaml2obj %s | llvm-dwarfdump - | FileCheck %s
+
+# CHECK:      DW_TAG_variable
+# CHECK-NEXT:   DW_AT_location (DW_OP_GNU_implicit_pointer 0x2a +4)
+
+--- !ELF
+FileHeader:
+  Class:           ELFCLASS64
+  Data:            ELFDATA2LSB
+  Type:            ET_DYN
+  Machine:         EM_X86_64
+DWARF:
+  debug_abbrev:
+    - Table:
+        - Code:            0x00000001
+          Tag:             DW_TAG_compile_unit
+          Children:        DW_CHILDREN_yes
+          Attributes:
+            - Attribute:       DW_AT_language
+              Form:            DW_FORM_data2
+            - Attribute:       DW_AT_low_pc
+              Form:            DW_FORM_addr
+            - Attribute:       DW_AT_high_pc
+              Form:            DW_FORM_data4
+        - Code:            0x00000002
+          Tag:             DW_TAG_subprogram
+          Children:        DW_CHILDREN_yes
+          Attributes:
+            - Attribute:       DW_AT_low_pc
+              Form:            DW_FORM_addr
+            - Attribute:       DW_AT_high_pc
+              Form:            DW_FORM_data4
+            - Attribute:       DW_AT_frame_base
+              Form:            DW_FORM_exprloc
+        - Code:            0x00000003
+          Tag:             DW_TAG_formal_parameter
+          Children:        DW_CHILDREN_no
+          Attributes:
+            - Attribute:       DW_AT_location
+              Form:            DW_FORM_exprloc
+        - Code:            0x00000004
+          Tag:             DW_TAG_variable
+          Children:        DW_CHILDREN_no
+          Attributes:
+            - Attribute:       DW_AT_location
+              Form:            DW_FORM_exprloc
+  debug_info:
+    - Length:          52
+      Version:         5
+      UnitType:        DW_UT_compile
+      AbbrOffset:      0
+      AddrSize:        8
+      Entries:
+        - AbbrCode:        0x00000001
+          Values:
+            - Value:           0x000000000000000C
+            - Value:           0x0000000100000F50
+            - Value:           0x0000000000000034
+        - AbbrCode:        0x00000002
+          Values:
+            - Value:           0x0000000100000F50
+            - Value:           0x0000000000000034
+            - Value:           0x0000000000000001
+              BlockData:
+                - 0x56
+        - AbbrCode:        0x00000003
+          Values:
+            - Value:           0x0000000000000002
+              BlockData:
+                - 0x91
+                - 0x78
+        - AbbrCode:        0x00000004
+          Values:
+            - Value:           0x0000000000000006
+              BlockData:
+                - 0xf2 # DW_OP_GNU_implicit_pointer
+                - 0x2a # Section offset of parameter in the previous entry
+                - 0x00
+                - 0x00
+                - 0x00
+                - 0x04 # Pointer references location 4 bytes into value of previous entry
+        - AbbrCode:        0x00000000
+          Values:
+        - AbbrCode:        0x00000000
+          Values:
+...