[lldb] Support DW_OP_WASM_location in DWARFExpression

[paolosevMSFT]

Add support for DW_OP_WASM_location in DWARFExpression.

[llvmbot]

@llvm/pr-subscribers-lldb

Author: Paolo Severini (paolosevMSFT)

Changes

Add support for DW_OP_WASM_location in DWARFExpression.

---

Full diff: https://github.com/llvm/llvm-project/pull/78977.diff

1 Files Affected:

• (modified) lldb/source/Expression/DWARFExpression.cpp (+41)

diff --git a/lldb/source/Expression/DWARFExpression.cpp b/lldb/source/Expression/DWARFExpression.cpp
index fe4928d4f43a434..95033db5ed8f5a1 100644
--- a/lldb/source/Expression/DWARFExpression.cpp
+++ b/lldb/source/Expression/DWARFExpression.cpp
@@ -346,6 +346,16 @@ static offset_t GetOpcodeDataSize(const DataExtractor &data,
     return (offset - data_offset) + subexpr_len;
   }
 
+  case DW_OP_WASM_location: {
+    uint8_t wasm_op = data.GetU8(&offset);
+    if (wasm_op == 3) {
+      data.GetU32(&offset);
+    } else {
+      data.GetULEB128(&offset);
+    }
+    return offset - data_offset;
+  }
+
   default:
     if (!dwarf_cu) {
       return LLDB_INVALID_OFFSET;
@@ -2595,6 +2605,37 @@ bool DWARFExpression::Evaluate(
       break;
     }
 
+    case DW_OP_WASM_location: {
+      uint8_t wasm_op = opcodes.GetU8(&offset);
+      uint32_t index;
+
+      /* LLDB doesn't have an address space to represents WebAssembly locals,
+       * globals and operand stacks.
+       * We encode these elements into virtual registers:
+       *   | tag: 2 bits | index: 30 bits |
+       *   where tag is:
+       *    0: Not a WebAssembly location
+       *    1: Local
+       *    2: Global
+       *    3: Operand stack value
+       */
+      if (wasm_op == 3) {
+        index = opcodes.GetU32(&offset);
+        wasm_op = 2; // Global
+      } else {
+        index = opcodes.GetULEB128(&offset);
+      }
+
+      reg_num = (((wasm_op + 1) & 0x03) << 30) | (index & 0x3fffffff);
+
+      if (ReadRegisterValueAsScalar(reg_ctx, reg_kind, reg_num, error_ptr, tmp))
+        stack.push_back(tmp);
+      else
+        return false;
+
+      break;
+    }
+
     default:
       if (dwarf_cu) {
         if (dwarf_cu->GetSymbolFileDWARF().ParseVendorDWARFOpcode(

[JDevlieghere]

Is it possible to write a test for this in DWARFExpressionTest.cpp?

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[paolosevMSFT]

Adding unit tests turned out to be a little complicated.
There were already a couple of unit tests in file lldb/unittests/Expression/DWARFExpressionTest.cpp that actually were failing with these changes, so I replaced them.
From the tests I learned that the vendor-specific DWARF opcode DW_OP_WASM_location should be handled in plugin code and not in DWARFExpression.cpp.
Not sure what to do with the old tests, I don't think a DW_OP_WASM_location might ever be located in an ELF file, given that with "split symbols" Wasm uses the Wasm file format both for the file with the code and for the file with symbols. (Tested with emscripten).

[paolosevMSFT]

Hi @JDevlieghere, could you take a look at the latest changes and unit tests?

[xujuntwt95329]

Hi @JDevlieghere Could you please give us some suggestions about the next steps for this PR and #77949?

Thanks a lot!

[JDevlieghere]

Apologies for the delay. LGTM!

[paolosevMSFT]

Apologies for the delay. LGTM!

Thanks! I have not merged any changes in several years and I am not very familiar with the new process, now that llvm_project has moved to Github.
I have read https://llvm.org/docs/MyFirstTypoFix.html#myfirsttypofix-issues-after-landing-your-pr and if I am not wrong there is no safe gating on PR merge commits?
How do I make sure all tests pass in all platforms, before merging?

[tritao]

Given this was already approved, what's missing for the merge?