[lldb] Allow SwiftASTContext fallbacks for types from expressions

[adrian-prantl]

Generally, it would be feasible to rely on DWARF and reflectio metadata for JIT-compiled images, but in practice, not everything that an expression returns may be anchored by a variable, which makes having debug info for expression-defined types hit and miss. If an expression type is involved, LLDB will have done the costly SwiftASTContext initialization already, so there isn't much saved by avoiding the fallback in this case.

[adrian-prantl]

@swift-ci test

[augusto2112]

Generally, it would be feasible to rely on DWARF and reflectio metadata for JIT-compiled images, but in practice, not everything that an expression returns may be anchored by a variable

Hmm, could you give an example of something that an expression returns that is not anchored by a variable?

[adrian-prantl]

Generally, it would be feasible to rely on DWARF and reflectio metadata for JIT-compiled images, but in practice, not everything that an expression returns may be anchored by a variable

Hmm, could you give an example of something that an expression returns that is not anchored by a variable?

Totally! This is from TestSwiftExpr.py:

struct StructTest {
  func foo() {
    print("Stop here in method \(m_var)") //% self.expect("expr self", DATA_TYPES_DISPLAYED_CORRECTLY, substrs = ["StructTest"])
                                          //% self.expect("expr typealias foo = StructTest; self as foo", DATA_TYPES_DISPLAYED_CORRECTLY, substrs = ["foo"]) 
  }
  let m_var = 234
}

There is nothing holding on to foo so there is no DWARF being generated for it.

[augusto2112]

I agree with this if we legitimately can't use DWARF and reflection metadata for certain use-cases.

[augusto2112]

struct StructTest {
  func foo() {
    print("Stop here in method \(m_var)") //% self.expect("expr self", DATA_TYPES_DISPLAYED_CORRECTLY, substrs = ["StructTest"])
                                          //% self.expect("expr typealias foo = StructTest; self as foo", DATA_TYPES_DISPLAYED_CORRECTLY, substrs = ["foo"]) 
  }
  let m_var = 234
}

You mean the type alias foo right? $__lldb_result should be holding on to foo, no?

With the expression log enabled:

 Variables:
   [name=$__lldb_injected_self, type = t.StructTest]
   [name=$__lldb_result, type = __lldb_expr_3.foo, is_result]
   [name=$__lldb_error_result, type = any Swift.Error, is_error]

Is it because $__lldb_result is special in some way it's not enough to make DWARF hold on to the type?

[adrian-prantl]

Good question! I only looked at the expression log to see what DITypes are generated and there was no DILocalVariable for the result:

extension $__lldb_context {
  @LLDBDebuggerFunction @available(macOS 15.3.0, *)
  mutating func $__lldb_user_expr_1(_ $__lldb_arg : UnsafeMutablePointer<Any>) {
    
do {
/*__LLDB_USER_START__*/
#sourceLocation(file: "/var/folders/n4/dh4jjrps5mz1wbw_thlvls940000gn/T/expr3-631fed..cpp", line: 1)
typealias foo = StructTest; self as foo

/*__LLDB_USER_END__*/
} catch (let __lldb_tmp_error) {
  var $__lldb_error_result = __lldb_tmp_error
}

  }
}
@LLDBDebuggerFunction @available(macOS 15.3.0, *)
func $__lldb_expr(_ $__lldb_arg : UnsafeMutablePointer<Any>) {
  do {
    $__lldb_injected_self.$__lldb_user_expr_1(
      $__lldb_arg
    )
  }
}

[adrian-prantl]

define hidden swiftcc void @"$s1a10StructTestV13__lldb_expr_4E03$__c6_user_D2_1yySpyypGF"(ptr %0, ptr nocapture swiftself dereferenceable(8) %1) #0 !dbg !35 {
  %3 = alloca ptr, align 8
  call void @llvm.memset.p0.i64(ptr align 8 %3, i8 0, i64 8, i1 false)
  %4 = alloca ptr, align 8
  call void @llvm.memset.p0.i64(ptr align 8 %4, i8 0, i64 8, i1 false)
  store ptr %0, ptr %3, align 8, !dbg !53
    #dbg_value(ptr %3, !50, !DIExpression(DW_OP_deref), !54)
  store ptr %1, ptr %4, align 8, !dbg !53
    #dbg_value(ptr %4, !52, !DIExpression(DW_OP_deref, DW_OP_deref), !55)
  %5 = getelementptr inbounds i8, ptr %0, i64 32, !dbg !56
  %6 = load ptr, ptr %5, align 8, !dbg !56
  %7 = getelementptr inbounds i8, ptr %0, i64 24, !dbg !56
  %8 = load ptr, ptr %7, align 8, !dbg !56
  %9 = getelementptr inbounds i8, ptr %0, i64 8, !dbg !56
  %10 = load ptr, ptr %9, align 8, !dbg !56
  %11 = getelementptr inbounds i8, ptr %0, i64 16, !dbg !56
  %12 = load ptr, ptr %11, align 8, !dbg !56
  %13 = getelementptr inbounds %T1a10StructTestV, ptr %1, i32 0, i32 0, !dbg !59
  %14 = getelementptr inbounds %TSi, ptr %13, i32 0, i32 0, !dbg !59
  %15 = load i64, ptr %14, align 8, !dbg !59
  %16 = getelementptr inbounds %T1a10StructTestV, ptr %8, i32 0, i32 0, !dbg !59
  %17 = getelementptr inbounds %TSi, ptr %16, i32 0, i32 0, !dbg !59
  store i64 %15, ptr %17, align 8, !dbg !59
  ret void, !dbg !64
}

!50 = !DILocalVariable(name: "$__lldb_arg", arg: 1, scope: !35, file: !13, line: 4, type: !51)
!51 = !DIDerivedType(tag: DW_TAG_const_type, baseType: !39)
!52 = !DILocalVariable(name: "self", arg: 2, scope: !35, file: !13, line: 4, type: !47, flags: DIFlagArtificial)

[adrian-prantl]

@swift-ci test