[llvm-dev] Debug info scope of explicit casting type does not seem correct

Sat May 7 03:02:38 PDT 2016

Hi David,
OK, I got that DIE in Compile Unit scope may point to a DIE in subprogram scope.
But how about that we are emitting a subprogram entry that has no attributes?

0x0000002b:   DW_TAG_subprogram [3] *

0x0000002c:     DW_TAG_typedef [4]
                               DW_AT_type [DW_FORM_ref4]     (cu + 0x0040 => {0x00000040})
                               DW_AT_name [DW_FORM_strp]     ( .debug_str[0x00000060] = "T")
                               DW_AT_decl_file [DW_FORM_data1]       ("c:\temp\ICL\LB\retain.cpp")
                               DW_AT_decl_line [DW_FORM_data1]       (16)

Regards,
Amjad
From: David Blaikie [mailto:dblaikie at gmail.com]
Sent: Saturday, April 30, 2016 17:59
To: Aboud, Amjad <amjad.aboud at intel.com>
Cc: Adrian Prantl <aprantl at apple.com>; llvm-dev <llvm-dev at lists.llvm.org>; Clang Dev <cfe-dev at lists.llvm.org>
Subject: Re: Debug info scope of explicit casting type does not seem correct

Yep, seems fine/reasonable to me. The pointer type has no logical scope so it doesn't really matter where its DIE goes.

GCC does put the pointer type in the function scope in basic cases, but it can also end up in other places. For example:

blaikie at blaikie-linux:~/dev$ cat func.cpp

void sink(void*);

template<typename T>

void f1() {

  T *t;

  sink(&t);

}

void f2() {

  struct foo { };

  f1<foo>();

}

blaikie at blaikie-linux:~/dev$ g++-4.7 -std=c++11 func.cpp -g -c && llvm-dwarfdump-tot -debug-dump=info func.o | grep DW_TAG

0x0000000b: DW_TAG_compile_unit [1] *

0x0000002d:   DW_TAG_subprogram [2] * // f2

0x00000051:     DW_TAG_lexical_block [3]

0x00000062:     DW_TAG_structure_type [4]

0x0000006b:   DW_TAG_subprogram [5] * // f1

0x00000087:     DW_TAG_template_type_parameter [6]

0x0000008e:     DW_TAG_lexical_block [7] *

0x0000009f:       DW_TAG_variable [8]

0x000000ab:       DW_TAG_pointer_type [9]  // foo*

& in fact, if you introduce another template that's similar to f1, its variable's type is 0x00ab - so it points into f1's children. Seems GCC puts it in whatever scope first references the pointer type.

I think it's probably as sensible, and a bit simpler, to put any such basic types in the CU scope.

(granted, if GCC and Clang were really smart, the pointer type in f1 could be pointer to 0x0087 - but that would really bloat the debug info (now every different way of writing foo* in every different template would be a separate type description... - that would probably be not good))

On Sat, Apr 30, 2016 at 3:32 AM, Aboud, Amjad <amjad.aboud at intel.com<mailto:amjad.aboud at intel.com>> wrote:
Hi,
I am wondering if this behavior of creating debug info is correct.
A type in compile unit entry is pointing to a type under subprogram entry?!

This is the root cause of https://llvm.org/bugs/show_bug.cgi?id=27579

0x0000000b: DW_TAG_compile_unit [1] *

0x00000026:   DW_TAG_pointer_type [2]
                            DW_AT_type [DW_FORM_ref4]       (cu + 0x002c => {0x0000002c})

0x0000002b:   DW_TAG_subprogram [3] *

0x0000002c:     DW_TAG_typedef [4]
                               DW_AT_type [DW_FORM_ref4]     (cu + 0x0040 => {0x00000040})
                               DW_AT_name [DW_FORM_strp]     ( .debug_str[0x00000060] = "T")
                               DW_AT_decl_file [DW_FORM_data1]       ("c:\temp\ICL\LB\retain.cpp")
                               DW_AT_decl_line [DW_FORM_data1]       (16)

0x00000037:     NULL

command line:
clang -cc1 -triple i386-apple-ios9.0.0 -emit-obj -debug-info-kind=limited -O2 test.cpp –o - | llvm-dwarfdump -debug-dump=info -

> cat test.cpp
class A {
public:
  int x;
};

class B {
public:
  typedef A type;
};

template<typename X>
int foo(void* in) {
  typedef typename X::type T;
  const T* p = (T*) in;
  return p->x;
}

int bar() {
A a;
  return foo<B>(&a);
}

Reason for this behavior is the explicit cast “(T*)“, which leads into the following IR:

!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !1, producer: "clang version 3.9.0 (trunk 267335)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, retainedTypes: !3)
!3 = !{!4}
!4 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !5, size: 32, align: 32)  <----------------- No Scope, leads to compile unit scope!!
!5 = !DIDerivedType(tag: DW_TAG_typedef, name: "T", scope: !7, file: !6, line: 16, baseType: !20)
!7 = distinct !DISubprogram(name: "foo<B>", linkageName: "_Z3fooI1BEiPv", scope: !6, file: !6, line: 15, type: !8, isLocal: false, isDefinition: true, scopeLine: 15, flags: DIFlagPrototyped, isOptimized: true, unit: !0, templateParams: !12, variables: !15)

Thanks,
Amjad

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