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

David Blaikie via llvm-dev
Sun, 08 May 2016 15:11:52 -0000

That'd be a bug - we should finish adding attributes to it at some point.
If we can detect that it's never going to be handled (that it's been
entirely optimized away) by the other codepaths, we should do it up-front.
Otherwise we may need to add it to a list somewhere to be handled.

Ah, this probably regressed when we switched the direction of the link in
subprograms (from CU->subprogram to subprogram->CU). Adrian?

On Sun, May 8, 2016 at 1:13 AM, Aboud, Amjad <amjad.aboud at intel.com>
wrote:

> That happens because we create the subprogram below as a context to the
> “DW_TAG_typedef” that was created as a type to “DW_TAG_pointer_type” that
> was added to the retained type list because of the explicit cast to (T*).
>
>
>
> This is the code that creates DW_TAG_subprogram:
>
>
>
> DIE *DwarfUnit::getOrCreateSubprogramDIE(const DISubprogram *SP, bool
> Minimal) {
>
>
>
> ...
>
>
>
>   // DW_TAG_inlined_subroutine may refer to this DIE.
>
>   DIE &SPDie = createAndAddDIE(dwarf::DW_TAG_subprogram, *ContextDIE,
SP);
>
>
>
>   // Stop here and fill this in later, depending on whether or not this
>
>   // subprogram turns out to have inlined instances or not.
>
>   if (SP->isDefinition())
>
>     return &SPDie;                                <-------------  In
our
> case the function returns here, and does not apply attributes.
>
>
>
>   applySubprogramAttributes(SP, SPDie);
>
>   return &SPDie;
>
> }
>
>
>
> It assumes that later when the function is finalized the attributes will
> be applied, which is called from this code:
>
>
>
> void DwarfDebug::finishSubprogramDefinitions() {
>
>   for (auto &F : MMI->getModule()->functions())
>
>     if (auto *SP = F.getSubprogram())
>
>       if (ProcessedSPNodes.count(SP) &&
>
>           SP->getUnit()->getEmissionKind() != DICompileUnit::NoDebug)
>
>         forBothCUs(*CUMap.lookup(SP->getUnit()),
[&](DwarfCompileUnit &CU)
> {
>
>           CU.finishSubprogramDefinition(SP);          <-------------  In
> our case, this code will not be reached for the above subprogram.
>
>         });
>
> ...
>
> }
>
>
>
> However, the function was optimized out and the code will not call
> “finishSubprogramDefinition” for it, and attributes will not be added.
>
>
>
> Regards,
>
> Amjad
>
>
>
>
>
> *From:* David Blaikie [mailto:dblaikie at gmail.com]
> *Sent:* Saturday, May 07, 2016 21:38
>
> *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
>
>
>
> Under what conditions do we produce a subprogram with no attributes? At
> first glance that certainly seems not useful, but perhaps there's a
reason
> for it.
>
>
>
> On Sat, May 7, 2016 at 3:02 AM, Aboud, Amjad <amjad.aboud at
intel.com>
> wrote:
>
> 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>
> 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(&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", 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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