[llvm] r208502 - DwarfUnit: Make explicit a limitation/bug in enumeration constant emission.

[Adrian Prantl]

> On May 12, 2014, at 10:44 AM, David Blaikie <dblaikie at gmail.com> wrote:
> 
> On Mon, May 12, 2014 at 10:28 AM, Adrian Prantl <aprantl at apple.com> wrote:
>> 
>>> On May 11, 2014, at 10:04 AM, David Blaikie <dblaikie at gmail.com> wrote:
>>> 
>>> Author: dblaikie
>>> Date: Sun May 11 12:04:05 2014
>>> New Revision: 208502
>>> 
>>> URL: http://llvm.org/viewvc/llvm-project?rev=208502&view=rev
>>> Log:
>>> DwarfUnit: Make explicit a limitation/bug in enumeration constant emission.
>>> 
>>> Filed as PR19712, LLVM fails to detect the right type of an enum
>>> constant when a frontend does not provide an underlying type for the
>>> enumeration type.
>>> 
>>> Modified:
>>>   llvm/trunk/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
>>> 
>>> Modified: llvm/trunk/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
>>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/AsmPrinter/DwarfUnit.cpp?rev=208502&r1=208501&r2=208502&view=diff
>>> ==============================================================================
>>> --- llvm/trunk/lib/CodeGen/AsmPrinter/DwarfUnit.cpp (original)
>>> +++ llvm/trunk/lib/CodeGen/AsmPrinter/DwarfUnit.cpp Sun May 11 12:04:05 2014
>>> @@ -748,12 +748,17 @@ void DwarfUnit::addBlockByrefAddress(con
>>> /// Return true if type encoding is unsigned.
>>> static bool isUnsignedDIType(DwarfDebug *DD, DIType Ty) {
>>>  DIDerivedType DTy(Ty);
>>> -  if (DTy.isDerivedType())
>>> -    return isUnsignedDIType(DD, DD->resolve(DTy.getTypeDerivedFrom()));
>>> +  if (DTy.isDerivedType()) {
>>> +    if (DIType Deriv = DD->resolve(DTy.getTypeDerivedFrom()))
>>> +      return isUnsignedDIType(DD, Deriv);
>>> +    // FIXME: Enums without a fixed underlying type have unknown signedness
>>> +    // here, leading to incorrectly emitted constants.
>>> +    assert(DTy.getTag() == dwarf::DW_TAG_enumeration_type);
>>> +    return false;
>>> +  }
>> 
>> This should be relaxed to allow other DIDerivedTypes, including DIComposite types.
> 
> What relaxation did you have in mind? "isDerivedType" returns true for
> DICompositeTypes too, I believe - so perhaps it's already as relaxed
> as you were suggesting it should be?

It does, and they will run straight into the assertion (which should be relaxed, in the sense of "removed” ;-).
> 
>> I’m imagining a world where SROA does not elide debug info, and a function like this:
>> 
>> struct S { int c; };
>> int foo() {
>>  struct S s = { 42 };
>>  return s.c;
>> }
>> 
>> Walking the struct members to find their signedness will be fun!
> 
> Yeah... :)
> 
> - David
> 
>> 
>> cheers,
>> adrian
>>> 
>>>  DIBasicType BTy(Ty);
>>> -  if (!BTy.isBasicType())
>>> -    return false;
>>> +  assert(BTy.isBasicType());
>>>  unsigned Encoding = BTy.getEncoding();
>>>  assert(Encoding == dwarf::DW_ATE_unsigned ||
>>>         Encoding == dwarf::DW_ATE_unsigned_char ||