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

[Adrian Prantl]

> 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.
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!

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 ||