[llvm-dev] Representing X86 long double in Debug Info

[Reid Kleckner via llvm-dev]

On Mon, Nov 2, 2015 at 8:38 AM, Adrian Prantl via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Looking at the code in clang CGDebugInfo just passes through the width of
> the type as it is described by the TypeInfo, which in turn is defined by
> the Target. At the moment I do not understand why an x86_fp80 is reported
> to be 128 bits wide. (Since it’s a type natively supported by LLVM
> http://llvm.org/docs/LangRef.html#floating-point-types I would have
> expected it to be more like size=80, align=128)
>
> This looks like a bug to me and the debug info should describe an x86_fp80
> as being 80 bits wide, but I don’t understand the mechanics well enough to
> decide whether the TypeInfo is wrong here or if we should work around it in
> CGDebugInfo.
>

I think TypeInfo usually describes the answer that sizeof() is supposed to
give. sizeof() is typically a multiple of alignment, so if alignof() is
128, sizeof must be 128. Other common alignments are 32 and 64, which makes
sizeof() 96 and 128 respectively. In practice, sizeof(long double) is never
80.

Maybe CGDebugInfo should ask TargetInfo::getLongDoubleFormat() what model
is in use and generate dwarf accordingly. It's a bit of a hack, so I'm open
to better suggestions.
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