[LLVMdev] So what's the deal with debug_frame V eh_frame

[Rafael Espíndola]

On 19 March 2014 17:31, David Blaikie <dblaikie at gmail.com> wrote:
> While comparing debug info between GCC and Clang I found a section
> that only Clang produces and GCC never produces: debug_frame.
>
> It seems (though I haven't verified this with absolute certainty) as
> though GCC just always uses eh_frame. LLVM on the other hand sometimes
> uses eh_frame and sometimes uses debug_frame.
>
> Here's an example:
>
> int f1();
> int i = f1();
> void func() { }
>
> Compiled with -fno-exceptions -g.
>
> The first two lines produce a global ctor.
>
> Clang is generous enough to flag the LLVM IR function for this global
> ctor ("__cxx_global_var_init") as nounwind
> (tools/clang/lib/CodeGen/CGDeclCXX.cpp:246), though "func" gets
> flagged as nounwind but also as uwtable (why? I don't really
> understand these semantics & haven't tracked down where that attribute
> gets applied)

Is this x86-64? If so, uwtable is a abi requirement. It says that that
function should have an entry in .eh_table.

> Without 'func' in this translation unit, LLVM emits a debug_frame
> section (because no functions actually need an unwind table, but we're
> compiling with debug info (without debug info LLVM emits no _frame
> section at all)). With 'func', LLVM switches to emitting an eh_frame
> section in both (-g and no -g) cases.
>
> GCC on the other hand emits eh_frame in all 4 cases (with and without
> -g and with and without 'func' defined).
>
> Do we need all this logic? Should we just always use eh_frame? Should
> we not have the 'uwtable' attribute on 'func'? Why is 'func' different
> from the global ctor in this regard?

.debug_frame is a non-load section. It is therefore an optimization to
emit it instead of .eh_frame. It is very likely not really a relevant
one, so it might be possible to just always use .eh_frame.

Not adding uwtable for _GLOBAL__I_a might actually be a bug. I will
check what gcc does.

Cheers,
Rafael