[PATCH] Allocate stack storage for .block_descriptor and captured self.

John McCall rjmccall at apple.com
Mon Feb 25 17:34:51 PST 2013

On Feb 25, 2013, at 5:14 PM, Eric Christopher <echristo at gmail.com> wrote:
> On Mon, Feb 25, 2013 at 5:02 PM, John McCall <rjmccall at apple.com> wrote:
> On Feb 25, 2013, at 4:55 PM, Adrian Prantl <aprantl at apple.com> wrote:
> > here’s another patch for review:
> >
> > Allocate stack storage for .block_descriptor and captured self.
> > This way the register allocator will not optimize away the the
> > debug info for captured variables.
> Allocating stack storage is not the right way to fix this problem.
> The frontend is emitting the right intrinsics to say that the argument
> is being kept in an LLVM value, not in memory.  If that's not working,
> then basically all optimized debug info is useless.
> That's pretty accurate. The backend in a lot of places is depending upon an
> alloca existing in order to get a location for a variable. To make optimized
> debug info work we're going to need to fix that assumption.

When did this become true?  I know that at least *some* debugging used
to work in blocks, and we haven't been making allocas for them in a while.
Maybe it was a really brittle, but some things used to survive at least to the
point that GDB could consume them.

If we *must* do this as a temporary debug info workaround, then the
appropriate solution for things like the block descriptor is to make the
alloca, slam a value into it, and use that for the debug info intrinsic, but
not otherwise change IR-generation for the function.


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