[PATCH] TypeFinder: use iterative algorithm to avoid stack overflow.

[Will Dietz]

To elaborate:

Making a quick survey of bitcode files I have on hand suggests
types never nest more than ~250 deep, usually much less.

At these levels, the difference in stack usage of llvm-dis before
and after is 12KB vs 27KB.  This was measured with
the attached synthetic "types.small.bc" file (200 deep)
by using "ulimit -s" to find the small stack size that would run
successfully with and without this patch.

Scaling up the type nesting depth to admittedly
unrealistic depths (60,000) causes stack usage
to go over 4MB before this patch while still
running with 12KB afterwards.  This was measured
using the attached (xz-compressed) types.bc file.

Importantly, AFAICT there is no performance impact with this change.

In short, while the stack usage issue is unlikely to be an issue
for most users it seems like a strict improvement over
the current state and can be an important change in
environments with very limited stack or with particularly
worst-case scenario bitcode files and normal stack sizes.

Finally, I've also attached an updated patch that fixes
a minor whitespace issue that somehow managed
to sneak into the original.

Thank you for your time :).

~Will

On Thu, Oct 10, 2013 at 5:58 PM, Will Dietz <w at wdtz.org> wrote:
> See attached.
>
> Without this change, TypeFinder can use a large amount of stack space
> when processing modules with deeply nested type references.
>
> Okay to commit?
>
> ~Will
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