[LLVMdev] Continuing PR5680: preserve order of use lists in bitcode

[Duncan P. N. Exon Smith]

> On 2014-Jul-08, at 16:48, Chandler Carruth <chandlerc at google.com> wrote:
> 
> 
>> On Tue, Jul 8, 2014 at 4:29 PM, Duncan P. N. Exon Smith <duncan at exonsmith.com> wrote:
>> I'm looking to tackle PR5680 [1].  The primary goal is to prevent
>> behaviour changes in passes that depend on the order of use lists when
>> serializing/deserializing the IR to a bitcode file (.bc) between passes.
>> 
>> Here's a quick high-level summary:
>> 
>>   - When writing bitcode, calculate what the use list order will be
>>     after reading the bitcode.  (Assumption: the order is predictable.)
>> 
>>   - Write a use-list-order-diff in a new (optional) section in the
>>     bitcode.
>> 
>>   - When reading bitcode, apply the use-list-order-diff to restore the
>>     original use list order.
> 
> So, it may be totally obvious, but is there a reason not to embed the observed order of the use list in the bitcode more directly? This seems quite round-about, and I'm curious why its needed.

I think it would be harder (for the author *and* the compiler) to
read in the "correct" order on-the-fly.  In particular, I'm not
sure how to do it without explicitly storing full use-lists and
creating forward-refs to fill them up.

So this design (when it's turned on) trades increased writer
complexity for smaller file size and decreased reader complexity.

Make sense?