[cfe-dev] [RFC] cHash: Integration of AST hashing

[Alex Lorenz via cfe-dev]

I see, thanks. That sounds promising.

Is the hash stored in a central database or do you use a separate file for each translation unit (or maybe something else)?

> On 3 Aug 2017, at 12:39, Christian Dietrich <dietrich at sra.uni-hannover.de> wrote:
> 
> Alex Lorenz via cfe-dev <cfe-dev at lists.llvm.org> writes:
> 
>> Hey,
>> 
>> What impact does the hashing itself have on the build time? I.e. If I
>> were to fully rebuild a project from scratch, how much longer would it
>> take?
> 
> We have build over 2300 commits from the musl libc library. In total
> that are 5.6 million compiler invocations (all commits are built from
> scratch). On average, the AST hashing took 9ms. In comparision, the
> parsing is 187ms and the -O2 optimization and assembling 1082ms. So the
> hashing time alone is almost negliable. Furthermore, the hashing time
> does not depend on the syntactic elements in the AST, but only on the
> used parts. So even if you have a thousands of type declarations, a file
> with a single 'void foo() {}' takes no time.
> 
> We use a non-cryptographical hash (MurMur2) to speed things up a little
> bit. And since we do not have to defend against evil attackers, we
> believe a non-cryptographic hash is enough. However, for our current
> implementation, we still have a few optimizations that could cut down a
> little bit on the hashing[1].
> 
> chris
> 
> [1] We hash currently every type annotation for every AST note. However,
>    only the type fields for variable declarations and function
>    signatures are relevant, since all other type fields are derived
>    from them in a determinsitic fashion.
> -- 
> Christian Dietrich, M.Sc. (Scientific Staff)
> Institute for Systems Engineering (Systems and Computerarchitecture)
> Leibniz Universität Hannover
> Appelstraße 4
> 30167 Hannover, Germany
> 
> Tel:    +49 511 762-19737
> Fax:    +49 511 762-19733
> eMail:  dietrich at sra.uni-hannover.de
> WWW:    https://www.sra.uni-hannover.de