[cfe-dev] [RFC] Compressing AST files by default?

[Richard Smith via cfe-dev]

On Thu, Oct 20, 2016 at 2:23 AM, Ilya Palachev via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> Hi,
>
> It seems that compressing AST files with simple "gzip --fast" makes them
> 30-40% smaller.
> So the questions are:
>  1. Is current AST serialization format really non-compressed (only
> abbreviations in bit stream format)?
>  2. Is it worthwhile to compress AST by default (with -emit-ast)?
>  3. Will this break things like PCH?
>  4. What's the current trade-off between PCH compile time and disk usage?
> If AST compression makes compilation a bit slower, but reduces the disk
> usage significantly, will this be appropriate for users or not?
>
> LLVM already has a support for compression (functions compress/uncompress
> in include/llvm/Support/Compression.h).


The current AST format is designed for lazy, partial loading from disk; we
make heavy use of file offsets to pull in only the small portions of AST
files that are actually used. In a compilation using hundreds or thousands
of AST files, it's essential that we don't load any more than we need to
(just the file headers) since we should need essentially nothing from
almost all loaded files.

Any approach that requires the entire file to be decompressed seems like a
non-starter. I would expect you could get something like the 30-40%
improvements you're seeing under gzip by making better use of abbreviations
and using smarter representations generally. There is some easy low-hanging
fruit here.
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