<div dir="ltr">Hi!<br><div><div class="gmail_extra"><br><div class="gmail_quote">On 20 October 2016 at 18:12, Mehdi Amini via cfe-dev <span dir="ltr"><<a href="mailto:cfe-dev@lists.llvm.org" target="_blank">cfe-dev@lists.llvm.org</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><span class=""><br>
> On Oct 20, 2016, at 2:23 AM, Ilya Palachev via cfe-dev <<a href="mailto:cfe-dev@lists.llvm.org">cfe-dev@lists.llvm.org</a>> wrote:<br>
><br>
> Hi,<br>
><br>
> It seems that compressing AST files with simple "gzip --fast" makes them 30-40% smaller.<br>
> So the questions are:<br>
> 1. Is current AST serialization format really non-compressed (only abbreviations in bit stream format)?<br>
> 2. Is it worthwhile to compress AST by default (with -emit-ast)?<br>
> 3. Will this break things like PCH?<br>
> 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?<br>
<br>
</span>Is there a need for this disk usage? If the main use of AST files is C++ modules / PCH, what is a typical size for a module cache directory?<br>
(Compression is expensive)<br></blockquote><div><br><br></div><div>In some cases compression can actually improve the peformance, because in some cases the bottleneck is the I/O, and less data read from the disk and a fast decompression can be beneficial to the overall performance. <br><br></div><div>In case someone wants to do a whole project analysis on merged ASTs, this compression can be a very significant saving. Dumping all of LLVM and Clang TUs to the disk occupies about 45 GB of disk space at the moment.<br></div><div><br> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<br>
—<br>
Mehdi<br>
<div class="HOEnZb"><div class="h5"><br>
<br>
><br>
> LLVM already has a support for compression (functions compress/uncompress in include/llvm/Support/<wbr>Compression.h).<br>
><br>
> Best regards,<br>
> Ilya Palachev<br>
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