[cfe-dev] 'too many sections' again when building ASTMatchers/Dynamic/Registry.cpp on Windows

[Samuel Benzaquen]

On Thu, Aug 29, 2013 at 3:49 PM, Kristof Beyls <kristof.beyls at arm.com>wrote:

> Hi Samuel,
>
> We've observed that in the past few days, debug-builds with Visual Studio
> have started failing again because too many sections are generated for
> ASTMatchers/Dynamic/Registry.cpp.
>
> This has been observed and discussed before in this email thread:
> http://lists.cs.uiuc.edu/pipermail/cfe-dev/2013-June/030102.html.
>
> It seems that since then, bit by bit, the number of sections for
> Registry.cpp
> has been creeping up. I've done a quick experiment on linux to measure how
> the size and the number of sections in Registry.cpp.o has been increasing
> over
> time. The table does show that your fix in r183768 did reduce the number of
> sections a lot; but since then, the number of sections has been increasing
> steadily and quite rapidly.
>
>                 Size of Register.cpp.o
> svn id      bytes               sections
> 189448  27122264        28108
> 189353  27204656        28259
> 189031  26473624        27284
> 187500  26451288        26952
> 186000  21940824        23119
> 183769  19289872        20544
> 183768  33647984        35738
>
> You did say on the previous email thread that you had a few more ideas on
> how
> to reduce the number of sections in Register.cpp.o. I'm wondering if you
> would
> be able to share the ideas you've got?
>

The problem is that there are a lot of AST nodes, and each node is getting
a whole bunch of classes instantiated, many of them with virtual methods.
For example, there are around 200 symbols related to clang::WhileStmt. It
instantiates TrueMatcher<clang::WhileStmt>, DynCastMatcher<clang::Stmt, ,
lang::WhileStmt>, BindableMatcher<clang::WhileStmt>,
MatcherInterface<clang::WhileStmt>,
SingleNodeMatcherInterface<clang::WhileStmt>,
VariadicOperatorMatcherInterface<clang::WhileStmt>,
Matcher<clang::WhileStmt>::WrappedMatcher, Matcher<clang::WhileStmt>,
IdMatcher<clang::WhileStmt>.
All of these have virtual methods, which means that there are 2 symbols per
constructor, 3 for the destructor and 1 for the vtable. This is without
counting the actual methods in the class.
We then do more method/class instantiations for the dynamic binding.

The main fix I wanted to try is to reduce the number of classes that get
instantiated. On the previous fix I removed vector<T> and list<T> for each
AST node. I just replaced those uses with plain arrays.
I will do another round of fixes. Do you have a target size/sections ?

At the moment, it seems clang cannot be build with visual studio in debug
> mode; and the large file size of Register.cpp.o and the large number of
> sections seems to be slowing down the compile process on other platforms
> too.
>

In the worst case we could use plan A, which was splitting Register.cpp
into many smaller files. We don't need to have all this code in the same
compilation unit. I wanted to avoid doing this because it doesn't help to
the total size of the objects and makes the code more complex.

_Sam


>
> Thanks,
>
> Kristof
>
>
>
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