[LLVMbugs] [Bug 20756] New: Create a helper pragma to make unity builds easire

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=20756

            Bug ID: 20756
           Summary: Create a helper pragma to make unity builds easire
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: jpakkane at gmail.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Unity builds are a method of massively speeding up C++ compilations, up to and
including 90% faster. The basic idea is that you don't compile files a.cc,
b.cc, c.cc etc in separate invocations. Instead you create a file that looks
like this:

#include<a.cc>
#include<b.cc>
#include<c.cc>
etc

and then compile only this file. Some build systems (such as Meson) can even
generate this "unity file" automatically.

The big downside of this is that if two files have, say, a static function
doStuff, then the unity build will fail due to symbol crashes. This is tedious
and one of the main reasons unity builds are not used all that much.

A syntactically simple fix for this would be to create a new pragma called
unityfence that would be used like this:

#include<a.cc>
#pragma unityfence
#include<b.cc>
#pragma unityfence
#include<c.cc>
#pragma unityfence
etc

When the compiler encounters this pragma it would do the following:

- unset any "using" declarations encountered
- would remove static and anonymous symbols from the list of encountered
symbols but would keep them around and link already encountered usages to them
- hide other information which could cause build failures

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