2ca4be97 - clang-cl: Implement initial limited support for precompiled headers.

Tue, 01 Mar 2016 23:16:44 +0000

clang-cl: Implement initial limited support for precompiled headers.In the gcc precompiled header model, one explicitly runs clang with `-xc++-header` on a .h file to produce a gch file, and then includes the headerwith `-include foo.h` and if a .gch file exists for that header it gets used.This is documented athttp://clang.llvm.org/docs/UsersManual.html#precompiled-headerscl.exe's model is fairly different, and controlled by the two flags /Yc and/Yu. A pch file is generated as a side effect of a regular compilation when/Ycheader.h is passed. While the compilation is running, the compiler keepstrack of #include lines in the main translation unit and writes everything upto an `#include "header.h"` line into a pch file. Conversely, /Yuheader.h tellsthe compiler to skip all code in the main TU up to and including `#include"header.h"` and instead load header.pch. (It's also possible to use /Yc and /Yuwithout an argument, in that case a `#pragma hrdstop` takes the role ofcontrolling the point where pch ends and real code begins.)This patch implements limited support for this in that it requires the pchheader to be passed as a /FI force include flag – with this restriction,it can be implemented almost completely in the driver with fairly small amountsof code. For /Yu, this is trivial, and for /Yc a separate pch action is addedthat runs before the actual compilation. After r261774, the first failingcommand makes a compilation stop – this means if the pch fails to build themain compilation won't run, which is what we want. However, in /fallback buildswe need to run the main compilation even if the pch build fails so that themain compilation's fallback can run. To achieve this, add a ForceSuccessCommandthat pretends that the pch build always succeeded in /fallback builds (the maincompilation will then fail to open the pch and run the fallback cl.exeinvocation).If /Yc /Yu are used in a setup that clang-cl doesn't implement yet, clang-clwill now emit a "not implemented yet; flag ignored" warning that can bedisabled using -Wno-clang-cl-pch.Since clang-cl doesn't yet serialize some important things (most notably`pragma comment(lib, ...)`, this feature is disabled by default and onlyenabled by an internal driver flag. Once it's more stable, this internal flagwill disappear.(The default stdafx.h setup passes stdafx.h as explicit argument to /Yc but notas /FI – instead every single TU has to `#include ` as first thing itdoes. Implementing support for this should be possible with the approach inthis patch with minimal frontend changes by passing a --stop-at / --start-atflag from the driver to the frontend. This is left for a follow-up. I don'tthink we ever want to support `#pragma hdrstop`, and supporting it with thisapproach isn't easy: This approach relies on the driver knowing the pchfilename in advance, and `#pragma hdrstop(out.pch)` can set the outputfilename, so the driver can't know about it in advance.)clang-cl now also honors /Fp and puts pch files in the same spot that cl.exewould put them, but the pch file format is of course incompatible. This hasramifications on /fallback, so /Yc /Yu aren't passed through to cl.exe in/fallback builds.http://reviews.llvm.org/D17695llvm-svn: 262420 List of files: /llvm-project-15.0.7/clang/test/Driver/Inputs/pchfile.cpp

Changes in pchfile.cpp

2ca4be97 - clang-cl: Implement initial limited support for precompiled headers.