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

In the gcc precompiled header model, one explicitly runs clang with `-x
c++-header` on a .h file to produce a gch file, and then includes the header
with `-include foo.h` and if a .gch file exists for that header it gets used.
This is documented at
http://clang.llvm.org/docs/UsersManual.html#precompiled-headers

cl.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 keeps
track of #include lines in the main translation unit and writes everything up
to an `#include "header.h"` line into a pch file. Conversely, /Yuheader.h tells
the 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 /Yu
without an argument, in that case a `#pragma hrdstop` takes the role of
controlling the point where pch ends and real code begins.)

This patch implements limited support for this in that it requires the pch
header to be passed as a /FI force include flag – with this restriction,
it can be implemented almost completely in the driver with fairly small amounts
of code. For /Yu, this is trivial, and for /Yc a separate pch action is added
that runs before the actual compilation. After r261774, the first failing
command makes a compilation stop – this means if the pch fails to build the
main compilation won't run, which is what we want. However, in /fallback builds
we need to run the main compilation even if the pch build fails so that the
main compilation's fallback can run. To achieve this, add a ForceSuccessCommand
that pretends that the pch build always succeeded in /fallback builds (the main
compilation will then fail to open the pch and run the fallback cl.exe
invocation).

If /Yc /Yu are used in a setup that clang-cl doesn't implement yet, clang-cl
will now emit a "not implemented yet; flag ignored" warning that can be
disabled 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 only
enabled by an internal driver flag. Once it's more stable, this internal flag
will disappear.

(The default stdafx.h setup passes stdafx.h as explicit argument to /Yc but not
as /FI – instead every single TU has to `#include <stdafx.h>` as first thing it
does. Implementing support for this should be possible with the approach in
this patch with minimal frontend changes by passing a --stop-at / --start-at
flag from the driver to the frontend. This is left for a follow-up. I don't
think we ever want to support `#pragma hdrstop`, and supporting it with this
approach isn't easy: This approach relies on the driver knowing the pch
filename in advance, and `#pragma hdrstop(out.pch)` can set the output
filename, 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.exe
would put them, but the pch file format is of course incompatible. This has
ramifications on /fallback, so /Yc /Yu aren't passed through to cl.exe in
/fallback builds.

http://reviews.llvm.org/D17695


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@262420 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 14 insertions, 0 deletions

diff --git a/include/clang/Basic/DiagnosticDriverKinds.td b/include/clang/Basic/DiagnosticDriverKinds.td
index c6e6eaf47c..ce455ae976 100644
--- a/include/clang/Basic/DiagnosticDriverKinds.td
+++ b/include/clang/Basic/DiagnosticDriverKinds.td
@@ -98,6 +98,20 @@ def err_drv_unknown_argument : Error<"unknown argument: '%0'">;
 def warn_drv_unknown_argument_clang_cl : Warning<
   "unknown argument ignored in clang-cl: '%0'">,
   InGroup<UnknownArgument>;
+
+def warn_drv_ycyu_no_arg_clang_cl : Warning<
+  "support for '%0' without a filename not implemented yet; flag ignored">,
+  InGroup<ClangClPch>;
+def warn_drv_ycyu_different_arg_clang_cl : Warning<
+  "support for '/Yc' and '/Yu' with different filenames not implemented yet; flags ignored">,
+  InGroup<ClangClPch>;
+def warn_drv_ycyu_no_fi_arg_clang_cl : Warning<
+  "support for '%0' without a corresponding /FI flag not implemented yet; flag ignored">,
+  InGroup<ClangClPch>;
+def warn_drv_yc_multiple_inputs_clang_cl : Warning<
+  "support for '/Yc' with more than one source file not implemented yet; flag ignored">,
+  InGroup<ClangClPch>;
+
 def err_drv_invalid_value : Error<"invalid value '%1' in '%0'">;
 def err_drv_invalid_int_value : Error<"invalid integral value '%1' in '%0'">;
 def err_drv_invalid_remap_file : Error<