[PATCH] D50640: Fix for bug 38508 - Don't do PCH processing when only generating preprocessor output

[Mike Rice via Phabricator via cfe-commits]

mikerice added a comment.

In https://reviews.llvm.org/D50640#1201216, @thakis wrote:

> How does the gcc driver codepath handle this?


Interestingly I'd say.  So the gcc PCH model uses -include pch.h to use a PCH.  In the driver, -include pch.h is handled by locating a matching pch file (say pch.h.gch) and if found adds -include_pch pch.h.gch instead of -include pch.h.  This occurs even when only preprocessing (-E).  When preprocessing the compiler then handles the -include_pch option by opening pch.h.gch, finding the original file that created it (pch.h) and doing what -include pch.h does.   Seems like it would be better handled by the driver but maybe there is a reason for it.

When we only allowed PCH with a single /FI the model was the same as gcc and it worked similarly.  Since the through header/MS model allows creation of a PCH from any tokens not just a single #include, we can't handle it this way anymore.


https://reviews.llvm.org/D50640