http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54780
Bug #: 54780 Summary: G++ does not find precompiled headers in some cases Classification: Unclassified Product: gcc Version: 4.7.2 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ AssignedTo: unassig...@gcc.gnu.org ReportedBy: jpakk...@gmail.com Created attachment 28326 --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=28326 Some dummy sources demonstrating the issue Using GCC of Ubuntu Quantal, version gcc version 4.7.2 (Ubuntu/Linaro 4.7.2-2ubuntu1). Under some circumstances g++ does not find precompiled headers that are in the search path. I have attached a simple test case that demonstrates the issue. Just run compile.sh that comes with it and watch the output. In the test case there is a common.h header that includes a bunch of STL headers. It is in a subdirectory called hdir. The file is first precompiled and placed in the subdirectory pchdir. The script then compiles a bunch of files that include common.h but do very little else. Both pchdir and hdir are passed in with -I. The precompiled header is found and compilation is fast. Next the script copies common.h to the main directory and compiles the sources again using the exact same compiler switches. What happens is that g++ does not find the precompiled header, probably because it first looks in the source dir and finds common.h but not the corresponding pch and just stops looking. This makes the compilation very slow. Then the script copies the precompiled header to the main directory and compiles the files again. Now g++ finds the pch and compilation is again fast. Summing all this up: precompiled headers can not be used if the header is in the source directory and the pch is in some other directory which is included with -I. This is a problem when doing out-of-source builds.