Hi Eric, Eric Blake wrote: > diff --git a/lib/ctype.in.h b/lib/ctype.in.h > index ccf222d..fdf9e1e 100644 > --- a/lib/ctype.in.h > +++ b/lib/ctype.in.h > @@ -23,12 +23,12 @@ > * <http://www.opengroup.org/onlinepubs/9699919799/basedefs/ctype.h.html> > */ > > -#ifndef _GL_CTYPE_H > - > #if __GNUC__ >= 3 > @PRAGMA_SYSTEM_HEADER@ > #endif > > +#ifndef _GL_CTYPE_H > + > /* Include the original <ctype.h>. */ > /* The include_next requires a split double-inclusion guard. */ > #...@include_next@ @NEXT_CTYPE_H@
Did you know that GCC has a special optimization for include files? It recognizes the idiom that the file starts with a #ifndef FOO directive and ends with the corresponding #endif directive. When the file is then #included a second time and FOO is defined, GCC skips the file entirely: it is not parsed again. You find this implemented in gcc-4.4.3/libcpp/files.c (field 'cmacro') and gcc-4.4.3/libcpp/directives.c (field 'mi_cmacro'). This optimization exists in GCC for a long time. I find it sad that patch destroys this optimization without a very good reason. The optimization brings some speed. I measured the saving to be 0.0012 seconds of compilation time for a 40 KB include file. How to reproduce: I took a copy of /usr/include/unistd.h and added the #if __GNUC__ >= 3 #pragma system_header #endif once inside, and once outside the outermost #ifndef. Then created a .c file that #includes this file 100 times. Average time of "gcc -c foo.c" (over 10 runs, real time) was: - inside: 0.067 sec - outside: 0.190 sec So, on average, on a 40 KB include file, it saves 0.0012 seconds of compilation time. Bruno
