On 5/2/19 10:16 AM, Bruno Haible wrote: > Hi Alex, > >> GNU Emacs spends about ~6s out of ~28s in its configuration process to >> determine supported C compiler warnings. > > It's good that you tackle this problem. Thanks! > >> This macro first checks if all the supplied warnings are >> supported, and then only checks them individually (via gl_WARN_ADD) if >> that check fails. > > So, the speed improvement exists only for the newest compilers; people > who use a compiler that was released 1 or 2 years before the Emacs release > will see no improvement. > > How about a modified algorithm? > 1) In a first pass, use the GCC's --help=warnings output to determine which > warnings are likely supported. Unfortunately, for clang, I don't know > of such a help option. > 2) When the first check (with all the options) fails, split that list into > 2 (or 3, or 4) chunks of nearly equal size, and perform the check > on each sub-list. And so on, recurse. > This way, you would still get some performance benefit if the compiler > supports 80% of the requested warnings but not all of them. > > Finally, since there is no semantic difference between gl_WARN_ADD and > gl_WARN_ADD_MULTIPLE, except that the latter allows multiple options, how > about extending the gl_WARN_ADD macro (to allow multiple options) instead > of defining a different macro (gl_WARN_ADD_MULTIPLE)?
Mentioned 2 years ago (but being still too busy to make a gnulib module from it); Wget2's implementation using --help=warnings is here and FSF copyrighted: https://gitlab.com/gnuwget/wget2/blob/master/m4/wget_manywarnings.m4 And how to use / fine-tune it can be seen in L106 of https://gitlab.com/gnuwget/wget2/blob/master/configure.ac There is gcc and clang support. The configure.ac creates two sets of compiler flags, for wget2/libwget sources and for gnulib sources. Regards, Tim
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