Hi, On Sat, Mar 16, 2019 at 12:41 AM Doug Anderson <dianders@xxxxxxxxxxxx> wrote: > > Hi, > > On Fri, Mar 15, 2019 at 8:37 AM Masahiro Yamada > <yamada.masahiro@xxxxxxxxxxxxx> wrote: > > > > On Sat, Mar 16, 2019 at 12:27 AM Masahiro Yamada > > <yamada.masahiro@xxxxxxxxxxxxx> wrote: > > > > > > On Fri, Mar 15, 2019 at 8:42 AM Douglas Anderson <dianders@xxxxxxxxxxxx> wrote: > > > > > > > > During a simple no-op (nothing changed) build I saw 39 invocations of > > > > the C compiler with the argument "-print-file-name=include". We don't > > > > need to call the C compiler 39 times for this--one time will suffice. > > > > > > > > Let's change NOSTDINC_FLAGS to a simply expanded variable to avoid > > > > this since there doesn't appear to be any reason it should be > > > > recursively expanded. > > > > > > > > On my build this shaved ~400 ms off my "no-op" build. > > > > > > > > Note that the recursive expansion seems to date back to the (really > > > > old) commit e8f5bdb02ce0 ("[PATCH] Makefile include path ordering"). > > > > It's a little unclear to me if the point of that patch was to switch > > > > the variable to be recursively expanded (which it did) or to avoid > > > > directly assigning to NOSTDINC_FLAGS (AKA to switch to +=) because > > > > someone else (out of tree?) was setting it. I presume later since if > > > > the only goal was to switch to recursive expansion the patch would > > > > have just removed the ":". > > > > > > > > Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx> > > > > > > Applied to linux-kbuild. > > > Thanks. > > > > > > BTW, I noticed no one else > > appends NOSTDINC_FLAGS. > > > > > > So, does it make more sense to do as follows? > > > > > > NOSTDINC_FLAGS := -nostdinc -isystem $(shell $(CC) -print-file-name=include) > > It might. ...but go back to look at commit e8f5bdb02ce0 ("[PATCH] > Makefile include path ordering"). Why did they do a += there since > even then nobody was setting it. Try: > > $ git grep NOSTDINC_FLAGS e8f5bdb02ce0~ > > > Do you understand what was going on in that commit? No, I do not understand. Maybe, it fixed a problem in downstream code since there was no directory like include/asm-xen. If you do not want to take risk, I am fine with this patch, though. > -Doug -- Best Regards Masahiro Yamada