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Re: [cdt-dev] MSVC (was Build Again) (OT)
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On 2010-04-29 21:33, Axel Müller wrote:
Am Donnerstag, 29. April 2010 10:25:20 schrieb Jesper Eskilson:
On 2010-04-28 15:25, Doug Schaefer wrote:
It wouldn't be that complicated. You'd probably do it the same way you
pass the list of object files to the linker.
No, because the linker ALWAYS receives ALL the objects files. The rule
typically looks like this
foo.exe: $(OBJS)
$(LINK) $^ -o $@
If you would do the same with the compiler, you would *always* compile
*all* source files. Instead, compiler rules typically look something
like this:
.c.h:
$(CC) -c $< -o $@
This will invoke "cl -c foo.c -o foo.o", once for each source file which
is out of date. I cannot see how the /MP option allows cl to
parallellize over all the source files it should compile.
This is getting too off-topic for CDT, so I'll stop here. Sorry if I'm
not able to make myself clear.
I just had a look at our Makefiles. The structure is indeed like you said.
.c.h:
$(CC) -c $< -o $@
The compiler is invoked for every source file. When I add /MP3 to the compiler
flags three cl instances will run. I think one instance acts like a kind of
manager and two instances do actually compile. The compile time benefit is
measurable.
More info here:
http://msdn.microsoft.com/en-us/library/bb385193.aspx
(Yes, I know I said I would stop, but since Doug said this was on topic,
I'll reply anyway. :-))
From that page: "The /MP option can reduce the total time to compile
the source files on the command line." I cannot see anything about
cl.exe being able to parallellize over multiple *independently* launched
cl.exe instances. Maybe that's an undocumented feature. Or maybe NMAKE
is able to automatically "merge" independent compiler invocations into a
single one.
How does the process tree look like? Are all cl.exe processes owned by
NMAKE or is there a main cl.exe process which creates the others?
--
/Jesper
