Messages in this thread | | | Date | Tue, 31 Oct 2000 21:06:33 -0600 | Subject | Re: test10-pre7 | From | Peter Samuelson <> |
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[Russell King] > Since someone kindly enlightened me that LINK_FIRST was unsorted, I'm > finding it very hard to grasp what the difference is between an > unsorted LINK_FIRST and unsorted LINK_LAST list, and an unsorted > obj-y list. From what I understand, obj-y = $(LINK_FIRST) > $(LINK_LAST) ?
Not quite. If that's how you understand it, I see why you think it's a bad idea. Here's what is *really* happening:
obj-y = {subset of LINK_FIRST that is in obj-y} \ {subset of obj-y that is not in LINK_FIRST or LINK_LAST} \ {subset of LINK_LAST that is in obj-y}
GNU make has extensions that make this easy to implement -- no more verbose than the pseudocode, in fact.
The biggest difference between LINK_FIRST and obj-y is that LINK_FIRST is meant to be a static list, not dependent on CONFIG_*, and specifies *only* those objects which must be linked before (or after, for LINK_LAST) other objects. In the common case, most object files do *not* appear in LINK_FIRST or LINK_LAST, but just in O_OBJS.
In the pathological case of strict requirements for the whole directory, LINK_FIRST would contain all of obj-y. Keith and I think this is a rare case -- a more common case is the opposite: LINK_FIRST/LAST are empty because there are *no* ordering requirements.
Again, anything that appears in O_OBJS but not in LINK_FIRST is linked in arbitrary order. Anything that appears in LINK_FIRST but not in O_OBJS is ignored. That is why it can be a static list.
Since LINK_FIRST is a (usually short) static list, it is easy for the author to guarantee that it has no duplicate files in it. By contrast, O_OBJS (or obj-y) frequently has duplicates, because of things like
obj-$(CONFIG_FOO) := foo.o xxx.o obj-$(CONFIG_BAR) := bar.o xxx.o
where xxx.o is something like 8390 support for network cards.
Removing duplicates is a side effect of the GNU make 'sort' function, which is THE ONLY REASON we want to sort $(O_OBJS). The reordering is the "other" side effect, the less desirable one. GNU make does not provide a 'uniq-without-sort' function, and while one is trivial to write in e.g. shell, some of us consider a shell hack to be, well, more hackish than LINK_FIRST.
** BTW, the only reason I'm still posting to this thread, which seems pretty moot because "Linus Has Spoken", is that I believe there is still a lot of misunderstanding about what LINK_FIRST actually does. When I'm satisfied that the opponents truly *understand* LINK_FIRST and still oppose it, I'll shut up.
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