[bug #27714] expansion of $(shell) in target forces serialization of targets

[Mike Frysinger]

URL:
  <http://savannah.gnu.org/bugs/?27714>

                 Summary: expansion of $(shell) in target forces
serialization of targets
                 Project: make
            Submitted by: vapier
            Submitted on: Fri 16 Oct 2009 07:50:48 AM GMT
                Severity: 3 - Normal
              Item Group: Bug
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
       Component Version: 3.81
        Operating System: POSIX-Based
           Fixed Release: None
           Triage Status: None

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Details:

i noticed a package recently that would build serially even when i gave it -j
(on a quad machine).  looking into it, the reason was that $(shell) was
appended to a compiler flag leading to it being expanded when the target was
run:
CFLAGS += $(shell .....)

forcing the $(shell) to be evaluated immediately allowed things to build in
parallel:
SOME_CHECK := $(shell .....)
CFLAGS += $(SOME_CHECK)

behavior is seen with make-3.80 and make-3.81

here is a simple example:
$ cat makefile
t = 0 a b c 1 d e f 2 g h i 3 j k l 4 m n o 5 p q r 6
all: $(t)
        @echo
$(t):
        @printf $@ $(shell :)

$ while make -f makefile -j ; do : ; done
0abc1de2ghi3jkl4mno5pqr6
0abc1de2ghi3jkl4mno5pqr6
0abc1de2ghi3jkl4mno5pqr6
0abc1de2ghi3jkl4mno5pqr6
0abc1de2ghi3jkl4mno5pqr6
0abc1de2ghi3jkl4mno5pqr6
0abc1de2ghi3jkl4mno5pqr6
0abc1de2ghi3jkl4mno5pqr6
0abc1de2ghi3jkl4mno5pqr6
0abc1de2ghi3jkl4mno5pqr6
0abc1de2ghi3jkl4mno5pqr6

removing the $(shell :) allows for parallel evaluation:
0abc1de2gi3hjkl4mnoqr56p
0abc1de2ghi3jkl4mn5pqor6
0acde21gbhi3jkl4mno5pqr6
0abc1de2ghi3jkl4mn5qr6po
0abc1d2gehi3jkl4o5mpnqr6
0abc1de2ghi3jkl4mno5pq6r
0abc1de2ghi3jkl4mno5pqr6
0abc1de2ghi3jl4n5pkqorm6
0abc1de2ghi3jkl4mno5pqr6
0abc1de2ghi3jkl4mnpqor56




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