• Thomas Hebb's avatar
    tools build feature: Use CC and CXX from parent · e3232c2f
    Thomas Hebb authored
    commit c8c18867 ("tools build: Use the same CC for feature detection
    and actual build") changed these assignments from unconditional (:=) to
    conditional (?=) so that they wouldn't clobber values from the
    environment. However, conditional assignment does not work properly for
    variables that Make implicitly sets, among which are CC and CXX. To
    quote tools/scripts/Makefile.include, which handles this properly:
    
      # Makefiles suck: This macro sets a default value of $(2) for the
      # variable named by $(1), unless the variable has been set by
      # environment or command line. This is necessary for CC and AR
      # because make sets default values, so the simpler ?= approach
      # won't work as expected.
    
    In other words, the conditional assignments will not run even if the
    variables are not overridden in the environment; Make will set CC to
    "cc" and CXX to "g++" when it starts[1], meaning the variables are not
    empty by the time the conditional assignments are evaluated. This breaks
    cross-compilation when CROSS_COMPILE is set but CC isn't, since "cc"
    gets used for feature detection instead of the cross compiler (and
    likewise for CXX).
    
    To fix the issue, just pass down the values of CC and CXX computed by
    the parent Makefile, which gets included by the Makefile that actually
    builds whatever we're detecting features for and so is guaranteed to
    have good values. This is a better solution anyway, since it means we
    aren't trying to replicate the logic of the parent build system and so
    don't risk it getting out of sync.
    
    Leave PKG_CONFIG alone, since 1) there's no common logic to compute it
    in Makefile.include, and 2) it's not an implicit variable, so
    conditional assignment works properly.
    
    [1] https://www.gnu.org/software/make/manual/html_node/Implicit-Variables.html
    
    Fixes: c8c18867 ("tools build: Use the same CC for feature detection and actual build")
    Signed-off-by: default avatarThomas Hebb <tommyhebb@gmail.com>
    Acked-by: default avatarJiri Olsa <jolsa@kernel.org>
    Cc: David Carrillo-Cisneros <davidcc@google.com>
    Cc: Ian Rogers <irogers@google.com>
    Cc: Igor Lubashev <ilubashe@akamai.com>
    Cc: Namhyung Kim <namhyung@kernel.org>
    Cc: Quentin Monnet <quentin@isovalent.com>
    Cc: Song Liu <songliubraving@fb.com>
    Cc: Stephane Eranian <eranian@google.com>
    Cc: thomas hebb <tommyhebb@gmail.com>
    Link: http://lore.kernel.org/lkml/0a6e69d1736b0fa231a648f50b0cce5d8a6734ef.1595822871.git.tommyhebb@gmail.comSigned-off-by: default avatarArnaldo Carvalho de Melo <acme@redhat.com>
    e3232c2f
Makefile.feature 8.15 KB