tools build feature: Use CC and CXX from parent

commit c8c188679ccf ("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: c8c188679ccf ("tools build: Use the same CC for feature detection and actual build")
Signed-off-by: Thomas Hebb <[email protected]>
Acked-by: Jiri Olsa <[email protected]>
Cc: David Carrillo-Cisneros <[email protected]>
Cc: Ian Rogers <[email protected]>
Cc: Igor Lubashev <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Quentin Monnet <[email protected]>
Cc: Song Liu <[email protected]>
Cc: Stephane Eranian <[email protected]>
Cc: thomas hebb <[email protected]>
Link: http://lore.kernel.org/lkml/0a6e69d1736b0fa231a648f50b0cce5d8a6734ef.1595822871.git.tommyhebb@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

2 files changed, 1 insertions, 3 deletions

diff --git a/tools/build/Makefile.feature b/tools/build/Makefile.feature
index cb152370fdef..774f0b0ca28a 100644
--- a/tools/build/Makefile.feature
+++ b/tools/build/Makefile.feature
@@ -8,7 +8,7 @@ endif
 
 feature_check = $(eval $(feature_check_code))
 define feature_check_code
-  feature-$(1) := $(shell $(MAKE) OUTPUT=$(OUTPUT_FEATURES) CFLAGS="$(EXTRA_CFLAGS) $(FEATURE_CHECK_CFLAGS-$(1))" CXXFLAGS="$(EXTRA_CXXFLAGS) $(FEATURE_CHECK_CXXFLAGS-$(1))" LDFLAGS="$(LDFLAGS) $(FEATURE_CHECK_LDFLAGS-$(1))" -C $(feature_dir) $(OUTPUT_FEATURES)test-$1.bin >/dev/null 2>/dev/null && echo 1 || echo 0)
+  feature-$(1) := $(shell $(MAKE) OUTPUT=$(OUTPUT_FEATURES) CC=$(CC) CXX=$(CXX) CFLAGS="$(EXTRA_CFLAGS) $(FEATURE_CHECK_CFLAGS-$(1))" CXXFLAGS="$(EXTRA_CXXFLAGS) $(FEATURE_CHECK_CXXFLAGS-$(1))" LDFLAGS="$(LDFLAGS) $(FEATURE_CHECK_LDFLAGS-$(1))" -C $(feature_dir) $(OUTPUT_FEATURES)test-$1.bin >/dev/null 2>/dev/null && echo 1 || echo 0)
 endef
 
 feature_set = $(eval $(feature_set_code))
diff --git a/tools/build/feature/Makefile b/tools/build/feature/Makefile
index b1f0321180f5..93b590d81209 100644
--- a/tools/build/feature/Makefile
+++ b/tools/build/feature/Makefile
@@ -74,8 +74,6 @@ FILES=                                          \
 
 FILES := $(addprefix $(OUTPUT),$(FILES))
 
-CC ?= $(CROSS_COMPILE)gcc
-CXX ?= $(CROSS_COMPILE)g++
 PKG_CONFIG ?= $(CROSS_COMPILE)pkg-config
 LLVM_CONFIG ?= llvm-config
 CLANG ?= clang