Instruct Automake to use C++ compiler for *.S files?

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Instruct Automake to use C++ compiler for *.S files?



We have a C++ project. We need to assemble a *.S file through the compiler. In our GNUmakefile we use:


*.S


GNUmakefile


# ARM asm implementation.
aes-armv4.o : aes-armv4.S
$(CXX) $(CXXFLAGS) $(AES_FLAGS) -mfloat-abi=$(FP_ABI) -c $<



We have the following in our Makefile.am, which depends on AC_SUBST([AES_FLAGS], [-march=armv7-a -Wa,--noexecstack]) in configure.ac:


Makefile.am


AC_SUBST([AES_FLAGS], [-march=armv7-a -Wa,--noexecstack])


configure.ac


libaes_armv4_la_SOURCES = aes-armv4.S
libaes_armv4_la_CXXFLAGS = $(AM_CXXFLAGS) $(AES_FLAGS)



However, the resulting Makefile uses the C compiler and fails to use the flags we setup for the source file:


Makefile


C


make[1]: Entering directory '/home/build/cryptopp'
/bin/bash ./libtool --mode=compile gcc -DHAVE_CONFIG_H -I. -g -O2 -MT aes-armv4.lo -MD -MP -MF .deps/aes-armv4.Tpo -c -o aes-armv4.lo aes-armv4.S
libtool: compile: gcc -DHAVE_CONFIG_H -I. -g -O2 -MT aes-armv4.lo -MD -MP -MF .deps/aes-armv4.Tpo -c aes-armv4.S -fPIC -DPIC -o .libs/aes-armv4.o
libtool: compile: gcc -DHAVE_CONFIG_H -I. -g -O2 -MT aes-armv4.lo -MD -MP -MF .deps/aes-armv4.Tpo -c aes-armv4.S -o aes-armv4.o >/dev/null 2>&1
mv -f .deps/aes-armv4.Tpo .deps/aes-armv4.Plo



AES_FLAGS provides -march=armv7-a -Wa,--noexecstack. -march=armv7-a increases performance due to unaligned loads and -Wa,--noexecstack is a security requirement.


AES_FLAGS


-march=armv7-a -Wa,--noexecstack


-march=armv7-a


-Wa,--noexecstack



The manual does not state how to tell Autotools to use the C++ compiler in this instance. Also see 8.9 C++ Support.



How do I tell Automake to use the C++ compiler and flags for the source file?



Our configure.ac has the following. It lacks a reference to the C compiler and never touches AC_PROG_CC, CFLAGS or AM_CFLAGS because we are a C++ project:


configure.ac


AC_PROG_CC


CFLAGS


AM_CFLAGS


AC_PROG_CXX
AC_LANG([C++])

...
AC_SUBST([AES_FLAGS], [-march=armv7-a -Wa,--noexecstack])





gnu.org/software/automake/manual/html_node/…
– Sam Varshavchik
3 hours ago





Thanks @Sam, that was it. Do you want to answer?
– jww
3 hours ago




1 Answer
1



automake uses the CCAS macro to specify which compiler to run, using CCASFLAGS and AM_CCASFLAGS to specify any custom compilation options.


CCAS


CCASFLAGS


AM_CCASFLAGS



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