Difficulties configuring LTO on macOS with clang

[Barry Scott]

> On 9 Jul 2019, at 19:28, Scott Colby <scott at scolby.com> wrote:
> 
> Hello all,
> 
> I am having difficulty building Python with `--with-lto` on macOS 10.14.5. With a clean checkout of the CPython source at the tagged release for 3.7.4, I ran:
> 
> $ ./configure --prefix=/opt/python3.7.4 --enable-optimizations --enable-ipv6 --with-lto --with-openssl=$(brew --prefix openssl) CC=clang
> checking for git... found
> checking build system type... x86_64-apple-darwin18.6.0
> checking host system type... x86_64-apple-darwin18.6.0
> checking for python3.7... python3.7
> checking for --enable-universalsdk... no
> checking for --with-universal-archs... no
> checking MACHDEP... checking for --without-gcc... no
> checking for --with-icc... no
> checking for gcc... clang
> checking whether the C compiler works... yes
> checking for C compiler default output file name... a.out
> checking for suffix of executables...
> checking whether we are cross compiling... no
> checking for suffix of object files... o
> checking whether we are using the GNU C compiler... yes
> checking whether clang accepts -g... yes
> checking for clang option to accept ISO C89... none needed
> checking how to run the C preprocessor... clang -E
> checking for grep that handles long lines and -e... /usr/bin/grep
> checking for a sed that does not truncate output... /usr/bin/sed
> checking for --with-cxx-main=<compiler>... no
> checking for clang++... no
> configure:
> 
>  By default, distutils will build C++ extension modules with "clang++".
>  If this is not intended, then set CXX on the configure command line.
> 
> checking for the platform triplet based on compiler characteristics... darwin
> checking for -Wl,--no-as-needed... no
> checking for egrep... /usr/bin/grep -E
> checking for ANSI C header files... yes
> checking for sys/types.h... yes
> checking for sys/stat.h... yes
> checking for stdlib.h... yes
> checking for string.h... yes
> checking for memory.h... yes
> checking for strings.h... yes
> checking for inttypes.h... yes
> checking for stdint.h... yes
> checking for unistd.h... yes
> checking minix/config.h usability... no
> checking minix/config.h presence... no
> checking for minix/config.h... no
> checking whether it is safe to define __EXTENSIONS__... yes
> checking for the Android API level... not Android
> checking for --with-suffix...
> checking for case-insensitive build directory... yes
> checking LIBRARY... libpython$(VERSION)$(ABIFLAGS).a
> checking LINKCC... $(PURIFY) $(MAINCC)
> checking for GNU ld... no
> checking for --enable-shared... no
> checking for --enable-profiling... no
> checking LDLIBRARY... libpython$(VERSION)$(ABIFLAGS).a
> checking for ar... ar
> checking for readelf... no
> checking for a BSD-compatible install... /usr/local/bin/ginstall -c
> checking for a thread-safe mkdir -p... /usr/local/bin/gmkdir -p
> checking for --with-pydebug... no
> checking for --with-assertions... no
> checking for --enable-optimizations... yes
> checking for --with-lto... yes
> checking target system type... x86_64-apple-darwin18.6.0
> checking for -llvm-ar... no
> checking for llvm-ar... ''
> configure: error: llvm-ar is required for a --with-lto build with clang but could not be found.

I see /opt/local/bin/llvm-ar-mp-4.0 which ends up running the llvm-ar from 
/opt/local/libexec/llvm-4.0/bin

Just a guess but maybe you need to add /opt/local/libexec/llvm-4.0/bin/ to your path
as its where llvm-ar is.

Barry

> 
> If I change the command to `... CC=cc` configuration appears to happen normally.
> 
> On my version of macOS, cc is a symlink to clang.
> 
> Why is this happening? What should I be doing differently to explicitly specify clang as my compiler?
> 
> Thank you,
> Scott Colby
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>