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Prepending '+' to the recipe line when linking with GCC's -flto=jobserve

From: R. Diez
Subject: Prepending '+' to the recipe line when linking with GCC's -flto=jobserver
Date: Tue, 18 Feb 2020 16:47:05 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1

Hi all:

I am developing firmware similar to this one:

First, I am building a GCC 8.3.0 cross-compiler toolchain for ARM Cortex-M4F on Ubuntu 18.04.4 LTS, and I am then using it to build some embedded software with a cross-compiling Autoconf project.

GCC introduced some time ago option -flto=jobserver in order to use the GNU 
Make jobserver when parallelising LTO builds.

I am actually building many such Autoconf projects from a top-level makefile, so using the top-level jobserver prevents overloading the system with too many parallel jobs. That is what the jobserver was designed for.

When doing a recursive make, you need to place a '+' character at the beginning of the recipe line in order to let GNU Make pass the jobserver file descriptors to the child processes. Alternatively, if you reference variable $(MAKE) inside the recipe like, you get the same effect. Otherwise, GNU Make does not let child processes inherit those file descriptors (or so I heard, probably with "close fd on exec").

In my top-level makefile, I added those '+' prefixes manually, so all Autoconf-generated makefiles are building in parallel but using the top-level jobserver.

The trouble is, invoking GCC in the linking phase with -flto=jobserver is actually a similar "recursive make" situation: GCC needs to inherit those jobserver file descriptors.

However, Autoconf/Automake does not generate that '+' prefix for the linking 

Is there a way to overcome this limitation?

This will probably become a general issue in the future, as each stage could 
theoretically benefit from multi-core CPUs. See for example:

Thanks in advance,

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