Cfrgtkky

Is their location hardcoded into gcc code or does gcc just call as and we must have as location in our PATH variable?

And in the latter case, how could we create two completely separate gcc toolchains? I mean, how can we make gcc-A invoke as-A and gcc-B invoke as-B if as-A and as-B are both called as?

Some of the paths (e.g., to cc1) are compiled in. Others (e.g., as) use normal lookup in $PATH. This can vary depending on the options GCC is configured with.

You can tell fairly easily by running with strace, and grepping for exec|stat.

$ strace -f gcc foo.c -o foo |& grep exec

⋮

[pid 24943] execve("/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/cc1", …

That is a call to cc1 by a compiled-in path, as you can see from the
lack of looking for it. Its also not in $PATH.

[pid 24944] execve("/home/anthony/bin/as", ["as", "--64", "-o", "/tmp/ccCIrcGi.o", "/tmp/ccbw3PkL.s"], [/* 51 vars */]) = -1 ENOENT (No such file or directory)

[pid 24944] execve("/usr/local/bin/as", ["as", "--64", "-o", "/tmp/ccCIrcGi.o", "/tmp/ccbw3PkL.s"], [/* 51 vars */]) = -1 ENOENT (No such file or directory)

[pid 24944] execve("/usr/bin/as", ["as", "--64", "-o", "/tmp/ccCIrcGi.o", "/tmp/ccbw3PkL.s"], [/* 51 vars */]) = 0

That is looking for as in $PATH. You can tell because its trying each
location in $PATH in order.

I've omitted a lot of strace output—even with just stat and exec, its
several pages long.

Running gcc -v will show you some of the compiled-in paths (as part of the configure line).

How could we create two completely separate gcc toolchains?

Compile GCC from source twice, detailed instructions at: Multiple glibc libraries on a single host

Everything is hardcoded and highly coupled as far as I can see, I don't think there is any other decent solution.

Query the GCC search path

You can also query the GCC search path with:

gcc -print-search-dirs | grep -E '^programs' | tr ':' 'n'

sample output:

programs

 =/usr/lib/gcc/x86_64-linux-gnu/6/

/usr/lib/gcc/x86_64-linux-gnu/6/

/usr/lib/gcc/x86_64-linux-gnu/

/usr/lib/gcc/x86_64-linux-gnu/6/

/usr/lib/gcc/x86_64-linux-gnu/

/usr/lib/gcc/x86_64-linux-gnu/6/../../../../x86_64-linux-gnu/bin/x86_64-linux-gnu/6/

/usr/lib/gcc/x86_64-linux-gnu/6/../../../../x86_64-linux-gnu/bin/x86_64-linux-gnu/

/usr/lib/gcc/x86_64-linux-gnu/6/../../../../x86_64-linux-gnu/bin/

and a specific program with:

gcc -print-prog-name=cc1

sample output:

/usr/lib/gcc/x86_64-linux-gnu/6/cc1

There's an ad-hoc option for that: -B*prefix*, quoting gcc docs:

For each subprogram to be run, the compiler driver first tries the -B prefix, if any. If
that name is not found, or if -B was not specified, the driver tries two standard prefixes,
which are /usr/lib/gcc/ and /usr/local/lib/gcc/. [...]