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).