It'd be good to be a bit more descriptive than "compilation failed".

The autoconf syntax here is a little confusing. This message isn't from the attempt to optimize with -icf=safe, it's from actually compiling the test program. BOLT is a post-link optimizer, so we give it a compiled and linked executable as an argument. (There is also ICF support in many linkers, to make things more confusing.) I'm using autoconf's function for running test-compiles for the side effect of creating a binary to pass to BOLT, but we need to handle the case where the compile, for whatever reason, fails. I don't expect anyone to hit this error because if you couldn't compile programs you probably couldn't get this far in ./configure anyway, but it's good to have something for the "can't happen" case.

The pseudo-Python equivalent of this check is

py_bolt_icf_flag = "-icf=safe"

@autoconf.cache_check(
    "py_cv_bolt_icf_safe",
    f"whether {LLVM_BOLT} supports safe identical code folding",
)
def check_bolt_icf_safe():
    saved_cflags, saved_ldflags = CFLAGS, LDFLAGS
    CFLAGS, LDFLAGS = CFLAGS_NODIST, LDFLAGS_NODIST
    if autoconf.test_link(autoconf.make_program("", "")):
        py_cv_bolt_icf_safe = False
        p = subprocess.run(
            [LLVM_BOLT, "-icf=safe" "-o", "conftest.bolt", f"conftest{EXEEXT}"],
            stdout=autoconf.message_log, stderr=autoconf.message_log,
        )
        if p.returncode == 0:
            py_cv_bolt_icf_safe = True
    else:
        raise RuntimeError("could not compile empty test program")
    CFLAGS, LDFLAGS = saved_cflags, saved_ldflags
    return py_cv_bolt_icf_safe

if not check_bolt_icf_safe():
    py_bolt_icf_flag = ""

It does occur to me, writing this out, that you could theoretically hit this failure case if your compiler and linker don't support the flags needed to produce a BOLT-able binary (Wl,--emit-relocs -fno-pie -no-pie), because this is the first time we're asking autoconf to use those flags in a test-compile. I have no idea why you would ask for BOLT if you don't have a vaguely compatible compiler, though, and you'd previously get a random compile error at some later point in the process, so I don't think this is making anything worse... but we could change the message to "could not compile and link test program with flags needed for BOLT" or something.

I would also accept the argument that I need wider indentation or some comments or something to make the autoconf readable.

Consider using our WITH_SAVE_ENV macro; it automatically saves and restores CFLAGS, CPPFLAGS, LDFLAGS, and LIBS:

Moreover, the double brackets ([[]]) are not needed; a single pair ([]) is sufficient.