[Bug other/126875] New: [17 regression] Parallel make compare breaks bootstrap

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126875

            Bug ID: 126875
           Summary: [17 regression] Parallel make compare breaks bootstrap
           Product: gcc
           Version: 17.0
            Status: UNCONFIRMED
          Keywords: build
          Severity: normal
          Priority: P3
         Component: other
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ro at gcc dot gnu.org
                CC: iains at gcc dot gnu.org, ktkachov at gcc dot gnu.org
  Target Milestone: ---
            Target: *-apple-darwin17, *-*-solaris2.11

The new parallel make compare breaks bootstrap on Mac OS X 10.13 (Darwin 17):

Comparing stages 2 and 3
compare.42895.mk:5: *** missing separator.  Stop.
make[2]: *** [Makefile:30524: compare] Error 2

When one keeps the generated compare.<id>.mk, one finds that it starts with

.PHONY: all FORCE
FORCE:
compare/%: FORCE ; @f1=$$r/stage2-$*; f2=$$r/stage3-$*; /usr/local/bin/bash
/vol/gcc/src/hg/master/local-slow/contrib/compare-debug $$f1 $$f2
 > /dev/null 2>&1; st=$$?; if test $$st -eq 1; then case $* in
gcc/cc*-checksum.o | gcc/ada/*tools/* | gcc/m2/gm2-compiler-boot/M2Version* |
gcc/m2/gm2-compiler-boot/SYSTEM* | gcc/m2/gm2version* | gcc/cobol/parse.o) echo
warning: $* differs ;; *) echo $* differs >> "$$bad_compare.$$$$" ;; esac; elif
test $$st -ne 0; then echo "$* compare: error status $$st" >>
"$$bad_compare.$$$$"; fi

I.e. a newline before '>'.

The issue can be reproduces as easily as

$ echo -n '$$' | sed -e 's,/\$,$$,g' | cat -e
$$$

i.e. sed adds a trailing newline.  GNU sed in comparison does not:

$ echo -n '$$' | sed -e 's,/\$,$$,g' | cat -e
$$

Things would be even worse with Solaris /usr/bin/sed which produces no output
at all.

I haven't yet checked if the autoconf docs suggest an alternative/solution.