devel::host-compat-toolchain fails for aarch64 targets

[mahaase]

devel::host-compat-toolchain fails for aarch64 targets: gmp.h not found (GCC t-aarch64 ALL_SPPFLAGS typo)

Summary

devel::host-compat-toolchain cannot be built whenever its isolation toolchain targets an aarch64 machine. The build dies in devel::compat::gcc-cross-bare:

In file included from gcc/config/aarch64/aarch64-speculation.cc:22:
gcc/system.h:687:10: fatal error: gmp.h: No such file or directory
  687 | #include <gmp.h>
make[2]: *** [gcc/config/aarch64/t-aarch64:81: aarch64-speculation.o] Error 1

Failed package:
devel::host-compat-toolchain/devel::compat::cross-toolchain/devel::compat::gcc-cross/libs::compat::glibc/devel::compat::gcc-cross-bare

Root cause

gcc/config/aarch64/t-aarch64 has a typo in the rule that builds aarch64-speculation.o:

        $(COMPILER) -c $(ALL_COMPILERFLAGS) $(ALL_SPPFLAGS) $(INCLUDES) \
                                            ^^^^^^^^^^^^^^
          $(srcdir)/config/aarch64/aarch64-speculation.cc

Every other rule in that very same file uses $(ALL_CPPFLAGS). ALL_SPPFLAGS is undefined, so it expands to nothing and, because
ALL_CPPFLAGS = $(INCLUDES) $(CPPFLAGS), the whole $(CPPFLAGS) part is silently dropped from that one compile line.

recipes/devel/compat/gcc.yaml locates gmp/mpfr/mpc/isl by appending -I flags to CPPFLAGS (see the loop over $2..$5 in its buildScript), not by passing --with-gmp-include & friends to configure. Those include paths therefore never reach aarch64-speculation.cc.

Verified present in GCC 9.5.0 (the version devel::compat::gcc uses) and in GCC 11.4.0. It is also still present on current GCC master, where it has meanwhile been copy-pasted into a second rule (aarch64-elf-metadata.o), so an upstream GCC fix does not look imminent.

Who is affected

• Native aarch64 build hosts. recipes/devel/host-compat-toolchain.yaml configures the isolation toolchain with AUTOCONF_TARGET: "$(gen-autoconf,bob_isolation)", and $(gen-autoconf,...) derives the machine from the build machine. On an arm64 host that is aarch64-bob_isolation-linux-gnu, so t-aarch64 is used and the build breaks. As far as I can tell devel::host-compat-toolchain — and therefore basement::rootrecipe — simply cannot be built on aarch64 today.
• Any setup where the sandbox architecture is aarch64 (this is how we ran into it).

Why nobody hit this so far

• With an x86_64 target, GCC uses config/i386/t-i386 and never reads t-aarch64 at all.
• recipes/devel/gcc.yaml (the non-compat GCC) has the identical typo in its 11.4.0 source but is unaffected, because it passes --with-gmp-include=... to configure. That lands in GMPINC, which is part of $(INCLUDES) — and $(INCLUDES) is present on the broken compile line.

Two possible fixes

1. Patch the GCC source. Add a patch to the devel::compat::gcc series that turns $(ALL_SPPFLAGS) into $(ALL_CPPFLAGS). One-line, obviously
   correct, applies with zero fuzz.
2. Change the recipe. Make recipes/devel/compat/gcc.yaml pass --with-gmp-include/--with-gmp-lib (and mpfr/mpc/isl) like recipes/devel/gcc.yaml already does, instead of relying on CPPFLAGS. More invasive, but it removes the dependency on a GCC makefile detail and would also fix the linker side.

• Which of the two would you prefer for basement?
• If it is the patch: should the same patch also be added to the recipes/devel/gcc/ series for consistency, even though that GCC is not affected in practice? That would invalidate the cached artifacts of every GCC build.
• Note that either fix changes the variant-id of devel::compat::gcc and thus forces a one-time rebuild of the compat toolchain on x86_64 as well (bit-identical result, since t-aarch64 is not used there).