Check whether core MPS is freestanding

[rptb1]

Working on #133

Plan of work:

• Work out what compiler invocations can be used on available platforms to check dependencies on libraries
• Implement dummy freestanding plinth
• Resolve problems
• Add to CI checks

[rptb1]

gcc -nostdlib -DCONFIG_PLINTH_NONE -DCONFIG_PF_ANSI mps.c mpslibfs.c on Ubuntu 22 says:

/usr/bin/ld: warning: cannot find entry symbol _start; defaulting to 0000000000003070
/usr/bin/ld: /tmp/ccZWa646.o: in function `mps_arena_release':
mps.c:(.text+0x525): undefined reference to `_setjmp'
/usr/bin/ld: /tmp/ccZWa646.o: in function `mps_arena_start_collect':
mps.c:(.text+0x70d): undefined reference to `_setjmp'
/usr/bin/ld: /tmp/ccZWa646.o: in function `mps_arena_collect':
mps.c:(.text+0x850): undefined reference to `_setjmp'
/usr/bin/ld: /tmp/ccZWa646.o: in function `mps_arena_step':
mps.c:(.text+0x9a3): undefined reference to `_setjmp'
/usr/bin/ld: /tmp/ccZWa646.o: in function `mps_alloc':
mps.c:(.text+0x2369): undefined reference to `_setjmp'
/usr/bin/ld: /tmp/ccZWa646.o:mps.c:(.text+0x34bc): more undefined references to `_setjmp' follow
/usr/bin/ld: /tmp/ccZWa646.o: in function `VMInit':
mps.c:(.text+0xa08a2): undefined reference to `malloc'
/usr/bin/ld: /tmp/ccZWa646.o: in function `VMFinish':
mps.c:(.text+0xa0d31): undefined reference to `free'
collect2: error: ld returned 1 exit status

[rptb1]

/usr/bin/ld: /tmp/ccZWa646.o: in function VMInit': mps.c:(.text+0xa08a2): undefined reference to malloc'
/usr/bin/ld: /tmp/ccZWa646.o: in function VMFinish': mps.c:(.text+0xa0d31): undefined reference to free'

These are a consequence of CONFIG_PF_ANSI which says to use malloc and free to supply memory to the arena. A freestanding/embedded implementation is more likely to use the client arena. This check should probably do that too.

[rptb1]

These are a consequence of CONFIG_PF_ANSI which says to use malloc and free to supply memory to the arena.

These errors can be eliminated by excluding:

• vman.c, the ANSI virtual memory mapping, which calls malloc and free to fake virtual memory
• arenavm.c, the VM arena class, which needs a VM implementation
• sa.c, the Sparse Array implementation used by the VM arena class, which also needs a VM implementation

It's conceivable that a freestanding / embedded client of the MPS might provide its own VM, this this could be done better by proding a fake "vmfs.c" stub.

[rptb1]

See also design.mps.lib.goal which says:

The goals of the MPS library interface are:

• To control the dependency of the MPS on the hosted ISO C library so that the core MPS remains freestanding (see design.mps.exec-env).

• To allow the core MPS convenient access to ISO C functionality that is provided on freestanding platforms (see
  design.mps.exec-env).

These documents should be updated as part of this work.

[rptb1]

So now gcc -ffreestanding -nostdlib -DCONFIG_PLINTH_NONE -DCONFIG_PF_ANSI --entry mps_lib_assert_fail mps.c mpslibfs.c vmfs.c on Ubuntu 22 says:

/usr/bin/ld: /tmp/cct0FyDm.o: in function `mps_arena_release':
mps.c:(.text+0x525): undefined reference to `_setjmp'
/usr/bin/ld: /tmp/cct0FyDm.o: in function `mps_arena_start_collect':
mps.c:(.text+0x70d): undefined reference to `_setjmp'
/usr/bin/ld: /tmp/cct0FyDm.o: in function `mps_arena_collect':
mps.c:(.text+0x850): undefined reference to `_setjmp'
/usr/bin/ld: /tmp/cct0FyDm.o: in function `mps_arena_step':
mps.c:(.text+0x9a3): undefined reference to `_setjmp'
/usr/bin/ld: /tmp/cct0FyDm.o: in function `mps_alloc':
mps.c:(.text+0x2369): undefined reference to `_setjmp'
/usr/bin/ld: /tmp/cct0FyDm.o:mps.c:(.text+0x34bc): more undefined references to `_setjmp' follow
collect2: error: ld returned 1 exit status

And clang -ffreestanding -nostdlib -DCONFIG_PLINTH_NONE -DCONFIG_PF_ANSI -e mps_lib_assert_fail -Wno-return-type mps.c mpslibfs.c vmfs.c says:

/usr/bin/ld: /tmp/mps-df6167.o: in function `mps_arena_release':
mps.c:(.text+0x1ad0): undefined reference to `_setjmp'
/usr/bin/ld: /tmp/mps-df6167.o: in function `mps_arena_start_collect':
mps.c:(.text+0x20c0): undefined reference to `_setjmp'
/usr/bin/ld: /tmp/mps-df6167.o: in function `mps_arena_collect':
mps.c:(.text+0x2310): undefined reference to `_setjmp'
/usr/bin/ld: /tmp/mps-df6167.o: in function `mps_arena_step':
mps.c:(.text+0x254a): undefined reference to `_setjmp'
/usr/bin/ld: /tmp/mps-df6167.o: in function `mps_alloc':
mps.c:(.text+0x68d9): undefined reference to `_setjmp'
/usr/bin/ld: /tmp/mps-df6167.o:mps.c:(.text+0x88eb): more undefined references to `_setjmp' follow
/usr/bin/ld: /tmp/mps-df6167.o: in function `rootCreate':
mps.c:(.text+0x4daef): undefined reference to `memcpy'
/usr/bin/ld: /tmp/mps-df6167.o: in function `StackScan':
mps.c:(.text+0x8684c): undefined reference to `_setjmp'
clang: error: linker command failed with exit code 1 (use -v to see invocation)

so we've picked up a reference to memcpy, but man clang says:

   -ffreestanding
          Indicate  that  the  file should be compiled for a freestanding,
          not a hosted, environment. Note that it is assumed that a  free‐
          standing  environment will additionally provide memcpy, memmove,
          memset and memcmp implementations, as these are needed for effi‐
          cient codegen for many programs.

and it's not clear whether insertion of this dependency can be suppressed. It may not matter if we use gcc to check. In any case it comes from a transgression of MPS rules with a structure copy, here

mps/code/root.c

Line 204 in 9bc1f98

root->the = *theUnionP;

[rptb1]

Assigned priority "optional" since it seems very likely that the MPS core is still freestanding from the above. But it would still be good to have a CI test for it.

[rptb1]

Revisit this when #38 is resolved. That will probably eliminate the dependencies on setjmp so that the build commands above succeed. Then we can add a simple CI check script.

[rptb1]

The index to source code will need updating (see also #153 ) and the manual for the new preprocessor symbols.