[gdb/tdep] Fix 'Unexpected register class' assert in amd64_push_arguments

[gdb/tdep] Fix 'Unexpected register class' assert in amd64_push_arguments

Atm, when executing gdb.base/infcall-nested-structs.exp on x86_64-linux, we get:
...
FAIL: gdb.base/infcall-nested-structs.exp: l=c++: types-tc-tf: \
  p/d check_arg_struct_02_01 (ref_val_struct_02_01)
FAIL: gdb.base/infcall-nested-structs.exp: l=c++: types-ts-tf: \
  p/d check_arg_struct_02_01 (ref_val_struct_02_01)
FAIL: gdb.base/infcall-nested-structs.exp: l=c++: types-ti-tf: \
  p/d check_arg_struct_02_01 (ref_val_struct_02_01)

                === gdb Summary ===

nr of expected passes            9255
nr of unexpected failures        3
nr of expected failures          142
...

The 3 FAILs are reported as PR tdep/25096.

The 142 XFAILs are for a gdb assertion failure, reported in PR tdep/24104,
which should have been KFAILs since there's a problem in gdb rather than in
the environment.

A minimal version of the assertion failure looks like this. Consider test.c:
...
struct s { struct { } es1; long f; };
struct s ref = { {}, 'f' };

int __attribute__((noinline,noclone)) check (struct s arg)
{ return arg.f == 'f'; }

int main (void)
{ return check (ref); }
...

When calling 'check (ref)' from main, we have '1' as expected:
...
$ g++ test3.c -g && ( ./a.out; echo $? )
1
...

But when calling 'check (ref)' from the gdb prompt, we get:
...
$ gdb a.out -batch -ex start -ex "p check (ref)"
Temporary breakpoint 1 at 0x4004f7: file test.c, line 8.

Temporary breakpoint 1, main () at test.c:8
8       { return check (ref); }
src/gdb/amd64-tdep.c:982: internal-error: \
  CORE_ADDR amd64_push_arguments(regcache*, int, value**, CORE_ADDR, \
                                 function_call_return_method): \
  Assertion `!"Unexpected register class."' failed.
...

The assert happens in this loop in amd64_push_arguments:
...
          for (j = 0; len > 0; j++, len -= 8)
            {
              int regnum = -1;
              int offset = 0;

              switch (theclass[j])
                {
                case AMD64_INTEGER:
                  regnum = integer_regnum[integer_reg++];
                  break;

                case AMD64_SSE:
                  regnum = sse_regnum[sse_reg++];
                  break;

                case AMD64_SSEUP:
                  gdb_assert (sse_reg > 0);
                  regnum = sse_regnum[sse_reg - 1];
                  offset = 8;
                  break;

                default:
                  gdb_assert (!"Unexpected register class.");
                }
		...
            }
...
when processing theclass[0], which is AMD64_NO_CLASS:
...
(gdb) p theclass
$1 = {AMD64_NO_CLASS, AMD64_INTEGER}
...

The layout of struct s is that the empty field es1 occupies one byte (due to
c++) at offset 0, and the long field f occupies 8 bytes at offset 8.

When compiling at -O2, we can see from the disassembly of main:
...
  4003f0:       48 8b 3d 41 0c 20 00    mov    0x200c41(%rip),%rdi \
                                               # 601038 <ref+0x8>
  4003f7:       e9 e4 00 00 00          jmpq   4004e0 <_Z5check1s>
  4003fc:       0f 1f 40 00             nopl   0x0(%rax)
...
that check is called with field f passed in %rdi, meaning that the
classification in theclass is correct, it's just not supported in the loop in
amd64_push_arguments mentioned above.

Fix the assert by implementing support for 'AMD64_NO_CLASS' in that loop.

This exposes 9 more FAILs of the PR tdep/25096 type, so mark all 12 of them as
KFAIL.

Tested on x86_64-linux.

Tested with g++ 4.8.5, 7.4.1, 8.3.1, 9.2.1.  With 4.8.5, 3 of the 12 KFAILs
are KPASSing.

Tested with clang++ 5.0.2 (which requires removing additional_flags=-Wno-psabi
and adding additional_flags=-Wno-deprecated).

gdb/ChangeLog:

2019-10-16  Tom de Vries  <tdevries@suse.de>

	PR tdep/24104
	* amd64-tdep.c (amd64_push_arguments): Handle AMD64_NO_CLASS in loop
	that handles 'theclass'.

gdb/testsuite/ChangeLog:

2019-10-16  Tom de Vries  <tdevries@suse.de>

	PR tdep/24104
	* gdb.base/infcall-nested-structs.exp: Remove XFAIL for PR tdep/24104.
	Add KFAIL for PR tdep/25096.

Change-Id: I8b66345bbf5c00209ca75b1209fd4d60b36e9ede