[gdb/tdep] Fix inferior call arg passing for amd64

[gdb/tdep] Fix inferior call arg passing for amd64

We currently have 12 KFAILS in gdb.base/infcall-nested-structs.exp for
PR tdep/25096.

A minimal version of the failure looks like this.  Consider test.c:
...
struct s { int c; struct { int a; float b; } s1; };
struct s ref = { 0, { 'a', 'b' } };

int __attribute__((noinline,noclone)) check (struct s arg)
{ return arg.s1.a == 'a' && arg.s1.b == 'b' && arg.c == 0; }

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

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

But when calling 'check (ref)' from the gdb prompt, we get '0':
...
$ gdb a.out -batch -ex start -ex "p check (ref)"
Temporary breakpoint 1 at 0x400518: file test.c, line 8.

Temporary breakpoint 1, main () at test.c:8
8       { return check (ref); }
$1 = 0
...

The layout of struct s is this:
- the field c occupies 4 bytes at offset 0,
- the s1.a field occupies 4 bytes at offset 4, and
- the s1.b field occupies 4 bytes at offset 8.

When compiling at -O2, we can see from the disassembly of main:
...
  4003f0:       48 8b 3d 31 0c 20 00    mov    0x200c31(%rip),%rdi \
                                               # 601028 <ref>
  4003f7:       f3 0f 10 05 31 0c 20    movss  0x200c31(%rip),%xmm0 \
                                               # 601030 <ref+0x8>
  4003fe:       00
  4003ff:       e9 ec 00 00 00          jmpq   4004f0 <_Z5check1s>
...
that check is called with fields c and s1.a passed in %rdi, and s1.b passed
in %xmm0.

However, the classification in theclass (a variable representing the first and
second eightbytes, to put it in SYSV X86_64 psABI terms) in
amd64_push_arguments is incorrect:
...
(gdb) p theclass
$1 = {AMD64_INTEGER, AMD64_INTEGER}
...
and therefore the struct is passed using %rdi and %rsi instead of using %rdi
and %xmm0, which explains the failure.

The reason that we're misclassifying the argument in amd64_classify_aggregate
has to do with how nested struct are handled.

Rather than using fields c and s1.a for the first eightbyte, and using field
s1.b for the second eightbyte, instead field c is used for the first
eightbyte, and fields s1.a and s1.b are classified together in an intermediate
eightbyte, which is then used to merge with both the first and second
eightbyte.

Fix this by factoring out a new function amd64_classify_aggregate_field, and
letting it recursively handle fields of nested structs.

Tested on x86_64-linux.

Tested with g++ 4.8.5, 7.4.1, 8.3.1, 9.2.1.

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/25096
	* amd64-tdep.c (amd64_classify_aggregate_field): Factor out of ...
	(amd64_classify_aggregate): ... here.
	(amd64_classify_aggregate_field): Handled fiels of nested structs
	recursively.

gdb/testsuite/ChangeLog:

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

	PR tdep/25096
	* gdb.base/infcall-nested-structs.exp: Remove PR25096 KFAILs.

Change-Id: Id55c74755f0a431ce31223acc86865718ae0c123