X86 64 abi misaligned packed

[kojix2]

Hello. While reading Crystal source code on Raptor Mini, Raptor pointed out a possible bug, so I tried to fix it. I don't fully understand the issue, but since I'm submitting this PR, not Raptor, I'll explain it.

Packed structs are not always naturally aligned and may contain misaligned fields. In the x86_64 ABI, such values should be passed indirectly through memory, not through registers, in Crystal::ABI::X86_64#classify. This PR fixes an incorrect slot range that was used when setting the Memory class for this case.

Also updated has_misaligned_fields? to correctly handle nested structs and arrays. It now passes the offset through recursive calls to check all fields properly.

@[Link(ldflags: "-L#{__DIR__} -lfoo")]
lib LibX
  @[Packed]
  struct Inner
    x : UInt16
    y : UInt8
  end

  struct Outer
    a : UInt8
    b : Inner
  end

  fun foo(x : Outer) : Int32
end

x = LibX::Outer.new
x.a = 1_u8
x.b.x = 2_u16
x.b.y = 3_u8

pp sizeof(LibX::Inner)
pp sizeof(LibX::Outer)
pp LibX.foo(x)

#include <stdint.h>

#pragma pack(push, 1)
struct Inner {
  uint16_t x;
  uint8_t y;
};
#pragma pack(pop)

struct Outer {
  uint8_t a;
  struct Inner b;
};

int foo(struct Outer x) {
  return (int)x.a + (int)x.b.x + (int)x.b.y;
}

gcc -fPIC -shared -o libfoo.so foo.c

Before

Index out of bounds (IndexError)
  from /crystal/src/indexable.cr:1002:11 in 'unify'
  from /crystal/src/compiler/crystal/codegen/abi/x86_64.cr:120:13 in 'classify'
  from /crystal/src/compiler/crystal/codegen/abi/x86_64.cr:155:20 in 'classify'
  from /crystal/src/compiler/crystal/codegen/abi/x86_64.cr:155:20 in 'classify'
  from /crystal/src/llvm/type.cr:34:5 in 'classify'
  from /crystal/src/compiler/crystal/codegen/abi/x86_64.cr:88:21 in 'abi_info'
  from /crystal/src/compiler/crystal/codegen/call.cr:149:7 in 'visit'
  from /crystal/src/compiler/crystal/syntax/visitor.cr:27:12 in 'accept'
  from /crystal/src/compiler/crystal/codegen/codegen.cr:2581:9 in 'codegen_assign'
  from /crystal/src/compiler/crystal/syntax/visitor.cr:27:12 in 'accept'
  from /crystal/src/enumerable.cr:510:7 in '??'
  from /crystal/src/compiler/crystal/syntax/visitor.cr:27:12 in 'accept'
  from /crystal/src/compiler/crystal/codegen/codegen.cr:2550:7 in 'codegen'
  from /crystal/src/compiler/crystal/compiler.cr:236:15 in 'compile'
  from /crystal/src/compiler/crystal/command.cr:340:5 in 'run'
  from /crystal/src/compiler/crystal/command.cr:55:3 in '__crystal_main'
  from /crystal/src/crystal/main.cr:129:5 in 'main'
  from /crystal/src/crystal/system/unix/main.cr:10:3 in 'main'
  from src/env/__libc_start_main.c:95:2 in 'libc_start_main_stage2'
Error: you've found a bug in the Crystal compiler. Please open an issue, including source code that will allow us to reproduce the bug: https://github.com/crystal-lang/crystal/issues

After

3
4
6