[Lldb-commits] [lldb] r318262 - Roll back r318260 because it is causing the windows bot to

[Jason Molenda via lldb-commits]

> On Nov 15, 2017, at 1:38 AM, Pavel Labath <labath at google.com> wrote:
> 
> On 15 November 2017 at 03:41, Jason Molenda via lldb-commits
> <lldb-commits at lists.llvm.org> wrote:
>> Author: jmolenda
>> Date: Tue Nov 14 19:41:47 2017
>> New Revision: 318262
>> 
>> URL: http://llvm.org/viewvc/llvm-project?rev=318262&view=rev
>> Log:
>> Roll back r318260 because it is causing the windows bot to
>> break.  The alignas(__uint128_t) is not recognized with MSVC
>> it looks like.  Zachary, is there a similar type on windows?
>> I suppose I can go with alignas(16) here but I'd prefer to
>> specify the type alignment that I want & let the ABI dictate
>> how much padding is required.
> 
> 
> Are you sure about that? If I understand correctly, this register
> context is used on the host, so, if you're cross-debugging, you'll
> have the *host* ABI dictating the layout of the mac arm64 target
> registers, which sounds like a bad idea.


Thanks for the comment.  The darwin kernel defines this structure like

struct __darwin_arm_neon_state64
{
 __uint128_t __v[32];
 __uint32_t __fpsr;
 __uint32_t __fpcr;
};

Someone was experimenting with saving the fpu register context in a corefile and noticed that lldb's definition of this structure,

  struct VReg {
    uint8_t bytes[16];
  };

  // mirrors <mach/arm/thread_status.h> arm_neon_state64_t
  struct FPU {
    struct VReg v[32];
    uint32_t fpsr;
    uint32_t fpcr;
  };

doesn't require 16-byte alignment so it has a different size.  The kernel structure with __uint128_t will have 8 bytes of padding at the end (total size 528), lldb's definition doesn't require aligning to a 16-byte boundary so it is only 520 bytes long.

We use this FPU definition from RegisterContextDarwin_arm64.h in ObjectFileMachO.cpp when we're reading saved register contexts from a core file.  The core file register context will have 528 bytes which won't match the size of FPU from RegisterContextDarwin_arm64.h so it'll error out.


I'll go with Zachary's suggestion of using the llvm idiom to force 16-byte alignment, I think that's a good way to solve this.