Add more detailed comment for ORC stub code

[dongAxis]

Summary

0xF1C40000000025ff means
stub1:

jmpq    *ptr1(%rip)
.byte   0xC4         ; <- Invalid opcode padding.
.byte   0xF1

But 0xF1C40000000025ff is hard to understand what is means. So i use this patch to make code more readable.

[MaskRay]

I guess the code does not need to care about endianness because it is guaranteed to run on x86-64(?) static const or static constexpr should be better.

[dongAxis]

This comment was removed by dongAxis.

[lhames]

I think this would be better explained through comments. There is a comment regarding this code sequence at the top of the function, but it would make sense to add a second one directly above where we use the constant.

[dongAxis]

use static constexpr variable instead of local variable

[lhames]

Hi Axis,

If you would like to add a comment explaining the constant (with reference to the comment at the top of the function) I would be happy to land that, but I don't see any value to changing the code itself.

llvm/lib/ExecutionEngine/Orc/OrcABISupport.cpp

   uint64_t *Stub = reinterpret_cast<uint64_t *>(StubsBlock.base());
   uint64_t PtrOffsetField = static_cast<uint64_t>(NumPages * PageSize - 6)
                             << 16;
+  static constexpr uint8_t StubCode[8] = {
+      0xFF, 0x25, 0x00, 0x00, 0x00, 0x00,   // jmpq    *0x00 (%rip)
+      0xC4,                                 // invalid code: 0xC4, 0xF1
+      0xF1
+  };
   for (unsigned I = 0; I < NumStubs; ++I)
-    Stub[I] = 0xF1C40000000025ff | PtrOffsetField;
+    Stub[I] = *(uint64_t *)StubCode | PtrOffsetField;
 
   if (auto EC = sys::Memory::protectMappedMemory(
           StubsBlock, sys::Memory::MF_READ | sys::Memory::MF_EXEC))