libunwind: add conventional X86 register numbers for non-Darwin OSes

[emaste]

Summary

For historical reasons Darwin/i386 has ebp and esp swapped in the eh_frame register numbering. That is:

         Darwin      Other
Reg #    eh_frame    eh_frame    DWARF
=====    ========    ========    =====
  4        ebp         esp        esp
  5        esp         ebp        ebp

Although the UNW_X86_* constants are not intended to be coupled with DWARF / eh_frame numbering they are currently conflated in libunwind, so add the numbering for non-Darwin operating systems.

[compnerd]

include/libunwind.h

160 ↗

(On Diff #64476)

GCC has this behaviour, its not specific to Apple. The .eh_frame should use the "swapped" values. We really should add a comment explaining that this is backwards for historical reasons and we cannot fix this due to this now being fossilized into the ABI :-(.

[emaste]

ping?

[emaste]

include/libunwind.h

160 ↗

(On Diff #64476)

The existing case (EBP = 4, ESP = 5) does not work on FreeBSD.

[compnerd]

include/libunwind.h

160 ↗

(On Diff #64476)

Well, we could just special case FreeBSD then:

#if defined(__FreeBSD__)
UNW_X86_ESP = 4,
UNW_X86_EBP = 5,
#else
UNW_X86_EBP = 4,
UNW_X86_ESP = 5,
#endif

[emaste]

Well, we could just special case FreeBSD then:

Sure, I can do that for now. Are you aware of a canonical reference for the swapped registers though? I came across this issue when working on lldb as well, but could not find a definitive reference for the swapping.

[emaste]

Here's the thread on the lldb list: http://lists.llvm.org/pipermail/lldb-commits/Week-of-Mon-20140811/012309.html
and this: http://lists.llvm.org/pipermail/lldb-commits/Week-of-Mon-20140811/012324.html

I believe this is the cause. On OSX, esp=5; on linux, esp=4

[emaste]

provide special case for FreeBSD only

[compnerd]

Interesting. We can change the Linux behavior better after a better understanding of whats going on there. libunwind (GNU Savannah) indicates the behavior as currently specified, although you are showing proof that it behaves differently. However, I think that this would be sufficient for unblocking you.

libunwind/trunk/include/libunwind.h

 };
 
 // 32-bit x86 registers
 enum {
   UNW_X86_EAX = 0,
   UNW_X86_ECX = 1,
   UNW_X86_EDX = 2,
   UNW_X86_EBX = 3,
+#ifdef __FreeBSD__
+  UNW_X86_ESP = 4,
+  UNW_X86_EBP = 5,
+#else
   UNW_X86_EBP = 4,
   UNW_X86_ESP = 5,
+#endif
   UNW_X86_ESI = 6,
   UNW_X86_EDI = 7
 };
 
 // 64-bit x86_64 registers
 enum {
   UNW_X86_64_RAX = 0,
   UNW_X86_64_RDX = 1,