Newbie hsdis questions

[Ben Evans]

Hi,

I have some newbie questions about the output of the HS disassembler. It's 
possible that some of these are really more general x86-64 assembler 
questions, and if so, a pointer to a good reference would be much 
appreciated.

I'm running 6u18-b05 on x64 with these options:  -XX:+PrintCompilation 
-XX:+PrintAssembly -XX:+PrintSignatureHandlers -XX:+PrintNMethods

Here's a chunk of output I'm interested in:

384115 Compiled (c2) 496   nmethod com.db.e.d.m.p.TP::doExecute (833 
bytes)
 384116  total in heap  [0x00007fc3911f2a50,0x00007fc391208120] = 87760
 384117  relocation     [0x00007fc3911f2b58,0x00007fc3911f3630] = 2776
 384118  main code      [0x00007fc3911f3640,0x00007fc3911fc140] = 35584
 384119  stub code      [0x00007fc3911fc140,0x00007fc3911fc6c7] = 1415
 384120  constants      [0x00007fc3911fc6c7,0x00007fc3911fc6c8] = 1
 384121  scopes data    [0x00007fc3911fc6c8,0x00007fc391205598] = 36560
 384122  scopes pcs     [0x00007fc391205598,0x00007fc391206948] = 5040
 384123  dependencies   [0x00007fc391206948,0x00007fc3912069d0] = 136
 384124  handler table  [0x00007fc3912069d0,0x00007fc391207a68] = 4248
 384125  nul chk table  [0x00007fc391207a68,0x00007fc391207e08] = 928
 384126  oops           [0x00007fc391207e08,0x00007fc391208120] = 792
 384127 Decoding compiled method 0x00007fc3911f2a50:
 384128 Code:
 384129 [Disassembling for mach='i386:x86-64']
 384130 [Entry Point]
 384131   0x00007fc3911f3640: cmp    0x8(%rsi),%rax
 384132   0x00007fc3911f3644: jne    0x00007fc3910405e0  ; {runtime_call}
 384133   0x00007fc3911f364a: xchg   %ax,%ax
 384134   0x00007fc3911f364c: nopl   0x0(%rax)
 384135 [Verified Entry Point]
 384136   0x00007fc3911f3650: mov    %eax,-0x6000(%rsp)
 384137   0x00007fc3911f3657: push   %rbp

First of, I'm assuming that the proximity of line 384115 to line 384116 is 
a guarantee that the 2 lines are referring to the compilation of the same 
method, and that what follows is all about com.db.e.d.m.p.TP::doExecute. 
If not, how do I map back from an address to a nmethod name?

Main questions:

1) What do the entries in the table in lines 384116-384126 represent?
2) Lines 384131-384134 are presumably the x64 representation of a 
monomorphic call guard. Why are the xchg and nopl instructions (which are 
surely both effectively no-ops) present?
3) The addresses for lines 384131-384134 look strange to me, eg why isn't 
there at least 8 bytes required for the jne instruction? Is there a good 
reference for instruction lengths somewhere?

I have some questions which are more pertinent to my actual perf anomaly, 
but I want to make sure I understand the basics fully, and do my own 
investigations first - and try to conserve the group's time.

Thanks,

Ben
-- 
Ben Evans
eFX Algorithmic Trading
Deutsche Bank, London
Office: +44 (0)20 7541 3953


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