RFD: Enhance crash log output when disassembler is not present

[Andrew Haley]

Several times recently I have received a HotSpot error log from a user
who did not have hsdis installed. Which, in practice, means most end users.
The trouble it that the disassembly is unreadable.

The disassembly printed, when hsdis is not present, looks like this:

[MachCode]
[Instructions begin]
   ;; N526: #	out( B1 ) <- BLOCK HEAD IS JUNK  Freq: 1
    0x00007f0c40564c20: 9090 448b | 5608 443b

    0x00007f0c40564c28: ;   {runtime_call Shared Runtime ic_miss_blob}
    0x00007f0c40564c28: 5008 0f85 | b0d4 9aff
[Verified Entry Point]
   ;; B1: #	out( B34 B2 ) <- BLOCK HEAD IS JUNK  Freq: 1
    0x00007f0c40564c30: 8984 2400 | 80fe ff55 | 4883 ec60 | 4181 7f20 | 0100 0000 | 0f85 d806

    0x00007f0c40564c48: ;*synchronization entry
                        ; - java.lang.StringBuilder::append at -1 (line 246)
    0x00007f0c40564c48: 0000 8954 | 2410 488b | de4d 33c0 | 450f be00

    0x00007f0c40564c58: ;*getfield value {reexecute=0 rethrow=0 return_oop=0}
                        ; - java.lang.AbstractStringBuilder::ensureCapacityInternal at 1 (line 244)
                        ; - java.lang.AbstractStringBuilder::append at 7 (line 804)
                        ; - java.lang.StringBuilder::append at 2 (line 246)
    0x00007f0c40564c58: 448b 4614

    0x00007f0c40564c5c: ; implicit exception: dispatches to 0x00007f0c405651a8
    0x00007f0c40564c5c: 478b 4cc4

I'd like to change it in the following ways:

- Replace the ";" with a platform-defined comment character
- Replace the address with a comment
- Replace the hex format with a line parseable by an assembler, in a platform-defined format
- A few other minor things

So we end up with something which is like this: not quite as nice, but legal assembler
syntax, retaining all comments:

# [MachCode]
# [Instructions begin]
      .text
      .org 0x4c20
  ## N526: #	out( B1 ) <- BLOCK HEAD IS JUNK  Freq: 1
   /* 00007f0c40564c20: */ .byte 0x90, 0x90,  0x44, 0x8b,  0x56, 0x08,  0x44, 0x3b

   /* 00007f0c40564c28: */ #   {runtime_call Shared Runtime ic_miss_blob}
   /* 00007f0c40564c28: */ .byte 0x50, 0x08,  0x0f, 0x85,  0xb0, 0xd4,  0x9a, 0xff
# [Verified Entry Point]
  ## B1: #	out( B34 B2 ) <- BLOCK HEAD IS JUNK  Freq: 1
   /* 00007f0c40564c30: */ .byte 0x89, 0x84,  0x24, 0x00,  0x80, 0xfe,  0xff, 0x55,  0x48, 0x83,  0xec, 0x60,  0x41, 0x81,  0x7f, 0x20,  0x01, 0x00,  0x00, 0x00,  0x0f, 0x85,  0xd8, 0x06

   /* 00007f0c40564c48: */ #*synchronization entry
                           # - java.lang.StringBuilder::append at -1 (line 246)
   /* 00007f0c40564c48: */ .byte 0x00, 0x00, 0x89, 0x54, 0x24, 0x10, 0x48, 0x8b, 0x4d, 0xde, 0x33, 0xc0, 0x0f, 0x45, 0xbe, 0x00

And it can then be assembled with a platform assembler, requiring no other tools:

     4c20:	90                   	nop
     4c21:	90                   	nop
     4c22:	44 8b 56 08          	mov    0x8(%rsi),%r10d
     4c26:	44 3b 50 08          	cmp    0x8(%rax),%r10d
     4c2a:	0f 85 b0 d4 9a ff    	jne    0xffffffffff9b20e0
     4c30:	89 84 24 00 80 fe ff 	mov    %eax,-0x18000(%rsp)
     4c37:	55                   	push   %rbp
     4c38:	48 83 ec 60          	sub    $0x60,%rsp
     ...

Obviously this is a little scrappy at present, and needs a good deal of tidying
up to be suitable for mainline. I'm asking for an "in principle" answer here:
if I could do a neat job, maintaining readability but making sure that in the
absence of hsdis we can print something that a maintenance programmer can easily
assemble and dump, would people be interested?

-- 
Andrew Haley  (he/him)
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
https://keybase.io/andrewhaley
EAC8 43EB D3EF DB98 CC77 2FAD A5CD 6035 332F A671