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Re: worrying differences in object code due to different build host!



Here's the first interesting part of a diff of the "objdump --disassemble"
between two i386 kernels built from the same source within a few minutes
of each other, but the one on the left was built on a native i386
machine and the one on the right was built on an amd64 machine (in a Xen
domU), running netbsd-4 and netbsd-5 respectively:

(everything before this appeared to just be differences in addresses
(direct and indirect) of other objects, e.g. in mov or call instructions,
etc.)

10942,10959c10942,10958
< c0108937:     09 c1                   or     %eax,%ecx
< c0108939:     89 0e                   mov    %ecx,(%esi)
< c010893b:     89 56 04                mov    %edx,0x4(%esi)
< c010893e:     a1 20 b0 47 c0          mov    0xc047b020,%eax
< c0108943:     01 c7                   add    %eax,%edi
< c0108945:     ff 07                   incl   (%edi)
< c0108947:     83 c4 2c                add    $0x2c,%esp
< c010894a:     89 d8                   mov    %ebx,%eax
< c010894c:     5b                      pop    %ebx
< c010894d:     5e                      pop    %esi
< c010894e:     5f                      pop    %edi
< c010894f:     5d                      pop    %ebp
< c0108950:     c3                      ret    
< c0108951:     7d 86                   jge    c01088d9 <crypto_newsession+0x69>
< c0108953:     a8 02                   test   $0x2,%al
< c0108955:     0f 85 7e ff ff ff       jne    c01088d9 <crypto_newsession+0x69>
< c010895b:     90                      nop
< c010895c:     8d 74 26 00             lea    0x0(%esi,%eiz,1),%esi
---
> c0108937:     89 0e                   mov    %ecx,(%esi)
> c0108939:     89 56 04                mov    %edx,0x4(%esi)
> c010893c:     a1 20 b0 47 c0          mov    0xc047b020,%eax
> c0108941:     01 c7                   add    %eax,%edi
> c0108943:     ff 07                   incl   (%edi)
> c0108945:     83 c4 2c                add    $0x2c,%esp
> c0108948:     89 d8                   mov    %ebx,%eax
> c010894a:     5b                      pop    %ebx
> c010894b:     5e                      pop    %esi
> c010894c:     5f                      pop    %edi
> c010894d:     5d                      pop    %ebp
> c010894e:     c3                      ret    
> c010894f:     7d 88                   jge    c01088d9 <crypto_newsession+0x69>
> c0108951:     a8 02                   test   $0x2,%al
> c0108953:     75 84                   jne    c01088d9 <crypto_newsession+0x69>
> c0108955:     8d 74 26 00             lea    0x0(%esi,%eiz,1),%esi
> c0108959:     8d bc 27 00 00 00 00    lea    0x0(%edi,%eiz,1),%edi

Then right away we're back in sync again except for address differences.

There's a big diff further on that's a bit confusing to me.  I'll just
post the first few lines of each hunk because I'm not so sure that the
disassembler didn't get confused here, though if it did then it soon got
un-confused again:

38172,38335c38171,38332
< c011f5fa:     74 1a                   je     c011f616 <ip_output+0x986>
< c011f5fc:     8b 4d 94                mov    -0x6c(%ebp),%ecx
< c011f5ff:     31 d2                   xor    %edx,%edx
< c011f601:     8b 81 60 01 00 00       mov    0x160(%ecx),%eax
< c011f607:     89 d1                   mov    %edx,%ecx
< c011f609:     25 80 00 00 00          and    $0x80,%eax
< c011f60e:     09 c1                   or     %eax,%ecx
< c011f610:     0f 84 c7 03 00 00       je     c011f9dd <ip_output+0xd4d>
[[ .... ]]
---
> c011f5fa:     74 10                   je     c011f60c <ip_output+0x97c>
> c011f5fc:     8b 45 94                mov    -0x6c(%ebp),%eax
> c011f5ff:     80 b8 60 01 00 00 00    cmpb   $0x0,0x160(%eax)
> c011f606:     0f 89 d1 03 00 00       jns    c011f9dd <ip_output+0xd4d>
> c011f60c:     f6 43 15 02             testb  $0x2,0x15(%ebx)
> c011f610:     8b 4d 94                mov    -0x6c(%ebp),%ecx
> c011f613:     8d 45 e0                lea    -0x20(%ebp),%eax


However it does sync up again soon with only diffs in addresses.

Eventually we get to a change that really does seem to screw up the
disassembler, though only for a short time:

126630,126634c126627,126631
< c016e7ad:     89 7c 24 0c             mov    %edi,0xc(%esp)
< c016e7b1:     89 14 24                mov    %edx,(%esp)
< c016e7b4:     89 44 24 08             mov    %eax,0x8(%esp)
< c016e7b8:     8b 85 98 fe ff ff       mov    -0x168(%ebp),%eax
< c016e7be:     89 44 24 04             mov    %eax,0x4(%esp)
---
> c016e7ad:     89 74 24 0c             mov    %esi,0xc(%esp)
> c016e7b1:     89 7c 24 04             mov    %edi,0x4(%esp)
> c016e7b5:     89 44 24 08             mov    %eax,0x8(%esp)
> c016e7b9:     8b 85 5c fe ff ff       mov    -0x1a4(%ebp),%eax
> c016e7bf:     89 04 24                mov    %eax,(%esp)

After this the diff eventually syncs up again until this curious
out-of-order sequence with different register assignments:

172665,172669c172660,172664
< c0194ad5:     0f 84 7d 03 00 00       je     c0194e58 <rf_AutoConfigureDisks+0x418>
< c0194adb:     c7 45 d4 00 00 00 00    movl   $0x0,-0x2c(%ebp)
< c0194ae2:     31 f6                   xor    %esi,%esi
< c0194ae4:     c7 45 d8 ff ff ff ff    movl   $0xffffffff,-0x28(%ebp)
< c0194aeb:     c7 45 dc ff 7f 00 00    movl   $0x7fff,-0x24(%ebp)
---
> c0194ad5:     0f 84 79 03 00 00       je     c0194e54 <rf_AutoConfigureDisks+0x414>
> c0194adb:     31 ff                   xor    %edi,%edi
> c0194add:     c7 45 d0 ff ff ff ff    movl   $0xffffffff,-0x30(%ebp)
> c0194ae4:     c7 45 d4 ff 7f 00 00    movl   $0x7fff,-0x2c(%ebp)
> c0194aeb:     c7 45 dc 00 00 00 00    movl   $0x0,-0x24(%ebp)


and on and on it goes....

-- 
						Greg A. Woods
						Planix, Inc.

<woods%planix.com@localhost>       +1 250 762-7675        http://www.planix.com/

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