[llvm-dev] [cfe-dev] How to debug if LTO generate wrong code?

[Mehdi Amini via llvm-dev]

Hi,


> On May 29, 2016, at 7:36 AM, Shi, Steven <steven.shi at intel.com> wrote:
> 
> Hi Mehdi,
> After deeper debug, I found my firmware LTO wrong code issue is related to X64 code model (-mcmodel=large) is always overridden as small (-mcmodel=small) if LTO build. And I don't know how to correctly specific the large code model for my X64 firmware LTO build. Appreciate if you could let me know it. 
>  
> You know, parts of my Uefi firmware (BIOS) have to been loaded to run in high address (larger than 2 GB) at the very beginning, and I need the code makes absolutely no assumptions about the addresses and data sections. But current LLVM LTO seems stick to use the small code model and generate many code with 32-bit RIP-relative addressing, which cause CPU exceptions when run in address larger than 2GB.
>  
> Below, I just simply reuse the Eli's codemodel1.c example (link: http://eli.thegreenplace.net/2012/01/03/understanding-the-x64-code-models <http://eli.thegreenplace.net/2012/01/03/understanding-the-x64-code-models>) to show the LLVM LTO code model issue.
> $ clang -g -O0 codemodel1.c -mcmodel=large -o codemodel1_large.bin
> $ clang -g -O0 codemodel1.c -mcmodel=small -o codemodel1_small.bin
> $ clang -g -O0 -flto codemodel1.c -mcmodel=large -o codemodel1_large_lto.bin
> $ clang -g -O0 -flto codemodel1.c -mcmodel=small -o codemodel1_small_lto.bin
>  
> You will see the codemodel1_large_lto.bin and codemodel1_small_lto.bin are exactly the same!
> And if you disassemble the codemodel1_large_lto.bin, you will see it uses the small code model (32-bit RIP-relative), not large, to do addressing as below.
>  
> $ objdump -dS codemodel1_large_lto.bin
>  
> int main(int argc, const char* argv[])
> {
>   4004f0:       55                      push   %rbp
>   4004f1:       48 89 e5                mov    %rsp,%rbp
>   4004f4:       48 83 ec 20             sub    $0x20,%rsp
>   4004f8:       c7 45 fc 00 00 00 00    movl   $0x0,-0x4(%rbp)
>   4004ff:       89 7d f8                mov    %edi,-0x8(%rbp)
>   400502:       48 89 75 f0             mov    %rsi,-0x10(%rbp)
>     int t = global_func(argc);
>   400506:       8b 7d f8                mov    -0x8(%rbp),%edi
>   400509:       e8 d2 ff ff ff          callq  4004e0 <global_func>
>   40050e:       89 45 ec                mov    %eax,-0x14(%rbp)
>     t += global_arr[7];
>   400511:       8b 04 25 4c 10 60 00    mov    0x60104c,%eax
>   400518:       03 45 ec                add    -0x14(%rbp),%eax
>   40051b:       89 45 ec                mov    %eax,-0x14(%rbp)
>     t += static_arr[7];
>   40051e:       8b 04 25 dc 11 60 00    mov    0x6011dc,%eax
>   400525:       03 45 ec                add    -0x14(%rbp),%eax
>   400528:       89 45 ec                mov    %eax,-0x14(%rbp)
>     t += global_arr_big[7];
>   40052b:       8b 04 25 6c 13 60 00    mov    0x60136c,%eax
>   400532:       03 45 ec                add    -0x14(%rbp),%eax
>   400535:       89 45 ec                mov    %eax,-0x14(%rbp)
>     t += static_arr_big[7];
>   400538:       8b 04 25 ac 20 63 00    mov    0x6320ac,%eax
>   40053f:       03 45 ec                add    -0x14(%rbp),%eax
>   400542:       89 45 ec                mov    %eax,-0x14(%rbp)
>     return t;
>   400545:       8b 45 ec                mov    -0x14(%rbp),%eax
>   400548:       48 83 c4 20             add    $0x20,%rsp
>   40054c:       5d                      pop    %rbp
>   40054d:       c3                      retq
>   40054e:       66 90                   xchg   %ax,%ax
>  
>  
> So, does LTO support large code model? How to correctly specify the LTO code model option?

Same answer as before: LTO is setup by the linker, so the option for that, if it exists, will be linker specific.

As far as I can tell, neither libLTO-based linker (ld64 on OS X for example), neither the gold plugin supports such an option and the code model is always "default". 

I don't know about lld, CC Rafael about that.

-- 
Mehdi




>  
>  
> Steven Shi
> Intel\SSG\STO\UEFI Firmware
>  
> Tel: +86 021-61166522
> iNet: 821-6522
>  
>  <>> -----Original Message-----
> > From: mehdi.amini at apple.com <mailto:mehdi.amini at apple.com> [mailto:mehdi.amini at apple.com <mailto:mehdi.amini at apple.com>]
> > Sent: Wednesday, May 18, 2016 4:02 AM
> > To: Umesh Kalappa <umesh.kalappa0 at gmail.com <mailto:umesh.kalappa0 at gmail.com>>
> > Cc: Shi, Steven <steven.shi at intel.com <mailto:steven.shi at intel.com>>; llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>>;
> > cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>
> > Subject: Re: [cfe-dev] [llvm-dev] How to debug if LTO generate wrong code?
> > 
> > 
> > > On May 17, 2016, at 11:21 AM, Umesh Kalappa
> > <umesh.kalappa0 at gmail.com <mailto:umesh.kalappa0 at gmail.com>> wrote:
> > >
> > > Steven,
> > >
> > > As mehdi stated , the optimisation level is specific to linker and it
> > > enables Inter-Pro  opts passes ,please  refer function
> > 
> > To be very clear: the -O option may trigger *linker* optimizations as well,
> > independently of LTO.
> > 
> > --
> > Mehdi
> > 
> > 
> > 

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