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

[Davide Italiano via cfe-dev]

On Sun, May 29, 2016 at 1:27 PM, Mehdi Amini via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> 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)
> 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.
>

Neither lld does (yet), to the best of my knowledge.

Cheers,

--
Davide