[cfe-dev] [llvm-dev] How to debug if LTO generate wrong code?
Rafael Espíndola via cfe-dev
cfe-dev at lists.llvm.org
Mon May 30 13:33:58 PDT 2016
We don't use cl::opt in gold, instead we parse the -plugin-opts that
gold passes the plugin (see process_plugin_option).
Cheers,
Rafael
On 30 May 2016 at 02:13, Mehdi Amini <mehdi.amini at apple.com> wrote:
>
> On May 29, 2016, at 5:44 PM, Shi, Steven <steven.shi at intel.com> wrote:
>
> (And I doubt the GNU linker supports LTO with LLVM).
> [Steven]: I’ve pushed GNU Binutils ld to support LLVM gold plugin, see
> detail in this bug https://sourceware.org/bugzilla/show_bug.cgi?id=20070.
> The new GNU ld linker works well with LLVM/Clang LTO when build IA32 code in
> my side. And from the ld owner input in the bug comments, the current X64
> LLVM LTO issue is in llvm LTO plugin.
>
>
> The fact that we don't support it for now seems to indicate that it is not a
> widely requested feature, especially considering that it is really a trivial
> option to add.
> What is the linker you're using? Are you building your own clang?
> [Steven]: I’m using the standard LLVM 3.8 with the above GNU new ld linker.
> I can build my own clang in my side if needed. I’m happy to know it is not
> difficult to enable the large code model in LLVM LTO and “it is really a
> trivial option to add”. Could you let me know how to enable it? My lots of
> work have been blocked by the large code model issue. Thank you!
>
>
>
> I can't test it locally, but here is a starting point in the gold plugin,
> inspired by the code present in clang:
>
>
>
> You need to use your linker-specific way of passing the option
> "-lto-use-large-codemodel=..." to the plugin.
>
> Let me know if it works for you!
>
> --
> Mehdi
>
>
>
>
> Steven Shi
> Intel\SSG\STO\UEFI Firmware
>
> Tel: +86 021-61166522
> iNet: 821-6522
>
> From: mehdi.amini at apple.com [mailto:mehdi.amini at apple.com]
> Sent: Monday, May 30, 2016 8:17 AM
> To: Shi, Steven <steven.shi at intel.com>
> Cc: Umesh Kalappa <umesh.kalappa0 at gmail.com>; eliben at gmail.com; llvm-dev
> <llvm-dev at lists.llvm.org>; cfe-dev at lists.llvm.org; Rafael Espíndola
> <rafael.espindola at gmail.com>
> Subject: Re: [cfe-dev] [llvm-dev] How to debug if LTO generate wrong code?
>
>
>
> On May 29, 2016, at 5:10 PM, Shi, Steven <steven.shi at intel.com> wrote:
>
> Hi Mehdi,
> GCC LTO seems support large code model in my side as below, if the code
> model is linker specific, does the GCC LTO use a special linker which is
> different from the one in GNU Binutils?
>
>
> I don't know anything about GCC.
> (And I doubt the GNU linker supports LTO with LLVM).
>
>
> I’m a bit surprised if both OS X ld64 and gold plugin do not support large
> code model in LTO. Since modern system widely use the 64bit, the code need
> to run in high address (larger than 2 GB) is a reasonable requirement.
>
>
> The fact that we don't support it for now seems to indicate that it is not a
> widely requested feature, especially considering that it is really a trivial
> option to add.
> What is the linker you're using? Are you building your own clang?
>
> --
> Mehdi
>
>
>
>
>
> $ gcc -g -O0 -flto codemodel1.c -mcmodel=large -o
> codemodel1_large_lto_gcc.bin
> $ objdump -dS codemodel1_large_lto_gcc.bin
>
> int main(int argc, const char* argv[])
> {
> 40048b: 55 push %rbp
> 40048c: 48 89 e5 mov %rsp,%rbp
> 40048f: 48 83 ec 20 sub $0x20,%rsp
> 400493: 89 7d ec mov %edi,-0x14(%rbp)
> 400496: 48 89 75 e0 mov %rsi,-0x20(%rbp)
> int t = global_func(argc);
> 40049a: 8b 45 ec mov -0x14(%rbp),%eax
> 40049d: 89 c7 mov %eax,%edi
> 40049f: 48 b8 76 04 40 00 00 movabs $0x400476,%rax
> 4004a6: 00 00 00
> 4004a9: ff d0 callq *%rax
> 4004ab: 89 45 fc mov %eax,-0x4(%rbp)
> t += global_arr[7];
> 4004ae: 48 b8 20 09 60 00 00 movabs $0x600920,%rax
> 4004b5: 00 00 00
> 4004b8: 8b 40 1c mov 0x1c(%rax),%eax
> 4004bb: 01 45 fc add %eax,-0x4(%rbp)
> t += static_arr[7];
> 4004be: 48 b8 c0 0a 60 00 00 movabs $0x600ac0,%rax
> 4004c5: 00 00 00
> 4004c8: 8b 40 1c mov 0x1c(%rax),%eax
> 4004cb: 01 45 fc add %eax,-0x4(%rbp)
> t += global_arr_big[7];
> 4004ce: 48 b8 60 0c 60 00 00 movabs $0x600c60,%rax
> 4004d5: 00 00 00
> 4004d8: 8b 40 1c mov 0x1c(%rax),%eax
> 4004db: 01 45 fc add %eax,-0x4(%rbp)
> t += static_arr_big[7];
> 4004de: 48 b8 a0 19 63 00 00 movabs $0x6319a0,%rax
> 4004e5: 00 00 00
> 4004e8: 8b 40 1c mov 0x1c(%rax),%eax
> 4004eb: 01 45 fc add %eax,-0x4(%rbp)
> return t;
> 4004ee: 8b 45 fc mov -0x4(%rbp),%eax
> }
>
> Steven Shi
> Intel\SSG\STO\UEFI Firmware
>
> Tel: +86 021-61166522
> iNet: 821-6522
>
> From: mehdi.amini at apple.com [mailto:mehdi.amini at apple.com]
> Sent: Monday, May 30, 2016 4:28 AM
> To: Shi, Steven <steven.shi at intel.com>
> Cc: Umesh Kalappa <umesh.kalappa0 at gmail.com>; eliben at gmail.com; llvm-dev
> <llvm-dev at lists.llvm.org>; cfe-dev at lists.llvm.org; Rafael Espíndola
> <rafael.espindola at gmail.com>
> Subject: Re: [cfe-dev] [llvm-dev] How to debug if LTO generate wrong code?
>
> 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.
>
> --
> 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]
>> Sent: Wednesday, May 18, 2016 4:02 AM
>> To: Umesh Kalappa <umesh.kalappa0 at gmail.com>
>> Cc: Shi, Steven <steven.shi at intel.com>; llvm-dev
>> <llvm-dev at lists.llvm.org>;
>> 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> 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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