[lld] r268056 - ELF: Add -O0 (produce output as fast as possible) mode.

Fri Apr 29 16:38:25 PDT 2016

On Windows, I imagine? On an SSD I assume?

Don't imagine there's much reason to believe it'd be substantially
different on Linux.

Thanks for comparing!

On Fri, Apr 29, 2016 at 4:35 PM, Rui Ueyama <ruiu at google.com> wrote:

> Touched a single cc file and and rerun ninja check-clang with and without
> -O0. The two cases were in the same ballpark.
>
> With -O0
>
> Testing Time: 71.21s
>   Expected Passes    : 9253
>   Expected Failures  : 16
>   Unsupported Tests  : 18
>
> real    2m8.528s
> user    30m7.967s
> sys     5m54.938s
>
> Without -O0
>
> Testing Time: 72.62s
>   Expected Passes    : 9253
>   Expected Failures  : 16
>   Unsupported Tests  : 18
>
> real    2m9.602s
> user    29m53.452s
> sys     5m51.387s
>
> On Fri, Apr 29, 2016 at 3:51 PM, David Blaikie <dblaikie at gmail.com> wrote:
>
>> Anyone want to test how fast this is to touch a single cc file, relink,
>> rerun all the clang+llvm tests using this option (preferrably on an SSD)?
>>
>> Be interesting to know if the increased code size slows down the test
>> execution enough to wash out the benefit of the improved link time (as is
>> the case with a shared library build - but of course there the overhead
>> (loading the shared libraries) is greater)
>>
>> On Fri, Apr 29, 2016 at 9:12 AM, Rui Ueyama via llvm-commits <
>> llvm-commits at lists.llvm.org> wrote:
>>
>>> Author: ruiu
>>> Date: Fri Apr 29 11:12:29 2016
>>> New Revision: 268056
>>>
>>> URL: http://llvm.org/viewvc/llvm-project?rev=268056&view=rev
>>> Log:
>>> ELF: Add -O0 (produce output as fast as possible) mode.
>>>
>>> This patch redefines the default optimization level as 1 and adds
>>> new level 0. In the command line, it is -O0. The flag disables
>>> costly but optional features so that the linker produces semantically
>>> correct but larger output quickly. Currently it only disables
>>> section merging.
>>>
>>> This flag is not intended to be used for final production linking.
>>> It is intended to be used in compile-link-test cycle.
>>>
>>> Time to link clang with debug info is about 2x faster with the flag.
>>>
>>>   Head:
>>>   13.24 seconds
>>>   Output size: 1227189664 bytes
>>>
>>>   With this patch:
>>>   7.41 seconds
>>>   Output size: 2490281784 bytes
>>>
>>> Differential Revision: http://reviews.llvm.org/D19705
>>>
>>> Modified:
>>>     lld/trunk/ELF/Driver.cpp
>>>     lld/trunk/ELF/InputFiles.cpp
>>>     lld/trunk/test/ELF/merge-string.s
>>>
>>> Modified: lld/trunk/ELF/Driver.cpp
>>> URL:
>>> http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/Driver.cpp?rev=268056&r1=268055&r2=268056&view=diff
>>>
>>> ==============================================================================
>>> --- lld/trunk/ELF/Driver.cpp (original)
>>> +++ lld/trunk/ELF/Driver.cpp Fri Apr 29 11:12:29 2016
>>> @@ -356,7 +356,7 @@ void LinkerDriver::readConfigs(opt::Inpu
>>>    Config->SoName = getString(Args, OPT_soname);
>>>    Config->Sysroot = getString(Args, OPT_sysroot);
>>>
>>> -  Config->Optimize = getInteger(Args, OPT_O, 0);
>>> +  Config->Optimize = getInteger(Args, OPT_O, 1);
>>>    Config->LtoO = getInteger(Args, OPT_lto_O, 2);
>>>    if (Config->LtoO > 3)
>>>      error("invalid optimization level for LTO: " + getString(Args,
>>> OPT_lto_O));
>>>
>>> Modified: lld/trunk/ELF/InputFiles.cpp
>>> URL:
>>> http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/InputFiles.cpp?rev=268056&r1=268055&r2=268056&view=diff
>>>
>>> ==============================================================================
>>> --- lld/trunk/ELF/InputFiles.cpp (original)
>>> +++ lld/trunk/ELF/InputFiles.cpp Fri Apr 29 11:12:29 2016
>>> @@ -143,6 +143,12 @@ elf::ObjectFile<ELFT>::getShtGroupEntrie
>>>
>>>  template <class ELFT> static bool shouldMerge(const typename ELFT::Shdr
>>> &Sec) {
>>>    typedef typename ELFT::uint uintX_t;
>>> +
>>> +  // We don't merge sections if -O0 (default is -O1). This makes
>>> sometimes
>>> +  // the linker significantly faster, although the output will be
>>> bigger.
>>> +  if (Config->Optimize == 0)
>>> +    return false;
>>> +
>>>    uintX_t Flags = Sec.sh_flags;
>>>    if (!(Flags & SHF_MERGE))
>>>      return false;
>>>
>>> Modified: lld/trunk/test/ELF/merge-string.s
>>> URL:
>>> http://llvm.org/viewvc/llvm-project/lld/trunk/test/ELF/merge-string.s?rev=268056&r1=268055&r2=268056&view=diff
>>>
>>> ==============================================================================
>>> --- lld/trunk/test/ELF/merge-string.s (original)
>>> +++ lld/trunk/test/ELF/merge-string.s Fri Apr 29 11:12:29 2016
>>> @@ -4,6 +4,8 @@
>>>  // RUN: llvm-readobj -s -section-data -t %t.so | FileCheck %s
>>>  // RUN: ld.lld -O1 %t.o -o %t.so -shared
>>>  // RUN: llvm-readobj -s -section-data -t %t.so | FileCheck
>>> --check-prefix=NOTAIL %s
>>> +// RUN: ld.lld -O0 %t.o -o %t.so -shared
>>> +// RUN: llvm-readobj -s -section-data -t %t.so | FileCheck
>>> --check-prefix=NOMERGE %s
>>>
>>>          .section       .rodata.str1.1,"aMS", at progbits,1
>>>         .asciz  "abc"
>>> @@ -55,6 +57,24 @@ zed:
>>>  // NOTAIL-NEXT:   0000: 61626300 626300                     |abc.bc.|
>>>  // NOTAIL-NEXT: )
>>>
>>> +// NOMERGE:      Name:    .rodata
>>> +// NOMERGE-NEXT: Type:    SHT_PROGBITS
>>> +// NOMERGE-NEXT: Flags [
>>> +// NOMERGE-NEXT:   SHF_ALLOC
>>> +// NOMERGE-NEXT:   SHF_MERGE
>>> +// NOMERGE-NEXT:   SHF_STRINGS
>>> +// NOMERGE-NEXT: ]
>>> +// NOMERGE-NEXT: Address:         0x1C8
>>> +// NOMERGE-NEXT: Offset:  0x1C8
>>> +// NOMERGE-NEXT: Size:    16
>>> +// NOMERGE-NEXT: Link: 0
>>> +// NOMERGE-NEXT: Info: 0
>>> +// NOMERGE-NEXT: AddressAlignment: 2
>>> +// NOMERGE-NEXT: EntrySize: 0
>>> +// NOMERGE-NEXT: SectionData (
>>> +// NOMERGE-NEXT:   0000: 61626300 61626300 62630000 14000000
>>> |abc.abc.bc......|
>>> +// NOMERGE-NEXT: )
>>> +
>>>  // CHECK:      Name: .rodata
>>>  // CHECK-NEXT: Type: SHT_PROGBITS
>>>  // CHECK-NEXT: Flags [
>>>
>>>
>>> _______________________________________________
>>> llvm-commits mailing list
>>> llvm-commits at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
>>>
>>
>>
>
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