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

[David Blaikie via llvm-commits]

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
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20160429/4377e0e9/attachment.html>