[libcxx-commits] [PATCH] D153991: [libc++] explicitly specialises `unique_ptr` when using `default_delete`

[Christopher Di Bella via Phabricator via libcxx-commits]

cjdb created this revision.
cjdb added reviewers: EricWF, ldionne.
Herald added a project: All.
cjdb requested review of this revision.
Herald added a project: libc++.
Herald added a subscriber: libcxx-commits.
Herald added a reviewer: libc++.

`default_delete` is a fairly heavy way to say `delete ptr` or `delete[]
ptr`, so this commit removes invoking the deleter at all. When using the
unstable ABI, we optimise a little further, by making `default_delete` a
static data member, which means that there's exactly one instance per
program for each `unique_ptr<T>`, as opposed to one instance per object.

When I built a debug Clang with the status quo, the stable change, and
the unstable change, I noticed that there was a total ~1% size
reduction. Here's what bloaty records:

**Status quo vs ABI stable change**

     FILE SIZE        VM SIZE
  --------------  --------------
   +0.5% +32.1Ki  [ = ]       0    .debug_abbrev
   [ = ]       0  +0.0%     +48    .bss
   [NEW]     +28  [ = ]       0    [Unmapped]
   -2.0%      -4  [ = ]       0    .debug_aranges
   -0.0%    -896  -0.0%    -896    .rodata
   -1.1%  -143Ki  [ = ]       0    .debug_rnglists
   -1.8%  -147Ki  -1.8%  -147Ki    .eh_frame_hdr
   -0.9%  -296Ki  [ = ]       0    .debug_addr
   -0.5%  -328Ki  [ = ]       0    .debug_str_offsets
   -1.7%  -442Ki  [ = ]       0    .symtab
   -1.8%  -590Ki  -1.8%  -590Ki    .eh_frame
   -0.5%  -738Ki  -0.5%  -738Ki    .text
   -0.7% -1.18Mi  [ = ]       0    .debug_line
   -1.8% -2.34Mi  [ = ]       0    .strtab
   -0.9% -3.81Mi  [ = ]       0    .debug_info
   -1.4% -3.82Mi  [ = ]       0    .debug_str
   -1.0% -13.7Mi  -0.6% -1.44Mi    TOTAL

ABI stable smaller.

**Status quo vs ABI unstable change**

     FILE SIZE        VM SIZE
  --------------  --------------
   +0.6% +41.7Ki  [ = ]       0    .debug_abbrev
   +0.0%    +448  +0.0%    +448    .dynstr
   +0.0%    +120  +0.0%    +120    .dynsym
   [ = ]       0  +0.0%     +32    .bss
   [NEW]     +28  [ = ]       0    [Unmapped]
   +0.0%     +24  +0.0%     +24    .gnu.hash
   +0.0%     +12  +0.0%     +12    .gnu.version
   +1.0%      +4  +1.0%      +4    [1 Others]
   -3.5%      -7  [ = ]       0    .debug_aranges
  -57.1%     -32 -57.1%     -32    [LOAD #2 [R]]
   -1.1%  -143Ki  [ = ]       0    .debug_rnglists
   -1.8%  -147Ki  -1.8%  -147Ki    .eh_frame_hdr
   -0.9%  -296Ki  [ = ]       0    .debug_addr
   -0.5%  -337Ki  [ = ]       0    .debug_str_offsets
   -1.7%  -442Ki  [ = ]       0    .symtab
   -1.8%  -589Ki  -1.8%  -589Ki    .eh_frame
   -0.5%  -747Ki  -0.5%  -747Ki    .text
   -0.7% -1.18Mi  [ = ]       0    .debug_line
   -1.8% -2.34Mi  [ = ]       0    .strtab
   -1.5% -3.90Mi  [ = ]       0    .debug_str
   -1.0% -3.92Mi  [ = ]       0    .debug_info
   -1.0% -13.9Mi  -0.6% -1.45Mi    TOTAL

ABI unstable smaller.

**Stable vs unstable**

     FILE SIZE        VM SIZE
  --------------  --------------
   +0.0%  +108Ki  [ = ]       0    .debug_info
   +0.0% +86.0Ki  [ = ]       0    .debug_str
   +0.0% +8.98Ki  +0.0% +8.98Ki    .text
   +0.0% +8.75Ki  [ = ]       0    .debug_str_offsets
   +0.0%    +558  [ = ]       0    .debug_line
   +133%     +32  +133%     +32    [LOAD #2 [R]]
   +0.0%     +28  [ = ]       0    .debug_rnglists
   [ = ]       0  +0.0%     +16    .bss
   +1.6%      +3  [ = ]       0    [1 Others]
   -1.0%      -4  -1.0%      -4    .gnu.version_r
   -0.0%     -12  -0.0%     -12    .gnu.version
   -0.0%     -24  -0.0%     -24    .gnu.hash
   -0.0%     -64  [ = ]       0    .debug_addr
   -0.0%     -64  -0.0%     -64    .eh_frame_hdr
   -0.0%    -120  -0.0%    -120    .dynsym
   -0.0%    -256  -0.0%    -256    .eh_frame
   -0.0%    -312  [ = ]       0    .symtab
   -0.0%    -448  -0.0%    -448    .dynstr
   -0.0%    -896  -0.0%    -896    .rodata
   -0.0% -1.09Ki  [ = ]       0    .strtab
   -0.1% -9.55Ki  [ = ]       0    .debug_abbrev
   +0.0%  +200Ki  +0.0% +7.25Ki    TOTAL

ABI unstable smaller.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D153991

Files:
  libcxx/include/__config
  libcxx/include/__memory/unique_ptr.h

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