[libcxx-commits] [PATCH] D75554: Partially inline basic_string::assign(__s [, __n]) methods.

[Martijn Vels via Phabricator via libcxx-commits]

mvels created this revision.
mvels added reviewers: EricWF, ldionne.
Herald added subscribers: libcxx-commits, dexonsmith.
Herald added a project: libc++.

This change adds constant value detection to the assign methods, which optimizes the assigment of compile time known constant string values.

The compiler elides either to the non-constant or constant code execution. The assign() methods are removed from the external template for the unstable ABI.

All assignment calls now fallback to the __assign_inlined() implementation code, which allows future changes where we do not call into the 'grow_by_and_replace' but specialize for SSO --> non SSO and non SSO --> non SSO growths.

Notice that this change does NOT take an early branch in the assign() methods. We may consider adding early branches, however, the compiler's inlining heuristics then go negative on inlining the code even as the constant optimization should be pretty clean cut. (this is likely only an issue for non FDO builds, but for now we use a single external implementation).

Benchmark STABLE / V1:

  -----------------------------------------------------------------------------------
  Benchmark                                        Time             CPU   Iterations
  -----------------------------------------------------------------------------------
  BM_StringAssignAsciiz_Empty_Opaque            6.61 ns         6.62 ns    105451520
  BM_StringAssignAsciiz_Empty_Transparent       6.43 ns         6.43 ns    108793856
  BM_StringAssignAsciiz_Small_Opaque            8.61 ns         8.61 ns     81317888
  BM_StringAssignAsciiz_Small_Transparent       8.33 ns         8.33 ns     84029440
  BM_StringAssignAsciiz_Large_Opaque            19.7 ns         19.7 ns     35692544
  BM_StringAssignAsciiz_Large_Transparent       19.2 ns         19.2 ns     36343808
  BM_StringAssignAsciiz_Huge_Opaque             1696 ns         1696 ns       401408
  BM_StringAssignAsciiz_Huge_Transparent        1743 ns         1743 ns       397312
  BM_StringAssignAsciizMix_Opaque               11.8 ns         11.8 ns     59224064
  BM_StringAssignAsciizMix_Transparent          11.9 ns         11.9 ns     58802176

Benchmark UNSTABLE / V2 without this change:

  -----------------------------------------------------------------------------------
  Benchmark                                        Time             CPU   Iterations
  -----------------------------------------------------------------------------------
  BM_StringAssignAsciiz_Empty_Opaque            6.24 ns         6.25 ns    109424640
  BM_StringAssignAsciiz_Empty_Transparent       5.92 ns         5.93 ns    118013952
  BM_StringAssignAsciiz_Small_Opaque            7.60 ns         7.60 ns     92065792
  BM_StringAssignAsciiz_Small_Transparent       7.80 ns         7.81 ns     89632768
  BM_StringAssignAsciiz_Large_Opaque            19.9 ns         19.9 ns     35254272
  BM_StringAssignAsciiz_Large_Transparent       19.1 ns         19.1 ns     36786176
  BM_StringAssignAsciiz_Huge_Opaque             1717 ns         1717 ns       389120
  BM_StringAssignAsciiz_Huge_Transparent        1723 ns         1722 ns       397312
  BM_StringAssignAsciizMix_Opaque               11.2 ns         11.2 ns     62922752
  BM_StringAssignAsciizMix_Transparent          11.1 ns         11.1 ns     62586880

Benchmark UNSTABLE / V2 with this change:

  -----------------------------------------------------------------------------------
  Benchmark                                        Time             CPU   Iterations
  -----------------------------------------------------------------------------------
  BM_StringAssignAsciiz_Empty_Opaque            4.92 ns         4.93 ns    136773632
  BM_StringAssignAsciiz_Empty_Transparent      0.834 ns        0.835 ns    837238784
  BM_StringAssignAsciiz_Small_Opaque            7.02 ns         7.03 ns     99655680
  BM_StringAssignAsciiz_Small_Transparent       1.10 ns         1.10 ns    634368000
  BM_StringAssignAsciiz_Large_Opaque            18.3 ns         18.3 ns     38576128
  BM_StringAssignAsciiz_Large_Transparent       14.3 ns         14.3 ns     48939008
  BM_StringAssignAsciiz_Huge_Opaque             1716 ns         1716 ns       393216
  BM_StringAssignAsciiz_Huge_Transparent        1688 ns         1688 ns       409600
  BM_StringAssignAsciizMix_Opaque               9.96 ns         9.95 ns     70467584
  BM_StringAssignAsciizMix_Transparent          4.34 ns         4.34 ns    164167680


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D75554

Files:
  libcxx/include/__string
  libcxx/include/string

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