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Use a sentinel for list iterators - #115

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list-sentinel
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Use a sentinel for list iterators#115
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list-sentinel

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@pauldreik

@pauldreik pauldreik commented Jul 30, 2026

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Summary

  • represent list links as a circular chain around an embedded, valueless sentinel
  • keep old end iterators current across insertion without a container tail pointer
  • preserve transferred element iterators across splice and move operations
  • reduce list iterators from two pointers to one
  • cover old-end behavior plus single, range, and whole-list splice traversal

Boost.Asio exposed the old representation when it retained an empty list end iterator, spliced in a recycled reactor node, and decremented the old iterator.

Validation

  • focused GCC 14 system and psychicstd list tests after the final rebase
  • focused strict-compatibility list tests after the final rebase
  • full 322-test in-tree GCC 14 suite
  • focused ASan and UBSan runs
  • 30-repetition compile-time comparison against main: no significant changes

@pauldreik
use a sentinel for list iterators
7148b29 
Represent list links as a circular chain around an embedded, valueless
sentinel. End iterators now point to a stable node, so an iterator obtained
before insertion sees the current tail and transferred element iterators follow
the destination after splice or move.

This removes the per-iterator tail snapshot and cuts iterators from two pointers
to one. It also replaces null endpoint cases with ordinary link updates without
growing the container.

Boost.Asio exposed the old-end issue when it spliced a recycled reactor queue
node into an empty list and decremented the previously obtained end iterator.
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Compile-time performance diff

Compiler: g++ (Ubuntu 13.3.0-6ubuntu2~24.04.1) 13.3.0.

Repetitions: 5 per configuration.

psychicstd compile time, main vs this PR (same runner). 🟢 faster · 🔴 slower · 🟡 within noise. A change is colored only when its bootstrap 95% CI excludes 0 and it clears ±5% / 3ms. vs system is this PR's speedup over system libstdc++, for context -- a regression is less alarming if psychicstd is still far ahead. Median system-time drift (noise proxy): 0.1%.

🔴 1 slower · 🟡 2 within noise.

benchmark main PR Δ (95% CI) vs system
iostream 75.5ms 79.7ms 🔴 +5.6% [+4.1, +7.1] 3.3x
🟡 2 benchmark(s) within noise
benchmark main PR Δ (95% CI) vs system
all-headers 209.1ms 212.1ms 🟡 +1.4% [-1.4, +4.6] 6.6x
wordcounter 177.7ms 178.9ms 🟡 +0.6% [-1.5, +4.3] 3.2x

Reproduce this on your machine: scripts/compare_compile_time_to_ref.py --reps 5 g++ origin/main

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eigen build-time diff (compile / run tests)

Compiler: c++ (Ubuntu 13.3.0-6ubuntu2~24.04.1) 13.3.0.

Repetitions: 2 per configuration.

eigen build time with psychicstd, main vs this PR (same runner). 🟢 faster · 🔴 slower · 🟡 within noise. A change is colored only when its bootstrap 95% CI excludes 0 and it clears ±5% / 3ms. vs system is this PR's speedup over system libstdc++, for context -- a regression is less alarming if psychicstd is still far ahead. Median system-time drift (noise proxy): 2.6%.

No significant changes (🟡 2 within noise).

🟡 2 benchmark(s) within noise
benchmark main PR Δ (95% CI) vs system
compile 16759.3ms 16478.8ms 🟡 -1.7% [-4.4, +1.1] 1.7x
run tests 37.0ms 36.2ms 🟡 -2.3% [-3.6, -0.9] 1.3x

Reproduce this on your machine: scripts/compare_realworld_to_ref.py eigen --build-type debug --reps 2

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rdfind build-time diff (configure / compile / run tests)

Compiler: c++ (Ubuntu 13.3.0-6ubuntu2~24.04.1) 13.3.0.

Repetitions: 5 per configuration.

rdfind build time with psychicstd, main vs this PR (same runner). 🟢 faster · 🔴 slower · 🟡 within noise. A change is colored only when its bootstrap 95% CI excludes 0 and it clears ±5% / 3ms. vs system is this PR's speedup over system libstdc++, for context -- a regression is less alarming if psychicstd is still far ahead. Median system-time drift (noise proxy): 0.9%.

No significant changes (🟡 3 within noise).

🟡 3 benchmark(s) within noise
benchmark main PR Δ (95% CI) vs system
compile 365.1ms 382.9ms 🟡 +4.9% [-20.7, +10.6] 3.9x
configure 1205.3ms 1221.9ms 🟡 +1.4% [-5.3, +66.1] 1.0x
run tests 1982.0ms 1843.0ms 🟡 -7.0% [-27.8, +27.6] 1.0x

Reproduce this on your machine: scripts/compare_realworld_to_ref.py rdfind --build-type debug --reps 5

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flatbuffers build-time diff (configure / compile / run tests)

Compiler: c++ (Ubuntu 13.3.0-6ubuntu2~24.04.1) 13.3.0.

Repetitions: 1 per configuration.

flatbuffers build time with psychicstd, main vs this PR (same runner). 🟢 faster · 🔴 slower · 🟡 within noise. A change is colored only when its bootstrap 95% CI excludes 0 and it clears ±5% / 3ms. vs system is this PR's speedup over system libstdc++, for context -- a regression is less alarming if psychicstd is still far ahead. Median system-time drift (noise proxy): 72.9%.

No significant changes (🟡 3 within noise).

🟡 3 benchmark(s) within noise
benchmark main PR Δ (95% CI) vs system
compile 23767.6ms 23773.0ms 🟡 +0.0% 2.2x
configure 434.8ms 428.5ms 🟡 -1.4% 1.1x
run tests 85.4ms 85.4ms 🟡 -0.0% 1.2x

Reproduce this on your machine: scripts/compare_realworld_to_ref.py flatbuffers --build-type debug --reps 1

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fmt build-time diff (compile / run tests)

Compiler: c++ (Ubuntu 13.3.0-6ubuntu2~24.04.1) 13.3.0.

Repetitions: 2 per configuration.

fmt build time with psychicstd, main vs this PR (same runner). 🟢 faster · 🔴 slower · 🟡 within noise. A change is colored only when its bootstrap 95% CI excludes 0 and it clears ±5% / 3ms. vs system is this PR's speedup over system libstdc++, for context -- a regression is less alarming if psychicstd is still far ahead. Median system-time drift (noise proxy): 20.1%.

No significant changes (🟡 2 within noise).

🟡 2 benchmark(s) within noise
benchmark main PR Δ (95% CI) vs system
compile 31064.7ms 31240.4ms 🟡 +0.6% [-0.6, +1.8] 1.6x
run tests 241.4ms 233.7ms 🟡 -3.2% [-9.1, +2.8] 1.7x

Reproduce this on your machine: scripts/compare_realworld_to_ref.py fmt --build-type debug --reps 2

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simdutf build-time diff (compile / run tests)

Compiler: c++ (Ubuntu 13.3.0-6ubuntu2~24.04.1) 13.3.0.

Repetitions: 1 per configuration.

simdutf build time with psychicstd, main vs this PR (same runner). 🟢 faster · 🔴 slower · 🟡 within noise. A change is colored only when its bootstrap 95% CI excludes 0 and it clears ±5% / 3ms. vs system is this PR's speedup over system libstdc++, for context -- a regression is less alarming if psychicstd is still far ahead. Median system-time drift (noise proxy): 2.7%.

No significant changes (🟡 2 within noise).

🟡 2 benchmark(s) within noise
benchmark main PR Δ (95% CI) vs system
compile 17901.2ms 18944.6ms 🟡 +5.8% 2.0x
run tests 40661.5ms 42226.6ms 🟡 +3.8% 1.0x

Reproduce this on your machine: scripts/compare_realworld_to_ref.py simdutf --build-type debug --reps 1

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catch2 build-time diff (compile / run tests)

Compiler: c++ (Ubuntu 13.3.0-6ubuntu2~24.04.1) 13.3.0.

Repetitions: 3 per configuration.

catch2 build time with psychicstd, main vs this PR (same runner). 🟢 faster · 🔴 slower · 🟡 within noise. A change is colored only when its bootstrap 95% CI excludes 0 and it clears ±5% / 3ms. vs system is this PR's speedup over system libstdc++, for context -- a regression is less alarming if psychicstd is still far ahead. Median system-time drift (noise proxy): 0.1%.

No significant changes (🟡 2 within noise).

🟡 2 benchmark(s) within noise
benchmark main PR Δ (95% CI) vs system
compile 15219.9ms 15238.5ms 🟡 +0.1% [-0.4, +0.7] 3.6x
run tests 4088.0ms 4085.4ms 🟡 -0.1% [-2.4, +1.9] 0.9x

Reproduce this on your machine: scripts/compare_realworld_to_ref.py catch2 --build-type debug --reps 3

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Compliance diff

No changes detected.

72 header(s) unchanged
header passes/sampled
algorithm 10/40
any 9/18
array 27/40
atomic 16/40
bit 2/15
bitset 9/40
cassert 3/9
cctype 2/5
cerrno 5/29
cfloat 5/29
chrono 1/40
ciso646 5/29
climits 15/40
clocale 5/29
cmath 1/19
compare 5/14
complex 17/40
concepts 2/35
condition_variable 13/26
csetjmp 3/9
csignal 5/29
cstddef 13/21
cstdint 3/5
cstdio 5/29
cstdlib 3/9
cstring 2/5
ctime 5/29
cwchar 5/29
deque 29/40
exception 16/21
filesystem 3/40
forward_list 9/40
fstream 14/40
functional 17/40
initializer_list 0/0
iomanip 22/40
ios 13/40
iostream 0/2
istream 23/40
iterator 5/40
limits 36/37
list 6/40
locale 9/40
map 13/40
memory 15/40
mutex 15/40
new 19/40
numeric 4/40
optional 11/40
ostream 31/40
random 4/40
ranges 0/40
ratio 13/13
regex 3/40
set 1/40
sstream 7/40
stack 16/36
stdexcept 9/9
streambuf 30/37
string 2/40
string_view 15/40
system_error 30/40
thread 6/40
tuple 10/40
type_traits 21/40
typeinfo 2/5
unordered_map 15/40
unordered_set 12/40
utility 8/40
valarray 1/40
vector 29/40
version 0/40

Reproduce this on your machine: LLVM_ROOT=$PWD/llvm-project cp base_cache.json .compliance_cache.json && tools/compliance.py --recheck

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Sanitizer diff (ASan/UBSan)

No changes detected.

72 header(s) unchanged
header passes/sampled
algorithm 4/25
any 6/18
array 17/25
atomic 12/25
bit 2/15
bitset 3/25
cassert 3/9
cctype 2/5
cerrno 4/25
cfloat 4/25
chrono 0/25
ciso646 4/25
climits 11/25
clocale 4/25
cmath 1/19
compare 5/14
complex 11/25
concepts 2/25
condition_variable 13/25
csetjmp 3/9
csignal 4/25
cstddef 13/21
cstdint 3/5
cstdio 4/25
cstdlib 3/9
cstring 2/5
ctime 4/25
cwchar 4/25
deque 17/25
exception 16/21
filesystem 2/25
forward_list 6/25
fstream 8/25
functional 10/25
initializer_list 0/0
iomanip 13/25
ios 11/25
iostream 0/2
istream 10/25
iterator 4/25
limits 24/25
list 4/25
locale 6/25
map 7/25
memory 8/25
mutex 13/25
new 6/25
numeric 3/25
optional 8/25
ostream 18/25
random 3/25
ranges 0/25
ratio 13/13
regex 1/25
set 0/25
sstream 5/25
stack 9/25
stdexcept 9/9
streambuf 18/25
string 1/25
string_view 9/25
system_error 20/25
thread 5/25
tuple 7/25
type_traits 14/25
typeinfo 2/5
unordered_map 10/25
unordered_set 7/25
utility 6/25
valarray 0/25
vector 18/25
version 0/25

Reproduce this on your machine: LLVM_ROOT=$PWD/llvm-project cp sanitize_base_cache.json .compliance_cache.sanitize.json && tools/compliance.py --sanitize --recheck

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cppcheck build-time diff (compile)

Compiler: c++ (Ubuntu 13.3.0-6ubuntu2~24.04.1) 13.3.0.

Repetitions: 2 per configuration.

cppcheck build time with psychicstd, main vs this PR (same runner). 🟢 faster · 🔴 slower · 🟡 within noise. A change is colored only when its bootstrap 95% CI excludes 0 and it clears ±5% / 3ms. vs system is this PR's speedup over system libstdc++, for context -- a regression is less alarming if psychicstd is still far ahead. Median system-time drift (noise proxy): 0.3%.

No significant changes (🟡 1 within noise).

🟡 1 benchmark(s) within noise
benchmark main PR Δ (95% CI) vs system
compile 48305.2ms 48693.4ms 🟡 +0.8% [+0.3, +1.3] 2.0x

Reproduce this on your machine: scripts/compare_realworld_to_ref.py cppcheck --build-type debug --reps 2

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@pauldreik