[wip] terminator perf experiments by panstromek · Pull Request #159928 · rust-lang/rust

panstromek · 2026-07-25T21:32:11Z

panstromek · 2026-07-25T21:33:06Z

hoping for no impact here for now

[wip] terminator perf experiments

rust-bors · 2026-07-25T23:41:48Z

☀️ Try build successful (CI)
Build commit: c9696d1 (c9696d172403366b5d28ed4bee1020bea22674ce)
Base parent: 008fa22 (008fa22ce3f8d3c8dfaca2e6486043c2b21851eb)

rust-timer · 2026-07-26T00:22:23Z

Finished benchmarking commit (c9696d1): comparison URL.

Overall result: ❌ regressions - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.3%	[0.3%, 0.3%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary 4.3%, secondary -4.1%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	4.3%	[4.3%, 4.3%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-4.1%	[-6.5%, -1.7%]	2
All ❌✅ (primary)	4.3%	[4.3%, 4.3%]	1

Cycles

Results (secondary -1.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	4.1%	[4.1%, 4.1%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-6.0%	[-6.0%, -6.0%]	1
All ❌✅ (primary)	-	-	0

Binary size

Results (primary 0.1%, secondary 0.1%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	0.1%	[0.0%, 0.2%]	41
Regressions ❌ (secondary)	0.1%	[0.0%, 0.1%]	41
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[0.0%, 0.2%]	41

Bootstrap: 489.346s -> 488.243s (-0.23%)
Artifact size: 387.69 MiB -> 387.82 MiB (0.03%)

panstromek · 2026-07-26T04:50:06Z

Good, but looks like the metadata is slightly bigger. Probably because it encodes enum instead of just values & targets arrays. We could optimize that by encoding it as two arrays in all cases.

panstromek · 2026-07-26T07:23:53Z

@bors try @rust-timer queue

[wip] terminator perf experiments

rust-bors · 2026-07-26T09:29:48Z

☀️ Try build successful (CI)
Build commit: a75e38e (a75e38eed3fb412c5725c75ad8ccb15cbc512bd2)
Base parent: fdbd8ee (fdbd8ee70d9b68d98be873e1095d74cf9441f0c8)

rust-timer · 2026-07-26T10:10:23Z

Finished benchmarking commit (a75e38e): comparison URL.

Overall result: ❌ regressions - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.2%	[0.2%, 0.2%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary 0.6%, secondary 4.4%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	3.0%	[3.0%, 3.0%]	1
Regressions ❌ (secondary)	4.4%	[4.4%, 4.4%]	1
Improvements ✅ (primary)	-1.7%	[-1.7%, -1.7%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.6%	[-1.7%, 3.0%]	2

Cycles

Results (secondary 3.4%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	3.4%	[3.0%, 3.8%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

Results (primary 0.1%, secondary 0.1%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	0.1%	[0.0%, 0.2%]	37
Regressions ❌ (secondary)	0.1%	[0.0%, 0.1%]	40
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[0.0%, 0.2%]	37

Bootstrap: 487.865s -> 487.9s (0.01%)
Artifact size: 387.69 MiB -> 387.68 MiB (-0.00%)

…get may_dangle drop impl This also enables TrustedLen impl, which is behind unstable flag, too, so this might have soundness or perf implications

panstromek · 2026-07-26T18:13:12Z

This is the first one that's supposed to have some impact - one byte down from TerminatorKind (and consequently Terminator and BasicBlockData), but achieved with ThinVec for Call args. That is less shrink_to_fit calls and possible reallocs, but also more heap usage for non-empty args.

@bors try @rust-timer queue

[wip] terminator perf experiments

rust-log-analyzer · 2026-07-26T19:13:36Z

The job x86_64-gnu-gcc failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

---- [run-make] tests/run-make-cargo/rustc-crates-on-stable stdout ----

error: rmake recipe failed to complete
status: exit status: 1
command: cd "/checkout/obj/build/x86_64-unknown-linux-gnu/test/run-make-cargo/rustc-crates-on-stable/rmake_out" && env -u RUSTFLAGS -u __RUSTC_DEBUG_ASSERTIONS_ENABLED -u __STD_DEBUG_ASSERTIONS_ENABLED -u __STD_REMAP_DEBUGINFO_ENABLED AR="ar" BUILD_ROOT="/checkout/obj/build/x86_64-unknown-linux-gnu" CARGO="/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools-bin/cargo" CC="cc" CC_DEFAULT_FLAGS="-ffunction-sections -fdata-sections -fPIC -m64" CXX="c++" CXX_DEFAULT_FLAGS="-ffunction-sections -fdata-sections -fPIC -m64" HOST_RUSTC_DYLIB_PATH="/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib" LD_LIBRARY_PATH="/checkout/obj/build/x86_64-unknown-linux-gnu/bootstrap-tools/x86_64-unknown-linux-gnu/release/build/run_make_support/a300432cf10d6d08/out:/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/lib/rustlib/x86_64-unknown-linux-gnu/lib" LD_LIB_PATH_ENVVAR="LD_LIBRARY_PATH" LLVM_BIN_DIR="/checkout/obj/build/x86_64-unknown-linux-gnu/ci-llvm/bin" LLVM_COMPONENTS="aarch64 aarch64asmparser aarch64codegen aarch64desc aarch64disassembler aarch64info aarch64utils abi aggressiveinstcombine all all-targets amdgpu amdgpuasmparser amdgpucodegen amdgpudesc amdgpudisassembler amdgpuinfo amdgputargetmca amdgpuutils analysis arm armasmparser armcodegen armdesc armdisassembler arminfo armutils asmparser asmprinter avr avrasmparser avrcodegen avrdesc avrdisassembler avrinfo binaryformat bitreader bitstreamreader bitwriter bpf bpfasmparser bpfcodegen bpfdesc bpfdisassembler bpfinfo cas cfguard cgdata codegen codegentypes core coroutines coverage csky cskyasmparser cskycodegen cskydesc cskydisassembler cskyinfo debuginfobtf debuginfocodeview debuginfodwarf debuginfodwarflowlevel debuginfogsym debuginfologicalview debuginfomsf debuginfopdb demangle dlltooldriver dtlto dwarfcfichecker dwarflinker dwarflinkerclassic dwarflinkerparallel dwp engine executionengine extensions filecheck frontendatomic frontenddirective frontenddriver frontendhlsl frontendoffloading frontendopenacc frontendopenmp fuzzercli fuzzmutate globalisel hexagon hexagonasmparser hexagoncodegen hexagondesc hexagondisassembler hexagoninfo hipstdpar instcombine instrumentation interfacestub interpreter ipo irprinter irreader jitlink libdriver lineeditor linker loongarch loongarchasmparser loongarchcodegen loongarchdesc loongarchdisassembler loongarchinfo lto m68k m68kasmparser m68kcodegen m68kdesc m68kdisassembler m68kinfo mc mca mcdisassembler mcjit mcparser mips mipsasmparser mipscodegen mipsdesc mipsdisassembler mipsinfo mirparser msp430 msp430asmparser msp430codegen msp430desc msp430disassembler msp430info native nativecodegen nvptx nvptxcodegen nvptxdesc nvptxinfo objcarcopts objcopy object objectyaml option orcdebugging orcjit orcshared orctargetprocess passes plugins powerpc powerpcasmparser powerpccodegen powerpcdesc powerpcdisassembler powerpcinfo profiledata remarks riscv riscvasmparser riscvcodegen riscvdesc riscvdisassembler riscvinfo riscvtargetmca runtimedyld sandboxir scalaropts selectiondag sparc sparcasmparser sparccodegen sparcdesc sparcdisassembler sparcinfo support supportlsp symbolize systemz systemzasmparser systemzcodegen systemzdesc systemzdisassembler systemzinfo tablegen target targetparser telemetry textapi textapibinaryreader transformutils vectorize webassembly webassemblyasmparser webassemblycodegen webassemblydesc webassemblydisassembler webassemblyinfo webassemblyutils windowsdriver windowsmanifest x86 x86asmparser x86codegen x86desc x86disassembler x86info x86targetmca xray xtensa xtensaasmparser xtensacodegen xtensadesc xtensadisassembler xtensainfo" LLVM_FILECHECK="/checkout/obj/build/x86_64-unknown-linux-gnu/ci-llvm/bin/FileCheck" PYTHON="/usr/bin/python3" RUSTC="/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" RUSTDOC="/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustdoc" SOURCE_ROOT="/checkout" TARGET="x86_64-unknown-linux-gnu" TARGET_EXE_DYLIB_PATH="/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/x86_64-unknown-linux-gnu/lib" __BOOTSTRAP_JOBS="4" __RMAKE_VERBOSE_SUBPROCESS_OUTPUT="1" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/run-make-cargo/rustc-crates-on-stable/rmake"
stdout: none
--- stderr -------------------------------
command failed at line 41
LD_LIBRARY_PATH="/checkout/obj/build/x86_64-unknown-linux-gnu/test/run-make-cargo/rustc-crates-on-stable/rmake_out:/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib:/checkout/obj/build/x86_64-unknown-linux-gnu/bootstrap-tools/x86_64-unknown-linux-gnu/release/build/run_make_support/a300432cf10d6d08/out:/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/lib/rustlib/x86_64-unknown-linux-gnu/lib" RUSTC="/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" RUSTC_STAGE="0" RUSTFLAGS="-D warnings -Zallow-features=" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools-bin/cargo" "build" "--manifest-path" "/checkout/Cargo.toml" "--no-default-features" "--target-dir" "target" "-p" "rustc_type_ir" "-p" "rustc_next_trait_solver" "-p" "rustc_pattern_analysis" "-p" "rustc_lexer" "-p" "rustc_abi" "-p" "rustc_parse_format" "-p" "rustc_hashes"
output status: `exit status: 101`
=== STDOUT ===



=== STDERR ===
   Compiling unicode-ident v1.0.24
---

error[E0725]: the feature `dropck_eyepatch` is not in the list of allowed features
##[error]   --> /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/thin-vec-0.2.19/src/lib.rs:154:43
    |
154 | #![cfg_attr(feature = "unstable", feature(dropck_eyepatch))]
    |                                           ^^^^^^^^^^^^^^^

error[E0658]: the `may_dangle` attribute has unstable semantics and may be removed in the future
##[error]    --> /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/thin-vec-0.2.19/src/lib.rs:1949:15
     |
1949 | unsafe impl<#[may_dangle] T> Drop for ThinVec<T> {
     |               ^^^^^^^^^^
     |
     = note: see issue #34761 <https://github.com/rust-lang/rust/issues/34761> for more information
     = help: add `#![feature(dropck_eyepatch)]` to the crate attributes to enable
     = note: this compiler was built on 2026-07-26; consider upgrading it if it is out of date

error[E0658]: use of unstable library feature `trusted_len`
##[error]    --> /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/thin-vec-0.2.19/src/lib.rs:2558:16
     |

Important

For more information how to resolve CI failures of this job, visit this link.

rust-bors · 2026-07-26T20:17:24Z

☀️ Try build successful (CI)
Build commit: 1338b91 (1338b91d2abc3d1e9cca32ce60d67321e973062f)
Base parent: 9451e06 (9451e069c34e1d7db23a155dcf9c25d1bc127055)

rust-timer · 2026-07-26T22:06:23Z

Finished benchmarking commit (1338b91): comparison URL.

Overall result: ❌ regressions - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@bors rollup=never rustc-perf
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.6%	[0.2%, 2.6%]	19
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.2%	[-0.2%, -0.2%]	1
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary 0.7%, secondary 0.4%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	1.0%	[0.5%, 2.1%]	12
Regressions ❌ (secondary)	1.8%	[1.2%, 2.4%]	2
Improvements ✅ (primary)	-3.1%	[-3.1%, -3.1%]	1
Improvements ✅ (secondary)	-2.3%	[-2.3%, -2.3%]	1
All ❌✅ (primary)	0.7%	[-3.1%, 2.1%]	13

Cycles

Results (secondary 3.7%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	3.7%	[2.2%, 5.2%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

Results (primary 0.1%, secondary 0.1%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	0.1%	[0.0%, 0.2%]	38
Regressions ❌ (secondary)	0.1%	[0.0%, 0.1%]	40
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[0.0%, 0.2%]	38

Bootstrap: 489.176s -> 487.69s (-0.30%)
Artifact size: 387.73 MiB -> 387.69 MiB (-0.01%)

panstromek · 2026-07-27T06:38:20Z

hm. I wonder what's up with the new solver that it stresses this code that much. That doesn't seem super related.

rust-bors · 2026-07-27T21:43:31Z

☔ The latest upstream changes (presumably #160021) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.

panstromek added 2 commits July 25, 2026 21:48

generalize Decodable impl to all array types

a98dccb

refactor SwitchTargets to use explicit heap/inline and shrink its size

b1fe807

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jul 25, 2026

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rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jul 25, 2026

rust-bors Bot pushed a commit that referenced this pull request Jul 25, 2026

Auto merge of #159928 - panstromek:shrink-terminator-kind, r=<try>

c9696d1

[wip] terminator perf experiments

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jul 26, 2026

use ThinVec for targets in InlineAsm

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rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jul 26, 2026

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rust-bors Bot pushed a commit that referenced this pull request Jul 26, 2026

Auto merge of #159928 - panstromek:shrink-terminator-kind, r=<try>

a75e38e

[wip] terminator perf experiments

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jul 26, 2026

panstromek added 5 commits July 26, 2026 18:53

update thinvec to latest version and enable it's unstable feature to …

cd40d4d

…get may_dangle drop impl This also enables TrustedLen impl, which is behind unstable flag, too, so this might have soundness or perf implications

wip thinvec args

2bd4272

tidy

caeee6a

fixup

e95621c

size assert

8a6e1dc

rustbot added the A-attributes Area: Attributes (`#[…]`, `#![…]`) label Jul 26, 2026

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rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jul 26, 2026

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rust-bors Bot pushed a commit that referenced this pull request Jul 26, 2026

Auto merge of #159928 - panstromek:shrink-terminator-kind, r=<try>

1338b91

[wip] terminator perf experiments

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rustbot added perf-regression Performance regression. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Jul 26, 2026

panstromek mentioned this pull request Jul 27, 2026

Use real ThinVec in StmtDebugInfos #159975

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rust-timer commented Jul 26, 2026

Overall result: ❌ regressions - no action needed

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Overall result: ❌ regressions - no action needed

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Overall result: ❌ regressions - please read:

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