Significant compilation time regression starting in v1.95.0-nightly (bevy)
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A-trait-system
C-bug
E-medium
E-needs-mcve
I-compiletime
P-high
T-compiler
T-types
- Dominant language
- Rust
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- 119k
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Description
Summary
Compilation time regression from 1.95 nightly onward.
1.94 nightly (nightly-2026-01-15) is not affected.
EDIT: this issue affects both LLVm and cranelift backends.
Command used
cargo build in a fresh (cargo init + cargo add bevy@0.18.1) bevy repository. Nothing fancy.
Expected vs Actual behaviour
- 1.94 stable cold compilation time: 48.5 seconds.
- 1.94 nightly cold compilation time: 46.9 seconds.
- 1.95 nightly cold compilation time: 85.8 seconds.
- 1.96 nightly cold compilation time: 86 seconds.
Configuration
[unstable] [build], [target] are empty. No env / local crate override. Built with time cargo build --profile x.
My .cargo/config.toml:
[profile.x]
inherits = "dev"
debug-assertions = true
incremental = false
# default, will change
opt-level = 0
debug = true
split-debuginfo = 'off'
strip = "none"
lto = false
panic = 'unwind'
codegen-units = 256
Afterwards, I toggled these options one by one (+ cranelift + Ctarget-cpu + Zthreads + Zshare-generics + linker + mold + wild) and results are consistent: compilation time roughly doubles in all scenarios, it does not seem to be related to an option.
Operating system
EndeavourOS (up to date)
HEAD
(version 1.94 nightly)
rustup toolchain install nightly-2026-01-15
rustup override set nightly-2026-01-15
(version 1.95 nightly)
rustup toolchain install nightly-2026-02-25
rustup override set nightly-2026-02-25
Additional context
Build comparison table
Stable 1.94
| stable | time (secs) | bin size (mo) | Disk deps (go) | Disk deps (go) w/ loop | bin size (mo) w/ loop | exec speed(ms)w/loop | Notes |
|---|---|---|---|---|---|---|---|
| REFERENCE | 48.5 | 3.8 | 5.6 | 6.9 | 1100 | 158 | The reference for all tests |
| codegen-units=1 | 53.3 | 3.8 | 4 | 4.7 | 784 | 158 | |
| codegen-units=16 | 46.7 | 3.8 | 4.4 | 5.5 | 887 | 158 | Sweet spot. |
| codegen-units=32 | 50 | 3.8 | 4 | 6.7 | 939 | 158 | |
| Ctarget-cpu=native | 48.7 | 3.8 | 6.9 | 6.9 | 1100 | 160 | |
| debug-assertions=false | 47.7 | - | - | 6.8 | 1100 | 141.6 | |
| debug-assertions=false + debug=false | 42.5 | - | - | 2.3 | 87 | 130 | |
| debug=false | 42.5 | 0.43 | 1.8 | 2.3 | 91 | 152 | |
| debug="line-tables-only | 43.6 | 0.43 | 1.9 | 3 | 271 | 152.4 | exec speed anomaly |
| split-debuginfo="unpacked" | 48.25 | - | - | 5.8 | 221 | 158 | |
| opt-level=1 | 83.7 | 3.8 | 5.1 | 7.7 | 1400 | 11.9 | |
| opt-level=2 | 95.8 | 3.8 | 5.7 | 8.7 | 1500 | 11.2 | |
| opt-level=3 | 96.4 | 3.8 | 5.8 | 8.9 | 1600 | 10.8 | |
| opt-level=3 + codegen-units=1 | 148 | - | - | 5.1 | 945 | 10.8 | Affects compilation time |
| opt-level=3 + codegen-units=16 | 92 | - | - | 7.3 | 1300 | 10.8 | |
| opt-level="s" | 78.9 | 3.8 | 4.4 | 6.5 | 1100 | 14.2 | |
| lto="fat" | (46 / ) 897 | 4.4 | 4.4 | 9.9 | 791 | 158 | Takes forever to build local crate |
| lto="thin" | (50 / ) 62.4 | 2.3 | 5.3 | 5 | 312 | 158 | Big local crates make compilation very long. |
| lto="thin" + codegen-units=16 | 60 | - | - | 4.5 | 255 | 158 | Big local crates make compilation very long. |
| panic="abort" | 37 / 51.95 | 3.8 | 5.5 | 7.6 | 1100 | 153 | Surprisingly, improves performance |
| strip="debuginfo" | 47.7 | 0.43 | 5.5 | 5 | 90 | 158 | |
| strip="symbols" | 47.8 | 0.345 | 4.8 | 4.9 | 55 | 158.4 | |
| REF1 = opt-level = 3 +lto = "thin" +debug = false +strip = "symbols" +panic = "abort" +codegen-units = 1 +split-debuginfo = "off" +Ctarget-cpu=native | 103 | 1.5 | 23 | 11.2 | BiS for release builds | ||
| REF1 but codegen-units = 16 | 72.8 | 1.7 | 37 | 11.1 | |||
| REF1 + built from RAM (build.target-dir = "/tmp/cargo-target") | 103 | 1.5 | 23 | 11.2 | df /tmp outputs tmpfs which indicates /tmp is in my RAM.No difference here means my SSD is not a bottleneck. | ||
| REF1 + mold linker | 103 | 1.5 | 23 | 11.2 | unaffected | ||
| REF1 + wild linker | 103 | 1.5 | 23 | 11.2 | unaffected |
Nightly 1.94
| nightly | time (secs) | bin size (mo) | Disk deps (go) | Disk deps (go)main | main bin size (mo) | exec speed(ms) | Notes |
|---|---|---|---|---|---|---|---|
| REFERENCE | 46.9 | 6.8 | 1.1 | 158 | |||
| codegen-units=1 | 52.9 | 5.2 | 912 | 157 | |||
| Zthreads=1 | 46.9 | 4.2 | 5.2 | 7.6 | 1200 | 158 | Seems to be the default value |
| Zthreads=8 | 33.8 | 4.2 | 5.2 | 7.6 | 1200 | 158 | != 1.95: here sweet spot in 16 |
| Zthreads=8 + codegen-units=16 | 31.4 | 4.2 | 5.2 | 7.6 | 1200 | 158 | |
| Zthreads=16 | 33.4 | 4.2 | 5.2 | 7.6 | 1200 | 158 | 9950X has 16 cores |
| Zthreads=32 | 40 | 4.2 | 5.2 | 7.6 | 1200 | 158 | 9950X has 32 threads |
| Zshare-generics=y | 48.6 | 4.2 | 5.2 | 6.1 | 1000 | - | No effect on REFERENCE configuration. |
| Ctarget-cpu=native | 49.2 | 4.2 | 7.6 | 7.6 | 1.2 | 166 | |
| debug=false | 43.15 | 0.44 | 2.1 | 2.3 | 107 | 155 | |
| opt-level=1 | 84 | 4.2 | 5.3 | 8.1 | 1500 | 12 | |
| opt-level=2 | 97.5 | 4.2 | 6 | 9.2 | 1600 | 10.8 | |
| opt-level=3 | 97.7 | 4.2 | 6 | 9.3 | 1700 | 11.1 | |
| REF1 = opt-level = 3 +lto = "thin" +debug = false +debug-assertions=falsestrip = "symbols" +panic = "abort" +codegen-units = 1 +split-debuginfo = "off"+ Ctarget-cpu=native+ Zthreads=8 | 1.6 | 23 | 10.9 | (release profile) | |||
| REF1 + Zshare-generics=y | 1.6 | 21 | 11.1 | ||||
| opt-level = 0 +lto = "off" +debug = line-tables-only +strip = false +panic = "unwind" +codegen-units = 16 +split-debuginfo = "unpacked"+ Ctarget-cpu=native+ Zthreads=8+ Zshare-generics=y | 3.2 | 186 | 163 | (dev profile) | |||
| REF2 = opt-level = 0debug = falsedebug-assertions=falsesplit-debuginfo = 'off'strip = falselto = "off"panic = 'abort'codegen-units = 16+ Ctarget-cpu=native+ Zthreads=8+ Zshare-generics=y | 29.34 | 2 | 84 | 117.6 | |||
| REF2 + debug-assertions=false | 29.7 | 2.1 | 88 | 145.2 | |||
| REF2 + mold | 25.74 | 2.1 | 116 | 118 | |||
| REF2 + wild | 24.93 | 2 | 85 | 116.8 | |||
| REF2 + cranelift | 29.5 | 2.7 | 376 | 195 | |||
| REF2 + cranelift + wild | 24.7 | 2.7 | 366 | 196 |
Nightly 1.95 (and 1.96)
| nightly | time (secs) | bin size (mo) | Disk deps (go) | Disk deps (go)main | main bin size (mo) | exec speed(ms) | Notes |
|---|---|---|---|---|---|---|---|
| REFERENCE | 86 | 4.2 | 5.2 | - | - | - | A lot longer than stable! |
| codegen-units=1 | |||||||
| Zthreads=1 | 86 | 4.2 | 5.2 | - | - | - | Seems to be the default value |
| Zthreads=8 | 66 | 4.2 | 5.2 | - | - | - | Sweet spot.From there : the higher the value, the longer it took. |
| Zthreads=16 | 86 | 4.2 | 5.2 | - | - | - | 9950X has 16 cores |
| Zthreads=32 | 95 | 4.2 | 5.2 | - | - | - | 9950X has 32 threads |
| Zshare-generics=y | 86 | 4.2 | 5.2 | - | - | - | No effect on REFERENCE configuration. |
| Ctarget-cpu=native | 86 | 4.2 | 7.6 | 7.6 | 1.2 | 166 | |
| debug=true | 94 | 7.5 | 1200 | 157.3 | |||
| debug=false | 79.8 | 0.44 | 2.1 | 2.3 | 103 | 155 | |
| opt-level=1 | 116.7 | 4.2 | 5.3 | 8.1 | 1500 | 12 | |
| opt-level=2 | 128 | 4.2 | 6 | 9.2 | 1600 | 10.8 | |
| opt-level=3 | 129.7 | 4.2 | 6 | 9.3 | 1700 | 11.1 | |
| REF1 = opt-level = 3 +lto = "thin" +debug = false +strip = "symbols" +panic = "abort" +codegen-units = 1 +split-debuginfo = "off"+ Ctarget-cpu=native+ Zthreads=8 | 123 | 1.6 | 23 | 10.9 | (release profile) | ||
| REF1 + Zshare-generics=y | 106.5 | 1.6 | 21 | 11.1 | |||
| opt-level = 0 +lto = "off" +debug = line-tables-only +strip = false +panic = "unwind" +codegen-units = 16 +split-debuginfo = "unpacked"+ Ctarget-cpu=native+ Zthreads=8+ Zshare-generics=y | 65 | 3.2 | 186 | 163 | (dev profile) | ||
| REF2 = opt-level = 0debug = falsesplit-debuginfo = 'off'strip = falselto = "off"panic = 'abort'codegen-units = 16+ Ctarget-cpu=native+ Zthreads=8+ Zshare-generics=y | 60.1 | 2.1 | 87 | 145.4 | (dev profile best of both worlds) | ||
| REF2 + mold | 56.3 | 2.2 | 121 | 146.8 | |||
| REF2 + wild | 56.2 | 2.1 | 88 | 145.8 | |||
| REF2 + cranelift | 61.7 | 2.8 | 402 | 195 | |||
| REF2 + cranelift + wild | 55.18 | 2.8 | 387 | 196 |
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
No Rust source file or failing test is named. Start by reproducing cargo build for the fresh Bevy 0.18.1 project with nightly-2026-01-15 and nightly-2026-02-25, then compare timing across the LLVM and Cranelift backends and the listed configuration changes. Done means identifying the regression's cause and a way to verify its effect on compilation time.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- rust
- Domain
- compilers, performance
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Quiet
- Clarity
- Mostly clear
- Newbie friendliness
- 38/100