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Fix WASM memory allocation failure (#4989)#7887

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alok-108 wants to merge 1 commit into
RustPython:mainfrom
alok-108:fix-wasm-memory-usage
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Fix WASM memory allocation failure (#4989)#7887
alok-108 wants to merge 1 commit into
RustPython:mainfrom
alok-108:fix-wasm-memory-usage

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@alok-108 alok-108 commented May 16, 2026 •
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Problem

Even a minimal script (a = 1) caused a memory allocation failure (~79MB) in WASM builds on constrained runtimes (wasmi). The root cause was unbounded heap growth, a large initial DataStack chunk, and missing size-optimized release profile.

Fixes

  1. Linker memory limits (.cargo/config.toml): Capped max memory at 64MB, stack at 1MB for wasm32-* targets.
  2. DataStack optimization (crates/vm/src/datastack.rs): Reduced MIN_CHUNK_SIZE to 4KB on wasm32 targets.
  3. WASM release profile (Cargo.toml): Added wasm-release with opt-level="s", LTO, strip.

Testing

  • Built minimal interpreter (without_stdlib) for wasm32-wasi with wasm-release profile.
  • Executed a = 1 successfully under both wasmtime and wasmi — no memory errors, allocation stays under 64MB.
  • All existing VM tests pass without regression.

Closes #4989

Summary by CodeRabbit

  • Chores
    • Optimized WebAssembly build configuration with enhanced linker settings for WASI targets.
    • Added optimized build profile for WebAssembly with size and performance tuning.
    • Improved memory allocation efficiency for WebAssembly builds.

Review Change Stack

@alok-108
Fix WASM memory allocation failure (RustPython#4989)
70f4503 
This commit addresses the unbounded memory allocation issue on WASM targets that caused minimal programs (like � = 1) to fail with ~79MB allocation errors in constrained runtimes like wasmi.

The fixes include:
1. Capping the max memory limit at 64MB and restricting stack size to 1MB via linker flags in .cargo/config.toml.
2. Reducing the minimum chunk size of the DataStack from 16KB to 4KB on WASM targets to shrink the initial memory footprint.
3. Adding a wasm-release profile in the root Cargo.toml optimized for size with opt-level = s, LTO enabled, and symbols stripped.
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coderabbitai Bot commented May 16, 2026 •
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📝 Walkthrough

Walkthrough

This PR optimizes RustPython VM for WebAssembly targets by adding a dedicated wasm-release build profile, configuring linker flags for WASI targets to constrain memory and stack, and reducing the minimum data-stack chunk size for wasm32 architecture to fit tighter memory constraints.

Changes

WebAssembly Optimization

Layer / File(s) Summary
Build profile for WASM
Cargo.toml		 New wasm-release profile inherits from release and tunes opt-level to "s", enables lto = true, sets codegen-units = 1, enables strip = true, and configures panic = "abort" for efficient wasm binaries.
Linker configuration for WASI targets
.cargo/config.toml		 New [target.wasm32-wasip1] and [target.wasm32-wasip2] sections pass linker arguments via rustflags to cap maximum memory and configure stack size limits.
Runtime memory allocation for wasm32
crates/vm/src/datastack.rs		 MIN_CHUNK_SIZE constant is conditionally set to 4 * 1024 for wasm32 targets and 16 * 1024 for other targets, reducing per-chunk memory overhead in constrained wasm environments.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

Poem

🐰 A rabbit hops through config lands,
Where wasm optimization stands,
Stack sizes shrink, profiles take flight,
WebAssembly shines so bright! ✨

🚥 Pre-merge checks | ✅ 5
✅ Passed checks (5 passed)
Check name Status Explanation
Title check ✅ Passed The title accurately summarizes the main objective: fixing WASM memory allocation failures through linker flags, reduced chunk sizes, and optimized build profile.
Docstring Coverage ✅ Passed No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check ✅ Passed Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check ✅ Passed Check skipped because no linked issues were found for this pull request.
Description Check ✅ Passed Check skipped - CodeRabbit’s high-level summary is enabled.

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✨ Finishing Touches
🧪 Generate unit tests (beta)
  • Create PR with unit tests

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@alok-108
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alok-108 commented May 16, 2026

Hi @youknowone, this PR fixes the old WASM memory issue (#4989). All 26 CI checks passed including wasm-wasi tests. Could you please take a look when you have time? Thanks!

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wasi build takes unexpectedly huge memory

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