JIT: cache the wasm image base in a local#131369
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Materializing the image base costs 6 bytes each time, since the global index is a padded relocation. When a wasm function needs it 3 or more times, cache it in a local instead. SPC R2R code section: 21,820,681 -> 21,010,052 (-810,629 bytes, -3.71%). Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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Pull request overview
This PR updates the WebAssembly JIT backend to optionally cache the module image base in a wasm local (per function/funclet) when it’s used multiple times, reducing repeated global.get materializations and improving code size.
Changes:
- Track per-funclet counts of relocatable handle constants to estimate repeated image base materializations.
- Reserve and initialize an extra wasm local for the image base when the use-count threshold is met, and emit
local.getfor subsequent uses. - Plumb a new per-funclet local index (
funWasmImageBaseLocalIndex) through funclet initialization, regalloc, prolog generation, and the wasm emitter.
Reviewed changes
Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.
Show a summary per file
| File | Description |
|---|---|
| src/coreclr/jit/regallocwasm.h | Adds per-funclet tracking state for counting image base materializations. |
| src/coreclr/jit/regallocwasm.cpp | Counts image base materializations and conditionally reserves a local slot + records its index during allocation. |
| src/coreclr/jit/flowgraph.cpp | Initializes the new wasm image base local index field for all FuncInfoDsc entries. |
| src/coreclr/jit/emitwasm.h | Adds an emitter helper for explicitly reading image base from the global. |
| src/coreclr/jit/emitwasm.cpp | Splits global vs cached-local emission paths for the image base. |
| src/coreclr/jit/compiler.h | Adds funWasmImageBaseLocalIndex to wasm-specific FuncInfoDsc state. |
| src/coreclr/jit/codegenwasm.cpp | Initializes the cached image base local in root/funclet prologs when present. |
| src/coreclr/jit/codegen.h | Declares the new wasm-only prolog helper for image base local initialization. |
| } | ||
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| //------------------------------------------------------------------------ | ||
| // emitImageBase: Emit the module base (imageBase global) onto the stack. |
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@adamperlin PTAL We could save even more space (another 300kb or so) by having |
I think the work of "code shrinking" in object writer would subsume this change. Hardcoding the global index is also an option in the interim, though less durable in the long term (since it will be made redundant by shrinking). |
Materializing the image base costs 6 bytes each time, since the global index is a padded relocation. When a wasm function needs it 3 or more times, cache it in a local instead.
SPC R2R code section: 21,820,681 -> 21,010,052 (-810,629 bytes, -3.71%).