# ANN: New package SnoopPrecompile

**URL:** <https://discourse.julialang.org/t/ann-new-package-snoopprecompile/84778>\
**Category:** Community\
**Tags:** package, ttfx, latency\
**Created:** [July 25, 2022, 9:40pm UTC](https://discourse.julialang.org/t/ann-new-package-snoopprecompile/84778 "2022-07-25T21:40:56Z")\
**Posts on this page:** 1\
**Showing post:** 4

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**Author:** ![tim.holy](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tim.holy/32/52_2.png) [@tim.holy](https://discourse.julialang.org/u/tim.holy)\
**Post date:** [July 29, 2022, 12:06pm UTC](https://discourse.julialang.org/t/ann-new-package-snoopprecompile/84778/4 "2022-07-29T12:06:57Z")

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It does 3 things:

1. run the block only when precompilling (it incorporates `if ccall(:jl_generating_output, Cint, ()) == 1`) so that if you’re running with `--compiled-modules=no` you don’t waste time
2. disables the interpreter when running the block (to ensure everything gets compiled)
3. intercept runtime dispatch to force precompilation of calls to methods defined in other packages

Item 3 is the really new thing. To explain in detail, any call made by runtime dispatch will break the chain of backedges. If those backedges don’t link back to the currently-precompiling package, the type-inferred code won’t be cached in the precompile file. Thus if the callee is a method in your package, no sweat (it’s already in your package so will be cached), but if the (runtime-dispatched) callee is in `Base` or elsewhere it won’t get precompiled by default on 1.8. `@precompile_all_calls` fixes this by snooping on inference and recording all new entrances to inference; once the `@precompile_all_calls` block exits it just iterates through the list of all newly-inferred MethodInstances and generates a manual `precompile(f, (argtypes...))` command (which on 1.8 does force caching even if the method is external).

None of this is a concern for inferrable dispatch, because on Julia 1.8 all inferrable dispatch gets cached regardless of module ownership. This is really just to scoop up those runtime-dispatched dependencies.

This really eliminates the need to ever statically generate those `precompile` directives, unless your workload has undesirable side-effects.

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_[View the full topic](https://discourse.julialang.org/t/ann-new-package-snoopprecompile/84778)._