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Assume slice len is bounded by allocation size

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Uses assume to check the length against a constant upper bound. The
inlined result then informs the optimizer of the sound value range.

This was tried with unreachable_unchecked before which introduces a
branch. This has the advantage of not being executed in sound code but
complicates basic blocks. It resulted in ~2% increased compile time in
some worst cases.

Add a codegen test for the assumption, testing the issue from #67186
This commit is contained in:
Andreas Molzer 2020-09-20 17:22:17 +02:00
parent d92d28e523
commit e44784b875
3 changed files with 51 additions and 3 deletions
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3
library/core/src/lib.rs
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@ -80,8 +80,9 @@
#![feature(constctlz)]
#![feature(const_panic)]
#![feature(const_pin)]
#![feature(const_fn_union)]
#![feature(const_fn)]
#![feature(const_fn_union)]
#![feature(const_assume)]
#![cfg_attr(not(bootstrap), feature(const_fn_floating_point_arithmetic))]
#![cfg_attr(not(bootstrap), feature(const_fn_fn_ptr_basics))]
#![feature(const_generics)]