Harden stable implementation of black_box #54
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added 2 commits
September 12, 2019 17:23
In Rust, `core::ptr::read_volatile` is not formally specified. However the current documentation mentions that "the semantics will almost always end up pretty similar to C11's definition of volatile". In C11, a volatile read *always* reads a value from memory. So this value *must* be in RAM in that form, at some time. This allows us to create a relatively efficient optimization barrier, by spilling the value to the stack, and immediadiately reloading it again. The benefit is that LTO could ever optimize the empty function away, even if #[inline(always)] is specified (although this may be very unlikely). However, `volatile` is concretely specified, so this optimization barrier will not be lost.
b43b139 effectively disables the branches produced by LLVM, so remove the statement that the non-`nightly` `black_box` does not protect against these optimizations.
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Relevant for #37: this code relies on the fact that LLVM cannot optimize through an explicit memory lookup. This always involves an L1 round-trip. Though afaik there is currently no way to kill the optimization in stable rust without using memory lookups or an opaque function. |
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This PR improves the implementation of
black_box, using the fact that the semantics ofvolatileare pretty well defined. With this change, you could probably even omit#[inline(never)], and then it may even be more efficient (I didn't benchmark).This patch also defines this case. ^
Example codegen: https://godbolt.org/z/3xYX1U