Merge pull request #2412 from nagisa/optimise-size

The optimize attribute

Author: Centril

text/2412-optimize-attr.md

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+- Feature Name: `optimize_attr`
+- Start Date: 2018-03-26
+- RFC PR: [rust-lang/rfcs#2412](https://github.com/rust-lang/rfcs/pull/2412)
+- Rust Issue: [rust-lang/rust#54882](https://github.com/rust-lang/rust/issues/54882)
+
+# Summary
+[summary]: #summary
+
+This RFC introduces the `#[optimize]` attribute for controlling optimization level on a per-item
+basis.
+
+# Motivation
+[motivation]: #motivation
+
+Currently, rustc has only a small number of optimization options that apply globally to the
+crate. With LTO and RLIB-only crates these options become applicable to a whole-program, which
+reduces the ability to control optimization even further.
+
+For applications such as embedded, it is critical, that they satisfy the size constraints. This
+means, that code must consciously pick one or the other optimization level. Absence of a method to
+selectively optimize different parts of a program in different ways precludes users from utilising
+the hardware they have to the greatest degree.
+
+With a C toolchain selective optimization is fairly easy to achieve by compiling the relevant
+codegen units (objects) with different options. In Rust ecosystem, where the concept of such units
+does not exist, an alternate solution is necessary.
+
+With the `#[optimize]` attribute it is possible to annotate the optimization level of separate
+items, so that they are optimized differently from the global optimization option.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+## `#[optimize(size)]`
+
+Sometimes, optimizations are a trade-off between execution time and the code size. Some
+optimizations, such as loop unrolling increase code size many times on average (compared to
+original function size) for marginal performance benefits. In case such optimization is not
+desirable…
+
+```rust
+#[optimize(size)]
+fn banana() {
+    // code
+}
+```
+
+…will instruct rustc to consider this trade-off more carefully and avoid optimising in a way that
+would result in larger code rather than a smaller one. It may also have effect on what instructions
+are selected to appear in the final binary.
+
+Note that `#[optimize(size)]` is a hint, rather than a hard requirement and compiler may still,
+while optimising, take decisions that increase function size compared to an entirely unoptimized
+result.
+
+Using this attribute is recommended when inspection of generated code reveals unnecessarily large
+function or functions, but use of `-O` is still preferable over `-C opt-level=s` or `-C
+opt-level=z`.
+
+## `#[optimize(speed)]`
+
+Conversely, when one of the global optimization options for code size is used (`-Copt-level=s` or
+`-Copt-level=z`), profiling might reveal some functions that are unnecessarily “hot”. In that case,
+those functions may be annotated with the `#[optimize(speed)]` to make the compiler make its best
+effort to produce faster code.
+
+```rust
+#[optimize(speed)]
+fn banana() {
+    // code
+}
+```
+
+Much like with `#[optimize(size)]`, the `speed` counterpart is also a hint and will likely not
+yield the same results as using the global optimization option for speed.
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+The `#[optimize(size)]` attribute applied to an item or expression will instruct the optimization
+pipeline to avoid applying optimizations that could result in a size increase and machine code
+generator to generate code that’s smaller rather than faster.
+
+The `#[optimize(speed)]` attribute applied to an item or expression will instruct the optimization
+pipeline to apply optimizations that are likely to yield performance wins and machine code
+generator to generate code that’s faster rather than smaller.
+
+The `#[optimize]` attributes are just a hint to the compiler and are not guaranteed to result in
+any different code.
+
+If an `#[optimize]` attribute is applied to some grouping item (such as `mod` or a crate), it
+propagates transitively to all items defined within the grouping item. Note, that a function is
+also a “grouping” item for the purposes of this RFC, and `#[optimize]` attribute applied to a
+function will propagate to other functions or closures defined within the body of the function.
+
+`#[optimize]` attribute may also be applied to a closure expression using the currently unstable
+`stmt_expr_attributes` feature.
+
+It is an error to specify multiple incompatible `#[optimize]` options to a single item or
+expression at once.  A more explicit `#[optimize]` attribute overrides a propagated attribute.
+
+`#[optimize(speed)]` is a no-op when a global optimization for speed option is set (i.e. `-C
+opt-level=1-3`). Similarly `#[optimize(size)]` is a no-op when a global optimization for size
+option is set (i.e. `-C opt-level=s/z`). `#[optimize]` attributes are no-op when no optimizations
+are done globally (i.e. `-C opt-level=0`). In all other cases the *exact* interaction of the
+`#[optimize]` attribute with the global optimization level is not specified and is left up to
+implementation to decide.
+
+`#[optimize]` attribute applied to non function-like items (such as `struct`) or non function-like
+expressions (i.e. not closures) is considered “unused” as of this RFC and should fire the
+`unused_attribute` lint (unless the same attribute was used for a function-like item or expression,
+via e.g.  propagation). Some future RFC may assign some behaviour to this attribute with respect to
+such definitions.
+
+# Implementation approach
+
+For the LLVM backend, these attributes may be implemented in a following manner:
+
+`#[optimize(size)]` – explicit function attributes exist at LLVM level. Items with
+`optimize(size)` would simply apply the LLVM attributes to the functions.
+
+`#[optimize(speed)]` in conjunction with `-C opt-level=s/z` – use a global optimization level of
+`-C opt-level=2/3` and apply the equivalent LLVM function attribute (`optsize`, `minsize`) to all
+items which do not have an `#[optimize(speed)]` attribute.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+* Not all of the alternative codegen backends may be able to express such a request, hence the
+“this is a hint” note on the `#[optimize]` attribute.
+    * As a fallback, this attribute may be implemented in terms of more specific optimization hints
+      (such as `inline(never)`, the future `unroll(never)` etc).
+
+# Rationale and alternatives
+[alternatives]: #alternatives
+
+Proposed is a very semantic solution (describes the desired result, instead of behaviour) to the
+problem of needing to sometimes inhibit some of the trade-off optimizations such as loop unrolling.
+
+Alternative, of course, would be to add attributes controlling such optimizations, such as
+`#[unroll(no)]` on top of a loop statement. There’s already precedent for this in the `#[inline]`
+annotations.
+
+The author would like to argue that we should eventually have *both*, the `#[optimize]` for
+people who look at generated code but are not willing to dig for exact reasons, and the targeted
+attributes for people who know *why* the code is not satisfactory.
+
+Furthermore, currently `optimize` is able to do more than any possible combination of targeted
+attributes would be able to such as influencing the instruction selection or switch codegen
+strategy (jump table, if chain, etc.) This makes the attribute useful even in presence of all the
+targeted optimization knobs we might have in the future.
+
+# Prior art
+[prior-art]: #prior-art
+
+* LLVM: `optsize`, `optnone`, `minsize` function attributes (exposed in Clang in some way);
+* GCC: `__attribute__((optimize))` function attribute which allows setting the optimization level
+and using certain(?) `-f` flags for each function;
+* IAR: Optimizations have a check box for “No size constraints”, which allows compiler to go out of
+its way to optimize without considering the size trade-off. Can only be applied on a
+per-compilation-unit basis. Enabled by default, as is appropriate for a compiler targeting
+embedded use-cases.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+* Should we also implement `optimize(always)`? `optimize(level=x)`?
+    * Left for future discussion, but should make sure such extension is possible.
+* Should there be any way to specify what global optimization for speed level is used in
+  conjunction with the optimization for speed option (e.g. `-Copt-level=s3` could be equivalent to
+  `-Copt-level=3` and `#[optimize(size)]` on the crate item);
+    * This may matter for users of `#[optimize(speed)]`.
+* Are the propagation and `unused_attr` approaches right?