Evaluate `std::fmt::Arguments::new_const()` during Compile Time #131663

veera-sivarajan · 2024-10-13T19:26:49Z

This PR aims to optimize calls to string formating macros without any arguments by evaluating std::fmt::Arguments::new_const() in a const context.

Currently,
println!("hola") compiles to std::io::_print(std::fmt::Arguments::new_const(&["hola\n"])).

With this PR,
println!("hola") compiles to std::io::_print(const { std::fmt::Arguments::new_const(&["hola\n"]) }).

This is accomplished in two steps:

Const stabilize std::fmt::Arguments::new_const().
Wrap calls to std::fmt::Arguments::new_const() in an inline const block when lowering the AST to HIR.

This reduces the generated code to a memcpy instead of multiple getelementptr and store instructions even with -C no-prepopulate-passes -C opt-level=0. Godbolt for code comparison: https://rust.godbolt.org/z/P7Px7de6c

This is a safe and sound transformation because std::fmt::Arguments::new_const() is a trivial constructor function taking a slice containing a 'static string literal as input.

CC #99012

rustbot · 2024-10-13T19:26:57Z

r? @chenyukang

rustbot has assigned @chenyukang.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

veera-sivarajan · 2024-10-13T19:27:28Z

library/core/src/fmt/mod.rs

@@ -333,7 +333,7 @@ pub struct Arguments<'a> {
 #[unstable(feature = "fmt_internals", issue = "none")]
 impl<'a> Arguments<'a> {
 #[inline]
- #[rustc_const_unstable(feature = "const_fmt_arguments_new", issue = "none")]
+ #[rustc_const_stable(feature = "const_fmt_arguments_new", since = "CURRENT_RUSTC_VERSION")]


Not sure if this warrants a separate stabilization PR because it's an internal only API behind [feature(fmt_internals)].

Kobzol · 2024-10-13T19:52:02Z

@bors try @rust-timer queue

rust-timer · 2024-10-13T19:52:04Z

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

…<try> Evaluate `std::fmt::Arguments::new_const()` during Compile Time Fixes rust-lang#128709 This PR aims to optimize calls to string formating macros without any arguments by evaluating `std::fmt::Arguments::new_const()` in a const context. Currently, `println!("hola")` compiles to `std::io::_print(std::fmt::Arguments::new_const(&["hola\n"]))`. With this PR, `println!("hola")` compiles to `std::io::_print(const { std::fmt::Arguments::new_const(&["hola\n"]) })`. This is accomplished in two steps: 1. Const stabilize `std::fmt::Arguments::new_const()`. 2. Wrap calls to `std::fmt::Arguments::new_const()` in an inline const block when lowering the AST to HIR. This reduces the generated code to a `memcpy` instead of multiple `getelementptr` and `store` instructions even with `-C no-prepopulate-passes -C opt-level=0`. Godbolt for code comparison: https://rust.godbolt.org/z/P7Px7de6c This is a safe and sound transformation because `std::fmt::Arguments::new_const()` is a trivial constructor function taking a slice containing a `'static` string literal as input. CC rust-lang#99012

bors · 2024-10-13T19:53:12Z

⌛ Trying commit 7638733 with merge ed038de...

rustbot · 2024-10-13T21:28:10Z

Some changes occurred in coverage tests.

cc @Zalathar

rustbot · 2024-10-14T14:15:28Z

This PR modifies tests/run-make/. If this PR is trying to port a Makefile
run-make test to use rmake.rs, please update the
run-make port tracking issue
so we can track our progress. You can either modify the tracking issue
directly, or you can comment on the tracking issue and link this PR.

cc @jieyouxu

Update Tests

deb0d0f

rustbot assigned chenyukang Oct 13, 2024

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Oct 13, 2024

veera-sivarajan commented Oct 13, 2024

View reviewed changes

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Oct 13, 2024