Reduce FormattingOptions to 64 bits #136974

m-ou-se · 2025-02-13T15:07:17Z

This is part of #99012

This reduces FormattingOptions from 6-7 machine words (384 bits on 64-bit platforms, 224 bits on 32-bit platforms) to just 64 bits (a single register on 64-bit platforms).

Before:

pub struct FormattingOptions {
    flags: u32, // only 6 bits used
    fill: char,
    align: Option<Alignment>,
    width: Option<usize>,
    precision: Option<usize>,
}

After:

pub struct FormattingOptions {
    /// Bits:
    ///  - 0-20: fill character (21 bits, a full `char`)
    ///  - 21: `+` flag
    ///  - 22: `-` flag
    ///  - 23: `#` flag
    ///  - 24: `0` flag
    ///  - 25: `x?` flag
    ///  - 26: `X?` flag
    ///  - 27: Width flag (if set, the width field below is used)
    ///  - 28: Precision flag (if set, the precision field below is used)
    ///  - 29-30: Alignment (0: Left, 1: Right, 2: Center, 3: Unknown)
    ///  - 31: Always set to 1
    flags: u32,
    /// Width if width flag above is set. Otherwise, always 0.
    width: u16,
    /// Precision if precision flag above is set. Otherwise, always 0.
    precision: u16,
}

m-ou-se · 2025-02-13T16:21:21Z

@bors try @rust-timer queue

bors · 2025-02-13T16:22:34Z

⌛ Trying commit c059377 with merge 47853a6...

Reduce FormattingOptions to 64 bits This reduces FormattingOptions from 6-7 machine words (384 bits on 64-bit platforms, 224 bits on 32-bit platforms) to just 64 bits (a single register on 64-bit platforms). This PR includes rust-lang#136932, which reduces the width and precision options to 16 bits, to make it all fit. Before: ```rust pub struct FormattingOptions { flags: u32, // only 6 bits used fill: char, align: Option<Alignment>, width: Option<usize>, precision: Option<usize>, } ``` After: ```rust pub struct FormattingOptions { /// Bits: /// - 0: `+` flag [rt::Flag::SignPlus] /// - 1: `-` flag [rt::Flag::SignMinus] /// - 2: `#` flag [rt::Flag::Alternate] /// - 3: `0` flag [rt::Flag::SignAwareZeroPad] /// - 4: `x?` flag [rt::Flag::DebugLowerHex] /// - 5: `X?` flag [rt::Flag::DebugUpperHex] /// - 6-7: Alignment (0: Left, 1: Right, 2: Center, 3: Unknown) /// - 8: Width flag (if set, the width field below is used) /// - 9: Precision flag (if set, the precision field below is used) /// - 10: unused /// - 11-31: fill character (21 bits, a full `char`) flags: u32, /// Width if width flag above is set. Otherwise, always 0. width: u16, /// Precision if precision flag above is set. Otherwise, always 0. precision: u16, } ```

bors · 2025-02-13T18:13:21Z

☀️ Try build successful - checks-actions
Build commit: 47853a6 (47853a630ac42e5c5ab5fcd1f07d37eef67f84e9)

rust-timer · 2025-02-13T19:31:14Z

Finished benchmarking commit (47853a6): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

@bors rollup=never
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

This is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

	mean	range	count
Regressions ❌ (primary)	0.6%	[0.6%, 0.6%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.4%	[-0.7%, -0.2%]	5
Improvements ✅ (secondary)	-0.5%	[-0.7%, -0.2%]	2
All ❌✅ (primary)	-0.2%	[-0.7%, 0.6%]	6

Max RSS (memory usage)

Results (primary 2.0%, secondary -1.2%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	2.0%	[1.9%, 2.1%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.2%	[-1.2%, -1.2%]	1
All ❌✅ (primary)	2.0%	[1.9%, 2.1%]	2

Cycles

Results (secondary -0.3%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	1.8%	[1.8%, 1.8%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.3%	[-2.3%, -2.3%]	1
All ❌✅ (primary)	-	-	0

Binary size

Results (primary -0.2%, secondary -0.1%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.1%	[0.0%, 0.3%]	6
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.2%	[-0.5%, -0.0%]	70
Improvements ✅ (secondary)	-0.1%	[-0.3%, -0.0%]	56
All ❌✅ (primary)	-0.2%	[-0.5%, 0.3%]	76

Bootstrap: 792.804s -> 787.91s (-0.62%)
Artifact size: 347.85 MiB -> 347.86 MiB (0.00%)

m-ou-se · 2025-02-13T20:24:58Z

Looks fine. Binary size improvements looking good too: https://perf.rust-lang.org/compare.html?start=54cdc751df770517e70db0588573e32e6a7b9821&end=47853a630ac42e5c5ab5fcd1f07d37eef67f84e9&stat=size%3Alinked_artifact&tab=compile

…=scottmcm Reduce formatting `width` and `precision` to 16 bits This is part of rust-lang#99012 This is reduces the `width` and `precision` fields in format strings to 16 bits. They are currently full `usize`s, but it's a bit nonsensical that we need to support the case where someone wants to pad their value to eighteen quintillion spaces and/or have eighteen quintillion digits of precision. By reducing these fields to 16 bit, we can reduce `FormattingOptions` to 64 bits (see rust-lang#136974) and improve the in memory representation of `format_args!()`. (See additional context below.) This also fixes a bug where the width or precision is silently truncated when cross-compiling to a target with a smaller `usize`. By reducing the width and precision fields to the minimum guaranteed size of `usize`, 16 bits, this bug is eliminated. This is a breaking change, but affects almost no existing code. --- Details of this change: There are three ways to set a width or precision today: 1. Directly a formatting string, e.g. `println!("{a:1234}")` 2. Indirectly in a formatting string, e.g. `println!("{a:width$}", width=1234)` 3. Through the unstable `FormattingOptions::width` method. This PR: - Adds a compiler error for 1. (`println!("{a:9999999}")` no longer compiles and gives a clear error.) - Adds a runtime check for 2. (`println!("{a:width$}, width=9999999)` will panic.) - Changes the signatures of the (unstable) `FormattingOptions::[get_]width` methods to use a `u16` instead. --- Additional context for improving `FormattingOptions` and `fmt::Arguments`: All the formatting flags and options are currently: - The `+` flag (1 bit) - The `-` flag (1 bit) - The `#` flag (1 bit) - The `0` flag (1 bit) - The `x?` flag (1 bit) - The `X?` flag (1 bit) - The alignment (2 bits) - The fill character (21 bits) - Whether a width is specified (1 bit) - Whether a precision is specified (1 bit) - If used, the width (a full usize) - If used, the precision (a full usize) Everything except the last two can simply fit in a `u32` (those add up to 31 bits in total). If we can accept a max width and precision of u16::MAX, we can make a `FormattingOptions` that is exactly 64 bits in size; the same size as a thin reference on most platforms. If, additionally, we also limit the number of formatting arguments, we can also reduce the size of `fmt::Arguments` (that is, of a `format_args!()` expression).

scottmcm · 2025-03-20T05:03:17Z

library/core/src/fmt/mod.rs

+        // Extract the debug upper/lower hex, zero pad, alternate, and plus/minus flags
+        // to stay compatible with older versions of Rust.
+        self.options.flags >> 21 & 0x3F


Checking: do we have pinning tests already that continue to pass after this change?

No, there's currently no test for this method. This method is deprecated, so perhaps we shouldn't put effort into trying to keep it compatible with older versions of Rust.

Maybe we should add a test, or change the method to just always return 0.

Added a test.

scottmcm · 2025-03-20T05:05:40Z

Looks good to me! I put some thoughts above, but r=me even if you look at them and decide they're not needed.

m-ou-se · 2025-03-21T16:11:31Z

@bors r=scottmcm

bors · 2025-03-21T16:11:34Z

📌 Commit d7685f0 has been approved by scottmcm

It is now in the queue for this repository.

bors · 2025-03-22T06:24:21Z

⌛ Testing commit d7685f0 with merge ce76ccc...

Reduce FormattingOptions to 64 bits This is part of rust-lang#99012 This reduces FormattingOptions from 6-7 machine words (384 bits on 64-bit platforms, 224 bits on 32-bit platforms) to just 64 bits (a single register on 64-bit platforms). Before: ```rust pub struct FormattingOptions { flags: u32, // only 6 bits used fill: char, align: Option<Alignment>, width: Option<usize>, precision: Option<usize>, } ``` After: ```rust pub struct FormattingOptions { /// Bits: /// - 0-20: fill character (21 bits, a full `char`) /// - 21: `+` flag /// - 22: `-` flag /// - 23: `#` flag /// - 24: `0` flag /// - 25: `x?` flag /// - 26: `X?` flag /// - 27: Width flag (if set, the width field below is used) /// - 28: Precision flag (if set, the precision field below is used) /// - 29-30: Alignment (0: Left, 1: Right, 2: Center, 3: Unknown) /// - 31: Always set to 1 flags: u32, /// Width if width flag above is set. Otherwise, always 0. width: u16, /// Precision if precision flag above is set. Otherwise, always 0. precision: u16, } ```

rust-log-analyzer · 2025-03-22T06:54:05Z

The job i686-mingw-2 failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

[RUSTC-TIMING] rustc_std_workspace_core test:false 0.657
error: could not compile `rustc-std-workspace-core` (lib)

Caused by:
  process didn't exit successfully: `D:\a\rust\rust\build\bootstrap\debug\rustc D:\a\rust\rust\build\bootstrap\debug\rustc --crate-name rustc_std_workspace_core --edition=2024 library\rustc-std-workspace-core\lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --crate-type lib --emit=dep-info,metadata,link -C opt-level=3 -C embed-bitcode=no -C codegen-units=1 -C debug-assertions=on --check-cfg cfg(docsrs,test) --check-cfg "cfg(feature, values())" -C metadata=2975d146d9ade100 -C extra-filename=-237cf34ade28b0c6 --out-dir D:\a\rust\rust\build\i686-pc-windows-gnu\stage0-std\i686-pc-windows-gnu\release\deps --target i686-pc-windows-gnu -L dependency=D:\a\rust\rust\build\i686-pc-windows-gnu\stage0-std\i686-pc-windows-gnu\release\deps -L dependency=D:\a\rust\rust\build\i686-pc-windows-gnu\stage0-std\release\deps --extern core=D:\a\rust\rust\build\i686-pc-windows-gnu\stage0-std\i686-pc-windows-gnu\release\deps\libcore-382cf6b153344497.rmeta --cfg=bootstrap -Csymbol-mangling-version=legacy --check-cfg=cfg(feature,values(any())) -Zunstable-options --check-cfg=cfg(bootstrap) -Zmacro-backtrace -Csplit-debuginfo=packed -Cprefer-dynamic -Zinline-mir -Zinline-mir-preserve-debug -Zmir_strip_debuginfo=locals-in-tiny-functions -Alinker-messages -Cforce-frame-pointers=yes "-Zcrate-attr=doc(html_root_url=\"https://doc.rust-lang.org/nightly/\")" -Z binary-dep-depinfo` (exit code: 0xc0000005, STATUS_ACCESS_VIOLATION)
warning: build failed, waiting for other jobs to finish...
[RUSTC-TIMING] compiler_builtins test:false 4.562
[RUSTC-TIMING] core test:false 28.553
Build completed unsuccessfully in 0:22:20
make: *** [Makefile:126: ci-mingw-x-2] Error 1
  local time: Sat Mar 22 06:53:51 CUT 2025
  network time: Sat, 22 Mar 2025 06:53:51 GMT
##[error]Process completed with exit code 2.
Post job cleanup.
[command]"C:\Program Files\Git\bin\git.exe" version

bors · 2025-03-22T06:54:45Z

💔 Test failed - checks-actions

m-ou-se · 2025-03-22T09:07:54Z

@bors retry

bors · 2025-03-22T10:56:17Z

⌛ Testing commit d7685f0 with merge 0ce1369...

bors · 2025-03-22T14:02:42Z

☀️ Test successful - checks-actions
Approved by: scottmcm
Pushing 0ce1369 to master...

github-actions · 2025-03-22T14:04:54Z

What is this?

This is an experimental post-merge analysis report that shows differences in test outcomes between the merged PR and its parent PR.

Comparing db68788 (parent) -> 0ce1369 (this PR)

Test differences

Show 200 test diffs

Additionally, 200 doctest diffs were found. These are ignored, as they are noisy.

Job group index

rust-timer · 2025-03-22T15:18:35Z

Finished benchmarking commit (0ce1369): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Our benchmarks found a performance regression caused by this PR.
This might be an actual regression, but it can also be just noise.

Next Steps:

If the regression was expected or you think it can be justified,
please write a comment with sufficient written justification, and add
@rustbot label: +perf-regression-triaged to it, to mark the regression as triaged.
If you think that you know of a way to resolve the regression, try to create
a new PR with a fix for the regression.
If you do not understand the regression or you think that it is just noise,
you can ask the @rust-lang/wg-compiler-performance working group for help (members of this group
were already notified of this PR).

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

Instruction count

This is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.2%	[0.2%, 0.2%]	3
Improvements ✅ (primary)	-0.4%	[-0.5%, -0.2%]	7
Improvements ✅ (secondary)	-1.5%	[-1.5%, -1.5%]	1
All ❌✅ (primary)	-0.4%	[-0.5%, -0.2%]	7

Max RSS (memory usage)

Results (primary -0.2%, secondary 4.4%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	7.1%	[4.9%, 9.3%]	2
Regressions ❌ (secondary)	4.4%	[3.2%, 5.6%]	2
Improvements ✅ (primary)	-3.8%	[-6.6%, -1.7%]	4
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.2%	[-6.6%, 9.3%]	6

Cycles

This benchmark run did not return any relevant results for this metric.

Binary size

Results (primary -0.2%, secondary -0.1%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.0%	[0.0%, 0.1%]	3
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.2%	[-0.5%, -0.0%]	81
Improvements ✅ (secondary)	-0.1%	[-0.3%, -0.0%]	53
All ❌✅ (primary)	-0.2%	[-0.5%, 0.1%]	84

Bootstrap: 775.231s -> 774.128s (-0.14%)
Artifact size: 365.57 MiB -> 365.52 MiB (-0.01%)

m-ou-se added T-libs Relevant to the library team, which will review and decide on the PR/issue. A-fmt Area: `core::fmt` S-experimental Status: Ongoing experiment that does not require reviewing and won't be merged in its current state. labels Feb 13, 2025

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m-ou-se mentioned this pull request Feb 14, 2025

Reduce formatting width and precision to 16 bits #136932

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m-ou-se force-pushed the fmt-options-64-bit branch 2 times, most recently from 4700995 to dd37150 Compare February 19, 2025 23:04

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Tracking issue for improving std::fmt::Arguments and format_args!() #99012

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m-ou-se added 2 commits March 21, 2025 17:05

Add todo comment on using a niche type for fmt flags.

9b7060a

Add test for Formatter flags.

d7685f0

m-ou-se force-pushed the fmt-options-64-bit branch from c5f6ecf to d7685f0 Compare March 21, 2025 16:06

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Mar 21, 2025

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bors added the merged-by-bors This PR was explicitly merged by bors. label Mar 22, 2025

bors merged commit 0ce1369 into rust-lang:master Mar 22, 2025
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redesign stage 0 std #119899

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Functionality for int_format_into for integer types #138338

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m-ou-se deleted the fmt-options-64-bit branch March 22, 2025 18:52

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Reduce FormattingOptions to 64 bits #136974

Reduce FormattingOptions to 64 bits #136974

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Reduce FormattingOptions to 64 bits #136974

Reduce FormattingOptions to 64 bits #136974

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Overall result: ❌✅ regressions and improvements - please read the text below

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Test differences

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Overall result: ❌✅ regressions and improvements - please read the text below

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