Auto merge of rust-lang#136264 - GuillaumeGomez:optimize-integers-to-string, r=Amanieu

Optimize `ToString` implementation for integers

Part of rust-lang#135543.

Follow-up of rust-lang#133247 and rust-lang#128204.

The benchmark results are:

| name| 1.87.0-nightly (3ea711f 2025-03-09) | With this PR | diff |
|-|-|-|-|
| bench_i16 | 32.06 ns/iter (+/- 0.12) | 17.62 ns/iter (+/- 0.03) | -45% |
| bench_i32 | 31.61 ns/iter (+/- 0.04) | 15.10 ns/iter (+/- 0.06) | -52% |
| bench_i64 | 31.71 ns/iter (+/- 0.07) | 15.02 ns/iter (+/- 0.20) | -52% |
| bench_i8 | 13.21 ns/iter (+/- 0.14) | 14.93 ns/iter (+/- 0.16) | +13% |
| bench_u16 | 31.20 ns/iter (+/- 0.06) | 16.14 ns/iter (+/- 0.11) | -48% |
| bench_u32 | 33.27 ns/iter (+/- 0.05) | 16.18 ns/iter (+/- 0.10) | -51% |
| bench_u64 | 31.44 ns/iter (+/- 0.06) | 16.62 ns/iter (+/- 0.21) | -47% |
| bench_u8 | 10.57 ns/iter (+/- 0.30) | 13.00 ns/iter (+/- 0.43) | +22% |

More information about it in [the original comment](rust-lang#136264 (comment)).

r? `@workingjubilee`

Author: bors

alloc/src/string.rs

@@ -2826,7 +2826,54 @@ impl SpecToString for bool {
     }
 }
 
+macro_rules! impl_to_string {
+    ($($signed:ident, $unsigned:ident,)*) => {
+        $(
+        #[cfg(not(no_global_oom_handling))]
+        #[cfg(not(feature = "optimize_for_size"))]
+        impl SpecToString for $signed {
+            #[inline]
+            fn spec_to_string(&self) -> String {
+                const SIZE: usize = $signed::MAX.ilog(10) as usize + 1;
+                let mut buf = [core::mem::MaybeUninit::<u8>::uninit(); SIZE];
+                // Only difference between signed and unsigned are these 8 lines.
+                let mut out;
+                if *self < 0 {
+                    out = String::with_capacity(SIZE + 1);
+                    out.push('-');
+                } else {
+                    out = String::with_capacity(SIZE);
+                }
+
+                out.push_str(self.unsigned_abs()._fmt(&mut buf));
+                out
+            }
+        }
+        #[cfg(not(no_global_oom_handling))]
+        #[cfg(not(feature = "optimize_for_size"))]
+        impl SpecToString for $unsigned {
+            #[inline]
+            fn spec_to_string(&self) -> String {
+                const SIZE: usize = $unsigned::MAX.ilog(10) as usize + 1;
+                let mut buf = [core::mem::MaybeUninit::<u8>::uninit(); SIZE];
+
+                self._fmt(&mut buf).to_string()
+            }
+        }
+        )*
+    }
+}
+
+impl_to_string! {
+    i8, u8,
+    i16, u16,
+    i32, u32,
+    i64, u64,
+    isize, usize,
+}
+
 #[cfg(not(no_global_oom_handling))]
+#[cfg(feature = "optimize_for_size")]
 impl SpecToString for u8 {
     #[inline]
     fn spec_to_string(&self) -> String {
@@ -2846,6 +2893,7 @@ impl SpecToString for u8 {
 }
 
 #[cfg(not(no_global_oom_handling))]
+#[cfg(feature = "optimize_for_size")]
 impl SpecToString for i8 {
     #[inline]
     fn spec_to_string(&self) -> String {

core/src/fmt/num.rs

@@ -208,7 +208,11 @@ macro_rules! impl_Display {
             fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                 #[cfg(not(feature = "optimize_for_size"))]
                 {
-                    self._fmt(true, f)
+                    const MAX_DEC_N: usize = $unsigned::MAX.ilog(10) as usize + 1;
+                    // Buffer decimals for $unsigned with right alignment.
+                    let mut buf = [MaybeUninit::<u8>::uninit(); MAX_DEC_N];
+
+                    f.pad_integral(true, "", self._fmt(&mut buf))
                 }
                 #[cfg(feature = "optimize_for_size")]
                 {
@@ -222,7 +226,11 @@ macro_rules! impl_Display {
             fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                 #[cfg(not(feature = "optimize_for_size"))]
                 {
-                    return self.unsigned_abs()._fmt(*self >= 0, f);
+                    const MAX_DEC_N: usize = $unsigned::MAX.ilog(10) as usize + 1;
+                    // Buffer decimals for $unsigned with right alignment.
+                    let mut buf = [MaybeUninit::<u8>::uninit(); MAX_DEC_N];
+
+                    f.pad_integral(*self >= 0, "", self.unsigned_abs()._fmt(&mut buf))
                 }
                 #[cfg(feature = "optimize_for_size")]
                 {
@@ -233,10 +241,13 @@ macro_rules! impl_Display {
 
         #[cfg(not(feature = "optimize_for_size"))]
         impl $unsigned {
-            fn _fmt(self, is_nonnegative: bool, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-                const MAX_DEC_N: usize = $unsigned::MAX.ilog(10) as usize + 1;
-                // Buffer decimals for $unsigned with right alignment.
-                let mut buf = [MaybeUninit::<u8>::uninit(); MAX_DEC_N];
+            #[doc(hidden)]
+            #[unstable(
+                feature = "fmt_internals",
+                reason = "specialized method meant to only be used by `SpecToString` implementation",
+                issue = "none"
+            )]
+            pub fn _fmt<'a>(self, buf: &'a mut [MaybeUninit::<u8>]) -> &'a str {
                 // Count the number of bytes in buf that are not initialized.
                 let mut offset = buf.len();
                 // Consume the least-significant decimals from a working copy.
@@ -301,13 +312,12 @@ macro_rules! impl_Display {
                 // SAFETY: All buf content since offset is set.
                 let written = unsafe { buf.get_unchecked(offset..) };
                 // SAFETY: Writes use ASCII from the lookup table exclusively.
-                let as_str = unsafe {
+                unsafe {
                     str::from_utf8_unchecked(slice::from_raw_parts(
                           MaybeUninit::slice_as_ptr(written),
                           written.len(),
                     ))
-                };
-                f.pad_integral(is_nonnegative, "", as_str)
+                }
             }
         })*