Avoid panic_bounds_check in fmt::write. by m-ou-se · Pull Request #78122 · rust-...

m-ou-se commented on Oct 20 •

Writing any fmt::Arguments would trigger the inclusion of usize formatting and padding code in the resulting binary, because indexing used in fmt::write would generate code using panic_bounds_check, which prints the index and length.

These bounds checks are not necessary, as fmt::Arguments never contains any out-of-bounds indexes.

This change replaces them with unsafe get_unchecked, to reduce the amount of generated code, which is especially important for embedded targets.

Demonstration of the size of and the symbols in a 'hello world' no_std binary:

Source code

Before:

   text	   data	    bss	    dec	    hex	filename
   6059	    736	      8	   6803	   1a93	before

0000000000001e00 T <T as core::any::Any>::type_id
0000000000003dd0 D core::fmt::num::DEC_DIGITS_LUT
0000000000001ce0 T core::fmt::num::imp::<impl core::fmt::Display for u64>::fmt
0000000000001ce0 T core::fmt::num::imp::<impl core::fmt::Display for usize>::fmt
0000000000001370 T core::fmt::write
0000000000001b30 t core::fmt::Formatter::pad_integral::write_prefix
0000000000001660 T core::fmt::Formatter::pad_integral
0000000000001350 T core::ops::function::FnOnce::call_once
0000000000001b80 t core::ptr::drop_in_place
0000000000001120 t core::ptr::drop_in_place
0000000000001c50 t core::iter::adapters::zip::Zip<A,B>::new
0000000000001c90 t core::iter::adapters::zip::Zip<A,B>::new
0000000000001b90 T core::panicking::panic_bounds_check
0000000000001c10 T core::panicking::panic_fmt
0000000000001130 t <&mut W as core::fmt::Write>::write_char
0000000000001200 t <&mut W as core::fmt::Write>::write_fmt
0000000000001250 t <&mut W as core::fmt::Write>::write_str

After:

   text	   data	    bss	    dec	    hex	filename
   3068	    600	      8	   3676	    e5c	after

0000000000001360 T core::fmt::write
0000000000001340 T core::ops::function::FnOnce::call_once
0000000000001120 t core::ptr::drop_in_place
0000000000001620 t core::iter::adapters::zip::Zip<A,B>::new
0000000000001660 t core::iter::adapters::zip::Zip<A,B>::new
0000000000001130 t <&mut W as core::fmt::Write>::write_char
0000000000001200 t <&mut W as core::fmt::Write>::write_fmt
0000000000001250 t <&mut W as core::fmt::Write>::write_str