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Change twice used large const table to static 

This table is used twice in core::num::dec2flt::algorithm::power_of_ten.
According to the semantics of const, a separate huge definition of the
table is inlined at both places.

    fn power_of_ten(e: i16) -> Fp {
        assert!(e >= table::MIN_E);
        let i = e - table::MIN_E;
        let sig = table::POWERS.0[i as usize];
        let exp = table::POWERS.1[i as usize];
        Fp { f: sig, e: exp }
    }

Theoretically this gets cleaned up by optimization passes, but in
practice I am experiencing a miscompile from LTO on this code. Making
the table a static, which would only be defined a single time and not
require attention from LTO, eliminates the miscompile and seems
semantically more appropriate anyway. A separate bug report on the LTO
bug is forthcoming.
This commit is contained in:
David Tolnay 2021-02-28 23:51:30 -08:00
parent 3b150b7a8f
commit bd51dea693
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2 changed files with 2 additions and 2 deletions
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2
library/core/src/num/dec2flt/table.rs
View File

@ -5,7 +5,7 @@ pub const MIN_E: i16 = -305;
pub const MAX_E: i16 = 305;
#[rustfmt::skip]
pub const POWERS: ([u64; 611], [i16; 611]) = (
pub static POWERS: ([u64; 611], [i16; 611]) = (
[
0xe0b62e2929aba83c,
0x8c71dcd9ba0b4926,

2
src/etc/dec2flt_table.py
View File

@ -113,7 +113,7 @@ def print_proper_powers():
print()
print("#[rustfmt::skip]")
typ = "([u64; {0}], [i16; {0}])".format(len(powers))
print("pub const POWERS: ", typ, " = (", sep='')
print("pub static POWERS: ", typ, " = (", sep='')
print(" [")
for z in powers:
print(" 0x{:x},".format(z.sig))