From 606f3158bf7dd2708b1efdde2e6de34f8eaebd7b Mon Sep 17 00:00:00 2001 From: Nicholas Nethercote Date: Fri, 29 Mar 2019 08:32:13 +1100 Subject: [PATCH] Optimize indentation in the pretty printer. Currently the pretty-printer calls `write!` for every space of indentation. On some workloads the indentation level can exceed 100, and a faster implementation reduces instruction counts by up to 7% on a few workloads. --- src/libsyntax/print/pp.rs | 22 +++++++++++++++++++--- 1 file changed, 19 insertions(+), 3 deletions(-) diff --git a/src/libsyntax/print/pp.rs b/src/libsyntax/print/pp.rs index d8a8cbb655b4b..45eb6995a7699 100644 --- a/src/libsyntax/print/pp.rs +++ b/src/libsyntax/print/pp.rs @@ -300,6 +300,8 @@ impl Default for BufEntry { } } +const SPACES: [u8; 128] = [b' '; 128]; + impl<'a> Printer<'a> { pub fn last_token(&mut self) -> Token { self.buf[self.right].token.clone() @@ -580,10 +582,24 @@ impl<'a> Printer<'a> { debug!("print String({})", s); // assert!(len <= space); self.space -= len; - while self.pending_indentation > 0 { - write!(self.out, " ")?; - self.pending_indentation -= 1; + + // Write the pending indent. A more concise way of doing this would be: + // + // write!(self.out, "{: >n$}", "", n = self.pending_indentation as usize)?; + // + // But that is significantly slower than using `SPACES`. This code is + // sufficiently hot, and indents can get sufficiently large, that the + // difference is significant on some workloads. + let spaces_len = SPACES.len() as isize; + while self.pending_indentation >= spaces_len { + self.out.write_all(&SPACES)?; + self.pending_indentation -= spaces_len; } + if self.pending_indentation > 0 { + self.out.write_all(&SPACES[0..self.pending_indentation as usize])?; + self.pending_indentation = 0; + } + write!(self.out, "{}", s) }