From fd7f605365b27bfdd3cd6763124e81bddd61dd28 Mon Sep 17 00:00:00 2001 From: Nicholas Nethercote Date: Thu, 4 Apr 2019 15:46:33 +1100 Subject: [PATCH] Increase `Span` from 4 bytes to 8 bytes. This increases the size of some important types, such as `ast::Expr` and `mir::Statement`. However, it drastically reduces how much the interner is used, and the fields are more natural sizes that don't require bit operations to extract. As a result, instruction counts drop across a range of workloads, by as much as 12% for incremental "check" builds of `script-servo`. Peak memory usage goes up a little for some cases, but down by more for some other cases -- as much as 18% for non-incremental builds of `packed-simd`. The commit also: - removes the `repr(packed)`, because it has negligible effect, but can cause undefined behaviour; - replaces explicit impls of common traits (`Copy`, `PartialEq`, etc.) with derived ones. --- src/librustc/mir/mod.rs | 2 +- src/libsyntax/ast.rs | 2 +- src/libsyntax_pos/hygiene.rs | 3 + src/libsyntax_pos/span_encoding.rs | 186 +++++++++++++---------------- 4 files changed, 90 insertions(+), 103 deletions(-) diff --git a/src/librustc/mir/mod.rs b/src/librustc/mir/mod.rs index 8424c096e88..3cab7a3812f 100644 --- a/src/librustc/mir/mod.rs +++ b/src/librustc/mir/mod.rs @@ -1735,7 +1735,7 @@ pub struct Statement<'tcx> { // `Statement` is used a lot. Make sure it doesn't unintentionally get bigger. #[cfg(target_arch = "x86_64")] -static_assert!(MEM_SIZE_OF_STATEMENT: mem::size_of::>() == 48); +static_assert!(MEM_SIZE_OF_STATEMENT: mem::size_of::>() == 56); impl<'tcx> Statement<'tcx> { /// Changes a statement to a nop. This is both faster than deleting instructions and avoids diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs index bcc8fdf8cd4..cf909e30e32 100644 --- a/src/libsyntax/ast.rs +++ b/src/libsyntax/ast.rs @@ -946,7 +946,7 @@ pub struct Expr { // `Expr` is used a lot. Make sure it doesn't unintentionally get bigger. #[cfg(target_arch = "x86_64")] -static_assert!(MEM_SIZE_OF_EXPR: std::mem::size_of::() == 88); +static_assert!(MEM_SIZE_OF_EXPR: std::mem::size_of::() == 96); impl Expr { /// Whether this expression would be valid somewhere that expects a value; for example, an `if` diff --git a/src/libsyntax_pos/hygiene.rs b/src/libsyntax_pos/hygiene.rs index 6331fe60886..1ffecea44ed 100644 --- a/src/libsyntax_pos/hygiene.rs +++ b/src/libsyntax_pos/hygiene.rs @@ -218,14 +218,17 @@ pub fn clear_markings() { } impl SyntaxContext { + #[inline] pub const fn empty() -> Self { SyntaxContext(0) } + #[inline] crate fn as_u32(self) -> u32 { self.0 } + #[inline] crate fn from_u32(raw: u32) -> SyntaxContext { SyntaxContext(raw) } diff --git a/src/libsyntax_pos/span_encoding.rs b/src/libsyntax_pos/span_encoding.rs index 743e61e3c22..525ec136232 100644 --- a/src/libsyntax_pos/span_encoding.rs +++ b/src/libsyntax_pos/span_encoding.rs @@ -9,122 +9,106 @@ use crate::hygiene::SyntaxContext; use rustc_data_structures::fx::FxHashMap; -use std::hash::{Hash, Hasher}; /// A compressed span. -/// Contains either fields of `SpanData` inline if they are small, or index into span interner. -/// The primary goal of `Span` is to be as small as possible and fit into other structures -/// (that's why it uses `packed` as well). Decoding speed is the second priority. -/// See `SpanData` for the info on span fields in decoded representation. -#[repr(packed)] -pub struct Span(u32); - -impl Copy for Span {} -impl Clone for Span { - #[inline] - fn clone(&self) -> Span { - *self - } -} -impl PartialEq for Span { - #[inline] - fn eq(&self, other: &Span) -> bool { - let a = self.0; - let b = other.0; - a == b - } -} -impl Eq for Span {} -impl Hash for Span { - #[inline] - fn hash(&self, state: &mut H) { - let a = self.0; - a.hash(state) - } +/// +/// `SpanData` is 12 bytes, which is a bit too big to stick everywhere. `Span` +/// is a form that only takes up 8 bytes, with less space for the length and +/// context. The vast majority (99.9%+) of `SpanData` instances will fit within +/// those 8 bytes; any `SpanData` whose fields don't fit into a `Span` are +/// stored in a separate interner table, and the `Span` will index into that +/// table. Interning is rare enough that the cost is low, but common enough +/// that the code is exercised regularly. +/// +/// An earlier version of this code used only 4 bytes for `Span`, but that was +/// slower because only 80--90% of spans could be stored inline (even less in +/// very large crates) and so the interner was used a lot more. +/// +/// Inline (compressed) format: +/// - `span.base_or_index == span_data.lo` +/// - `span.len_or_tag == len == span_data.hi - span_data.lo` (must be `<= MAX_LEN`) +/// - `span.ctxt == span_data.ctxt` (must be `<= MAX_CTXT`) +/// +/// Interned format: +/// - `span.base_or_index == index` (indexes into the interner table) +/// - `span.len_or_tag == LEN_TAG` (high bit set, all other bits are zero) +/// - `span.ctxt == 0` +/// +/// The inline form uses 0 for the tag value (rather than 1) so that we don't +/// need to mask out the tag bit when getting the length, and so that the +/// dummy span can be all zeroes. +/// +/// Notes about the choice of field sizes: +/// - `base` is 32 bits in both `Span` and `SpanData`, which means that `base` +/// values never cause interning. The number of bits needed for `base` +/// depends on the crate size. 32 bits allows up to 4 GiB of code in a crate. +/// `script-servo` is the largest crate in `rustc-perf`, requiring 26 bits +/// for some spans. +/// - `len` is 15 bits in `Span` (a u16, minus 1 bit for the tag) and 32 bits +/// in `SpanData`, which means that large `len` values will cause interning. +/// The number of bits needed for `len` does not depend on the crate size. +/// The most common number of bits for `len` are 0--7, with a peak usually at +/// 3 or 4, and then it drops off quickly from 8 onwards. 15 bits is enough +/// for 99.99%+ of cases, but larger values (sometimes 20+ bits) might occur +/// dozens of times in a typical crate. +/// - `ctxt` is 16 bits in `Span` and 32 bits in `SpanData`, which means that +/// large `ctxt` values will cause interning. The number of bits needed for +/// `ctxt` values depend partly on the crate size and partly on the form of +/// the code. No crates in `rustc-perf` need more than 15 bits for `ctxt`, +/// but larger crates might need more than 16 bits. +/// +#[derive(Clone, Copy, Eq, PartialEq, Hash)] +pub struct Span { + base_or_index: u32, + len_or_tag: u16, + ctxt_or_zero: u16 } +const LEN_TAG: u16 = 0b1000_0000_0000_0000; +const MAX_LEN: u32 = 0b0111_1111_1111_1111; +const MAX_CTXT: u32 = 0b1111_1111_1111_1111; + /// Dummy span, both position and length are zero, syntax context is zero as well. -/// This span is kept inline and encoded with format 0. -pub const DUMMY_SP: Span = Span(0); +pub const DUMMY_SP: Span = Span { base_or_index: 0, len_or_tag: 0, ctxt_or_zero: 0 }; impl Span { #[inline] - pub fn new(lo: BytePos, hi: BytePos, ctxt: SyntaxContext) -> Self { - encode(&match lo <= hi { - true => SpanData { lo, hi, ctxt }, - false => SpanData { lo: hi, hi: lo, ctxt }, - }) + pub fn new(mut lo: BytePos, mut hi: BytePos, ctxt: SyntaxContext) -> Self { + if lo > hi { + std::mem::swap(&mut lo, &mut hi); + } + + let (base, len, ctxt2) = (lo.0, hi.0 - lo.0, ctxt.as_u32()); + + if len <= MAX_LEN && ctxt2 <= MAX_CTXT { + // Inline format. + Span { base_or_index: base, len_or_tag: len as u16, ctxt_or_zero: ctxt2 as u16 } + } else { + // Interned format. + let index = with_span_interner(|interner| interner.intern(&SpanData { lo, hi, ctxt })); + Span { base_or_index: index, len_or_tag: LEN_TAG, ctxt_or_zero: 0 } + } } #[inline] pub fn data(self) -> SpanData { - decode(self) + if self.len_or_tag != LEN_TAG { + // Inline format. + debug_assert!(self.len_or_tag as u32 <= MAX_LEN); + SpanData { + lo: BytePos(self.base_or_index), + hi: BytePos(self.base_or_index + self.len_or_tag as u32), + ctxt: SyntaxContext::from_u32(self.ctxt_or_zero as u32), + } + } else { + // Interned format. + debug_assert!(self.ctxt_or_zero == 0); + let index = self.base_or_index; + with_span_interner(|interner| *interner.get(index)) + } } } -// Tags -const TAG_INLINE: u32 = 0; -const TAG_INTERNED: u32 = 1; -const TAG_MASK: u32 = 1; - -// Fields indexes -const BASE_INDEX: usize = 0; -const LEN_INDEX: usize = 1; -const CTXT_INDEX: usize = 2; - -// Tag = 0, inline format. -// ------------------------------------------------------------- -// | base 31:7 | len 6:1 | ctxt (currently 0 bits) | tag 0:0 | -// ------------------------------------------------------------- -// Since there are zero bits for ctxt, only SpanData with a 0 SyntaxContext -// can be inline. -const INLINE_SIZES: [u32; 3] = [25, 6, 0]; -const INLINE_OFFSETS: [u32; 3] = [7, 1, 1]; - -// Tag = 1, interned format. -// ------------------------ -// | index 31:1 | tag 0:0 | -// ------------------------ -const INTERNED_INDEX_SIZE: u32 = 31; -const INTERNED_INDEX_OFFSET: u32 = 1; - -#[inline] -fn encode(sd: &SpanData) -> Span { - let (base, len, ctxt) = (sd.lo.0, sd.hi.0 - sd.lo.0, sd.ctxt.as_u32()); - - let val = if (base >> INLINE_SIZES[BASE_INDEX]) == 0 && - (len >> INLINE_SIZES[LEN_INDEX]) == 0 && - (ctxt >> INLINE_SIZES[CTXT_INDEX]) == 0 { - (base << INLINE_OFFSETS[BASE_INDEX]) | (len << INLINE_OFFSETS[LEN_INDEX]) | - (ctxt << INLINE_OFFSETS[CTXT_INDEX]) | TAG_INLINE - } else { - let index = with_span_interner(|interner| interner.intern(sd)); - (index << INTERNED_INDEX_OFFSET) | TAG_INTERNED - }; - Span(val) -} - -#[inline] -fn decode(span: Span) -> SpanData { - let val = span.0; - - // Extract a field at position `pos` having size `size`. - let extract = |pos: u32, size: u32| { - let mask = ((!0u32) as u64 >> (32 - size)) as u32; // Can't shift u32 by 32 - (val >> pos) & mask - }; - - let (base, len, ctxt) = if val & TAG_MASK == TAG_INLINE {( - extract(INLINE_OFFSETS[BASE_INDEX], INLINE_SIZES[BASE_INDEX]), - extract(INLINE_OFFSETS[LEN_INDEX], INLINE_SIZES[LEN_INDEX]), - extract(INLINE_OFFSETS[CTXT_INDEX], INLINE_SIZES[CTXT_INDEX]), - )} else { - let index = extract(INTERNED_INDEX_OFFSET, INTERNED_INDEX_SIZE); - return with_span_interner(|interner| *interner.get(index)); - }; - SpanData { lo: BytePos(base), hi: BytePos(base + len), ctxt: SyntaxContext::from_u32(ctxt) } -} - #[derive(Default)] pub struct SpanInterner { spans: FxHashMap, -- 2.44.0