rustc_span: Add conveniences for working with span formats

Centralize span ctxt updates in Span::update_ctxt.
Stop requiring inline and interned ctxts in partially interned format to be synchronized.

Author: petrochenkov

compiler/rustc_expand/src/mbe/transcribe.rs

@@ -30,11 +30,11 @@ impl MutVisitor for Marker {
         // it's some advanced case with macro-generated macros. So if we cache the marked version
         // of that context once, we'll typically have a 100% cache hit rate after that.
         let Marker(expn_id, transparency, ref mut cache) = *self;
-        let data = span.data();
-        let marked_ctxt = *cache
-            .entry(data.ctxt)
-            .or_insert_with(|| data.ctxt.apply_mark(expn_id.to_expn_id(), transparency));
-        *span = data.with_ctxt(marked_ctxt);
+        span.update_ctxt(|ctxt| {
+            *cache
+                .entry(ctxt)
+                .or_insert_with(|| ctxt.apply_mark(expn_id.to_expn_id(), transparency))
+        });
     }
 }
 

compiler/rustc_span/src/lib.rs

@@ -520,8 +520,9 @@ impl SpanData {
     pub fn with_hi(&self, hi: BytePos) -> Span {
         Span::new(self.lo, hi, self.ctxt, self.parent)
     }
+    /// Avoid if possible, `Span::update_ctxt` should be preferred.
     #[inline]
-    pub fn with_ctxt(&self, ctxt: SyntaxContext) -> Span {
+    fn with_ctxt(&self, ctxt: SyntaxContext) -> Span {
         Span::new(self.lo, self.hi, ctxt, self.parent)
     }
     #[inline]
@@ -576,8 +577,9 @@ impl Span {
         self.data().with_hi(hi)
     }
     #[inline]
-    pub fn with_ctxt(self, ctxt: SyntaxContext) -> Span {
-        self.data_untracked().with_ctxt(ctxt)
+    pub fn with_ctxt(mut self, ctxt: SyntaxContext) -> Span {
+        self.update_ctxt(|_| ctxt);
+        self
     }
     #[inline]
     pub fn parent(self) -> Option<LocalDefId> {
@@ -1051,9 +1053,9 @@ impl Span {
     }
 
     #[inline]
-    pub fn apply_mark(self, expn_id: ExpnId, transparency: Transparency) -> Span {
-        let span = self.data();
-        span.with_ctxt(span.ctxt.apply_mark(expn_id, transparency))
+    pub fn apply_mark(mut self, expn_id: ExpnId, transparency: Transparency) -> Span {
+        self.update_ctxt(|ctxt| ctxt.apply_mark(expn_id, transparency));
+        self
     }
 
     #[inline]
@@ -1101,15 +1103,15 @@ impl Span {
     }
 
     #[inline]
-    pub fn normalize_to_macros_2_0(self) -> Span {
-        let span = self.data();
-        span.with_ctxt(span.ctxt.normalize_to_macros_2_0())
+    pub fn normalize_to_macros_2_0(mut self) -> Span {
+        self.update_ctxt(|ctxt| ctxt.normalize_to_macros_2_0());
+        self
     }
 
     #[inline]
-    pub fn normalize_to_macro_rules(self) -> Span {
-        let span = self.data();
-        span.with_ctxt(span.ctxt.normalize_to_macro_rules())
+    pub fn normalize_to_macro_rules(mut self) -> Span {
+        self.update_ctxt(|ctxt| ctxt.normalize_to_macro_rules());
+        self
     }
 }
 

compiler/rustc_span/src/span_encoding.rs

@@ -4,10 +4,10 @@ use crate::SPAN_TRACK;
 use crate::{BytePos, SpanData};
 
 use rustc_data_structures::fx::FxIndexSet;
-
 // This code is very hot and uses lots of arithmetic, avoid overflow checks for performance.
 // See https://github.com/rust-lang/rust/pull/119440#issuecomment-1874255727
 use rustc_serialize::int_overflow::DebugStrictAdd;
+use std::mem::transmute;
 
 /// A compressed span.
 ///
@@ -87,6 +87,99 @@ pub struct Span {
     ctxt_or_parent_or_marker: u16,
 }
 
+// Convenience structures for all span formats.
+#[derive(Clone, Copy)]
+struct InlineCtxt {
+    lo: u32,
+    len: u16,
+    ctxt: u16,
+}
+#[derive(Clone, Copy)]
+struct InlineParent {
+    lo: u32,
+    len_with_tag: u16,
+    parent: u16,
+}
+#[derive(Clone, Copy)]
+struct PartiallyInterned {
+    index: u32,
+    _marker1: u16,
+    ctxt: u16,
+}
+#[derive(Clone, Copy)]
+struct Interned {
+    index: u32,
+    _marker1: u16,
+    _marker2: u16,
+}
+enum Fmt<'a> {
+    InlineCtxt(&'a mut InlineCtxt),
+    InlineParent(&'a mut InlineParent),
+    PartiallyInterned(&'a mut PartiallyInterned),
+    Interned(&'a mut Interned),
+}
+
+impl InlineCtxt {
+    fn data(self) -> SpanData {
+        let len = self.len as u32;
+        debug_assert!(len <= MAX_LEN);
+        SpanData {
+            lo: BytePos(self.lo),
+            hi: BytePos(self.lo.debug_strict_add(len)),
+            ctxt: SyntaxContext::from_u32(self.ctxt as u32),
+            parent: None,
+        }
+    }
+    fn to_span(self) -> Span {
+        unsafe { transmute(self) }
+    }
+}
+impl InlineParent {
+    fn data(self) -> SpanData {
+        let len = (self.len_with_tag & !PARENT_TAG) as u32;
+        debug_assert!(len <= MAX_LEN);
+        let parent = LocalDefId { local_def_index: DefIndex::from_u32(self.parent as u32) };
+        SpanData {
+            lo: BytePos(self.lo),
+            hi: BytePos(self.lo.debug_strict_add(len)),
+            ctxt: SyntaxContext::root(),
+            parent: Some(parent),
+        }
+    }
+    fn to_span(self) -> Span {
+        unsafe { transmute(self) }
+    }
+}
+impl PartiallyInterned {
+    fn data(self) -> SpanData {
+        SpanData {
+            ctxt: SyntaxContext::from_u32(self.ctxt as u32),
+            ..with_span_interner(|interner| interner.spans[self.index as usize])
+        }
+    }
+    fn to_span(self) -> Span {
+        unsafe { transmute(self) }
+    }
+}
+impl Interned {
+    fn data(self) -> SpanData {
+        with_span_interner(|interner| interner.spans[self.index as usize])
+    }
+    fn to_span(self) -> Span {
+        unsafe { transmute(self) }
+    }
+}
+impl Fmt<'_> {
+    fn data(self) -> SpanData {
+        match self {
+            Fmt::InlineCtxt(span) => span.data(),
+            Fmt::InlineParent(span) => span.data(),
+            Fmt::PartiallyInterned(span) => span.data(),
+            Fmt::Interned(span) => span.data(),
+        }
+    }
+}
+
 // `MAX_LEN` is chosen so that `PARENT_TAG | MAX_LEN` is distinct from
 // `BASE_LEN_INTERNED_MARKER`. (If `MAX_LEN` was 1 higher, this wouldn't be true.)
 const MAX_LEN: u32 = 0b0111_1111_1111_1110;
@@ -111,42 +204,48 @@ impl Span {
             std::mem::swap(&mut lo, &mut hi);
         }
 
-        let (lo2, len, ctxt2) = (lo.0, hi.0 - lo.0, ctxt.as_u32());
-
+        // Small len may enable one of fully inline formats (or may not).
+        let (len, ctxt32) = (hi.0 - lo.0, ctxt.as_u32());
         if len <= MAX_LEN {
-            if ctxt2 <= MAX_CTXT && parent.is_none() {
-                // Inline-context format.
-                return Span {
-                    lo_or_index: lo2,
-                    len_with_tag_or_marker: len as u16,
-                    ctxt_or_parent_or_marker: ctxt2 as u16,
-                };
-            } else if ctxt2 == SyntaxContext::root().as_u32()
+            if ctxt32 <= MAX_CTXT && parent.is_none() {
+                return InlineCtxt { lo: lo.0, len: len as u16, ctxt: ctxt32 as u16 }.to_span();
+            } else if ctxt32 == 0
                 && let Some(parent) = parent
-                && let parent2 = parent.local_def_index.as_u32()
-                && parent2 <= MAX_CTXT
+                && let parent32 = parent.local_def_index.as_u32()
+                && parent32 <= MAX_CTXT
             {
-                // Inline-parent format.
-                return Span {
-                    lo_or_index: lo2,
-                    len_with_tag_or_marker: PARENT_TAG | len as u16,
-                    ctxt_or_parent_or_marker: parent2 as u16,
-                };
+                let len_with_tag = PARENT_TAG | len as u16;
+                return InlineParent { lo: lo.0, len_with_tag, parent: parent32 as u16 }.to_span();
             }
         }
 
-        // Partially-interned or fully-interned format.
-        let index =
-            with_span_interner(|interner| interner.intern(&SpanData { lo, hi, ctxt, parent }));
-        let ctxt_or_parent_or_marker = if ctxt2 <= MAX_CTXT {
-            ctxt2 as u16 // partially-interned
-        } else {
-            CTXT_INTERNED_MARKER // fully-interned
+        // Otherwise small ctxt may enable the partially inline format.
+        let index = |ctxt| {
+            with_span_interner(|interner| interner.intern(&SpanData { lo, hi, ctxt, parent }))
         };
-        Span {
-            lo_or_index: index,
-            len_with_tag_or_marker: BASE_LEN_INTERNED_MARKER,
-            ctxt_or_parent_or_marker,
+        if ctxt32 <= MAX_CTXT {
+            let index = index(SyntaxContext::from_u32(u32::MAX)); // any value, should never be read
+            PartiallyInterned { index, _marker1: BASE_LEN_INTERNED_MARKER, ctxt: ctxt32 as u16 }
+                .to_span()
+        } else {
+            let index = index(ctxt);
+            Interned { index, _marker1: BASE_LEN_INTERNED_MARKER, _marker2: CTXT_INTERNED_MARKER }
+                .to_span()
+        }
+    }
+
+    #[inline]
+    fn fmt(&mut self) -> Fmt<'_> {
+        if self.len_with_tag_or_marker != BASE_LEN_INTERNED_MARKER {
+            if self.len_with_tag_or_marker & PARENT_TAG == 0 {
+                Fmt::InlineCtxt(unsafe { transmute(self) })
+            } else {
+                Fmt::InlineParent(unsafe { transmute(self) })
+            }
+        } else if self.ctxt_or_parent_or_marker != CTXT_INTERNED_MARKER {
+            Fmt::PartiallyInterned(unsafe { transmute(self) })
+        } else {
+            Fmt::Interned(unsafe { transmute(self) })
         }
     }
 
@@ -162,39 +261,8 @@ impl Span {
     /// Internal function to translate between an encoded span and the expanded representation.
     /// This function must not be used outside the incremental engine.
     #[inline]
-    pub fn data_untracked(self) -> SpanData {
-        if self.len_with_tag_or_marker != BASE_LEN_INTERNED_MARKER {
-            if self.len_with_tag_or_marker & PARENT_TAG == 0 {
-                // Inline-context format.
-                let len = self.len_with_tag_or_marker as u32;
-                debug_assert!(len <= MAX_LEN);
-                SpanData {
-                    lo: BytePos(self.lo_or_index),
-                    hi: BytePos(self.lo_or_index.debug_strict_add(len)),
-                    ctxt: SyntaxContext::from_u32(self.ctxt_or_parent_or_marker as u32),
-                    parent: None,
-                }
-            } else {
-                // Inline-parent format.
-                let len = (self.len_with_tag_or_marker & !PARENT_TAG) as u32;
-                debug_assert!(len <= MAX_LEN);
-                let parent = LocalDefId {
-                    local_def_index: DefIndex::from_u32(self.ctxt_or_parent_or_marker as u32),
-                };
-                SpanData {
-                    lo: BytePos(self.lo_or_index),
-                    hi: BytePos(self.lo_or_index.debug_strict_add(len)),
-                    ctxt: SyntaxContext::root(),
-                    parent: Some(parent),
-                }
-            }
-        } else {
-            // Fully-interned or partially-interned format. In either case,
-            // the interned value contains all the data, so we don't need to
-            // distinguish them.
-            let index = self.lo_or_index;
-            with_span_interner(|interner| interner.spans[index as usize])
-        }
+    pub fn data_untracked(mut self) -> SpanData {
+        self.fmt().data()
     }
 
     /// Returns `true` if this is a dummy span with any hygienic context.
@@ -214,26 +282,26 @@ impl Span {
         }
     }
 
+    // For optimization we are interested in cases in which the context is inline and the context
+    // update doesn't change format. All non-inline or format changing scenarios require accessing
+    // interner and can fall back to `Span::new`.
+    pub fn update_ctxt(&mut self, update: impl FnOnce(SyntaxContext) -> SyntaxContext) {
+        // FIXME(#125017): Update ctxt inline without touching interner when possible.
+        let data = self.data();
+        *self = data.with_ctxt(update(data.ctxt));
+    }
+
     // Returns either syntactic context, if it can be retrieved without taking the interner lock,
     // or an index into the interner if it cannot.
-    fn inline_ctxt(self) -> Result<SyntaxContext, usize> {
-        Ok(if self.len_with_tag_or_marker != BASE_LEN_INTERNED_MARKER {
-            if self.len_with_tag_or_marker & PARENT_TAG == 0 {
-                // Inline-context format.
-                SyntaxContext::from_u32(self.ctxt_or_parent_or_marker as u32)
-            } else {
-                // Inline-parent format. We know that the SyntaxContext is root.
-                SyntaxContext::root()
+    fn inline_ctxt(mut self) -> Result<SyntaxContext, usize> {
+        match self.fmt() {
+            Fmt::InlineCtxt(InlineCtxt { ctxt, .. })
+            | Fmt::PartiallyInterned(PartiallyInterned { ctxt, .. }) => {
+                Ok(SyntaxContext::from_u32(*ctxt as u32))
             }
-        } else if self.ctxt_or_parent_or_marker != CTXT_INTERNED_MARKER {
-            // Partially-interned format. This path avoids looking up the
-            // interned value, and is the whole point of the
-            // partially-interned format.
-            SyntaxContext::from_u32(self.ctxt_or_parent_or_marker as u32)
-        } else {
-            // Fully-interned format.
-            return Err(self.lo_or_index as usize);
-        })
+            Fmt::InlineParent(_) => Ok(SyntaxContext::root()),
+            Fmt::Interned(span) => Err(span.index as usize),
+        }
     }
 
     /// This function is used as a fast path when decoding the full `SpanData` is not necessary.