rustc_span: Optimize span parent get/set methods

Author: petrochenkov

compiler/rustc_span/src/lib.rs

@@ -525,8 +525,9 @@ impl SpanData {
     fn with_ctxt(&self, ctxt: SyntaxContext) -> Span {
         Span::new(self.lo, self.hi, ctxt, self.parent)
     }
+    /// Avoid if possible, `Span::with_parent` should be preferred.
     #[inline]
-    pub fn with_parent(&self, parent: Option<LocalDefId>) -> Span {
+    fn with_parent(&self, parent: Option<LocalDefId>) -> Span {
         Span::new(self.lo, self.hi, self.ctxt, parent)
     }
     /// Returns `true` if this is a dummy span with any hygienic context.
@@ -580,14 +581,6 @@ impl Span {
     pub fn with_ctxt(self, ctxt: SyntaxContext) -> Span {
         self.map_ctxt(|_| ctxt)
     }
-    #[inline]
-    pub fn parent(self) -> Option<LocalDefId> {
-        self.data().parent
-    }
-    #[inline]
-    pub fn with_parent(self, ctxt: Option<LocalDefId>) -> Span {
-        self.data().with_parent(ctxt)
-    }
 
     #[inline]
     pub fn is_visible(self, sm: &SourceMap) -> bool {

compiler/rustc_span/src/span_encoding.rs

@@ -322,29 +322,28 @@ 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`.
     #[inline]
-    pub fn map_ctxt(self, update: impl FnOnce(SyntaxContext) -> SyntaxContext) -> Span {
-        match_span_kind! {
+    pub fn map_ctxt(self, map: impl FnOnce(SyntaxContext) -> SyntaxContext) -> Span {
+        let data = match_span_kind! {
             self,
             InlineCtxt(span) => {
-                let updated_ctxt32 = update(SyntaxContext::from_u16(span.ctxt)).as_u32();
-                // Any small new context including zero will preserve the format.
-                return if updated_ctxt32 <= MAX_CTXT {
-                    InlineCtxt::span(span.lo, span.len, updated_ctxt32 as u16)
+                // This format occurs 1-2 orders of magnitude more often than others (#125017),
+                // so it makes sense to micro-optimize it to avoid `span.data()` and `Span::new()`.
+                let new_ctxt = map(SyntaxContext::from_u16(span.ctxt));
+                let new_ctxt32 = new_ctxt.as_u32();
+                return if new_ctxt32 <= MAX_CTXT {
+                    // Any small new context including zero will preserve the format.
+                    InlineCtxt::span(span.lo, span.len, new_ctxt32 as u16)
                 } else {
-                    span.data().with_ctxt(SyntaxContext::from_u32(updated_ctxt32))
+                    span.data().with_ctxt(new_ctxt)
                 };
             },
-            InlineParent(_span) => {},
-            PartiallyInterned(_span) => {},
-            Interned(_span) => {},
-        }
+            InlineParent(span) => span.data(),
+            PartiallyInterned(span) => span.data(),
+            Interned(span) => span.data(),
+        };
 
-        let data = self.data_untracked();
-        data.with_ctxt(update(data.ctxt))
+        data.with_ctxt(map(data.ctxt))
     }
 
     // Returns either syntactic context, if it can be retrieved without taking the interner lock,
@@ -381,6 +380,56 @@ impl Span {
             }),
         }
     }
+
+    #[inline]
+    pub fn with_parent(self, parent: Option<LocalDefId>) -> Span {
+        let data = match_span_kind! {
+            self,
+            InlineCtxt(span) => {
+                // This format occurs 1-2 orders of magnitude more often than others (#126544),
+                // so it makes sense to micro-optimize it to avoid `span.data()` and `Span::new()`.
+                if parent.is_none() {
+                    // Only if the new parent is `None` the format will be preserved.
+                    return self;
+                } else if span.ctxt == 0
+                    && let Some(parent) = parent
+                    && let parent32 = parent.local_def_index.as_u32()
+                    && parent32 <= MAX_CTXT
+                {
+                    // Otherwise the new format may be `InlineParent` if the new parent is small.
+                    return InlineParent::span(span.lo, span.len, parent32 as u16);
+                }
+                span.data()
+            },
+            InlineParent(span) => span.data(),
+            PartiallyInterned(span) => span.data(),
+            Interned(span) => span.data(),
+        };
+
+        if let Some(old_parent) = data.parent {
+            // FIXME: Is this tracking necessary?
+            (*SPAN_TRACK)(old_parent);
+        }
+        data.with_parent(parent)
+    }
+
+    #[inline]
+    pub fn parent(self) -> Option<LocalDefId> {
+        let interned_parent =
+            |index: u32| with_span_interner(|interner| interner.spans[index as usize].parent);
+        let parent = match_span_kind! {
+            self,
+            InlineCtxt(_span) => return None,
+            InlineParent(span) => Some(LocalDefId { local_def_index: DefIndex::from_u32(span.parent as u32) }),
+            PartiallyInterned(span) => interned_parent(span.index),
+            Interned(span) => interned_parent(span.index),
+        };
+        if let Some(parent) = parent {
+            // FIXME: Is this tracking necessary?
+            (*SPAN_TRACK)(parent);
+        }
+        parent
+    }
 }
 
 #[derive(Default)]