rustc_mir: add a pass for fragmenting locals into their fields (aka SROA).

src/librustc/mir/mod.rs

@@ -948,10 +948,64 @@ pub struct VarDebugInfo<'tcx> {
     /// (see `LocalDecl`'s `source_info` field for more details).
     pub source_info: SourceInfo,
 
-    /// Where the data for this user variable is to be found.
-    /// NOTE(eddyb) There's an unenforced invariant that this `Place` is
-    /// based on a `Local`, not a `Static`, and contains no indexing.
-    pub place: Place<'tcx>,
+    /// How the contents of this user variable are represented.
+    pub contents: VarDebugInfoContents<'tcx>,
+}
+
+#[derive(Clone, Debug, RustcEncodable, RustcDecodable, HashStable, TypeFoldable)]
+pub enum VarDebugInfoContents<'tcx> {
+    /// The user variable's data is entirely in one `Place`.
+    // NOTE(eddyb) there's an unenforced invariant that this `Place` is
+    // contains no indexing (with a non-constant index).
+    Compact(Place<'tcx>),
+
+    /// The user variable's data is split across several fragments,
+    /// each described by a `VarDebugInfoFragment`.
+    /// See DWARF 5's "2.6.1.2 Composite Location Descriptions"
+    /// and LLVM's `DW_OP_LLVM_fragment` for more details on
+    /// the underlying debuginfo feature this relies on.
+    Composite {
+        /// Type of the original user variable.
+        ty: Ty<'tcx>,
+
+        /// All the parts of the original user variable, which ended
+        /// up in disjoint places, due to optimizations.
+        fragments: Vec<VarDebugInfoFragment<'tcx>>,
+    },
+}
+
+#[derive(Clone, RustcEncodable, RustcDecodable, HashStable, TypeFoldable)]
+pub struct VarDebugInfoFragment<'tcx> {
+    /// Where in the composite user variable this fragment is,
+    /// represented as a "projection" into the composite variable.
+    /// At lower levels, this corresponds to a byte/bit range.
+    // NOTE(eddyb) there's an unenforced invariant that this contains
+    // only `Field`s, and not into `enum` variants or `union`s.
+    // FIXME(eddyb) support this for `enum`s by either using DWARF's
+    // more advanced control-flow features (unsupported by LLVM?)
+    // to match on the discriminant, or by using custom type debuginfo
+    // with non-overlapping variants for the composite variable.
+    pub projection: Vec<ProjectionKind>,
+
+    /// Where the data for this fragment can be found.
+    // NOTE(eddyb) There's an unenforced invariant that this `Place` is
+    // contains no indexing (with a non-constant index).
+    pub contents: Place<'tcx>,
+}
+
+impl Debug for VarDebugInfoFragment<'_> {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> fmt::Result {
+        for elem in self.projection.iter() {
+            match elem {
+                ProjectionElem::Field(field, _) => {
+                    write!(fmt, ".{:?}", field.index())?;
+                }
+                _ => bug!("unsupported fragment projection `{:?}`", elem),
+            }
+        }
+
+        write!(fmt, " => {:?}", self.contents)
+    }
 }
 
 ///////////////////////////////////////////////////////////////////////////

src/librustc/mir/visit.rs

@@ -739,16 +739,31 @@ macro_rules! make_mir_visitor {
                 let VarDebugInfo {
                     name: _,
                     source_info,
-                    place,
+                    contents,
                 } = var_debug_info;
 
                 self.visit_source_info(source_info);
                 let location = START_BLOCK.start_location();
-                self.visit_place(
-                    place,
-                    PlaceContext::NonUse(NonUseContext::VarDebugInfo),
-                    location,
-                );
+                match contents {
+                    VarDebugInfoContents::Compact(place) => {
+                        self.visit_place(
+                            place,
+                            PlaceContext::NonUse(NonUseContext::VarDebugInfo),
+                            location,
+                        );
+                    }
+                    VarDebugInfoContents::Composite { ty, fragments } => {
+                        // FIXME(eddyb) use a better `TyContext` here.
+                        self.visit_ty(ty, TyContext::Location(location));
+                        for VarDebugInfoFragment { projection: _, contents } in fragments {
+                            self.visit_place(
+                                contents,
+                                PlaceContext::NonUse(NonUseContext::VarDebugInfo),
+                                location,
+                            );
+                        }
+                    }
+                }
             }
 
             fn super_source_scope(&mut self,

src/librustc_codegen_llvm/debuginfo/metadata.rs

@@ -1288,10 +1288,14 @@ fn generator_layout_and_saved_local_names(
 
     let state_arg = mir::Local::new(1);
     for var in &body.var_debug_info {
-        if var.place.local != state_arg {
+        let place = match var.contents {
+            mir::VarDebugInfoContents::Compact(place) => place,
+            mir::VarDebugInfoContents::Composite { .. } => continue,
+        };
+        if place.local != state_arg {
             continue;
         }
-        match var.place.projection[..] {
+        match place.projection[..] {
             [
                 // Deref of the `Pin<&mut Self>` state argument.
                 mir::ProjectionElem::Field(..),

src/librustc_codegen_llvm/debuginfo/mod.rs

@@ -34,6 +34,7 @@ use libc::c_uint;
 use log::debug;
 use std::cell::RefCell;
 use std::ffi::CString;
+use std::ops::Range;
 
 use rustc::ty::layout::{self, HasTyCtxt, LayoutOf, Size};
 use rustc_codegen_ssa::traits::*;
@@ -154,30 +155,40 @@ impl DebugInfoBuilderMethods for Builder<'a, 'll, 'tcx> {
         variable_alloca: Self::Value,
         direct_offset: Size,
         indirect_offsets: &[Size],
+        fragment: Option<Range<Size>>,
         span: Span,
     ) {
         assert!(!dbg_context.source_locations_enabled);
         let cx = self.cx();
 
         let loc = span_start(cx, span);
 
-        // Convert the direct and indirect offsets to address ops.
+        // Convert the direct/indirect offsets and fragment byte range to DWARF OPs.
         let op_deref = || unsafe { llvm::LLVMRustDIBuilderCreateOpDeref() };
         let op_plus_uconst = || unsafe { llvm::LLVMRustDIBuilderCreateOpPlusUconst() };
-        let mut addr_ops = SmallVec::<[_; 8]>::new();
+        let op_llvm_fragment = || unsafe { llvm::LLVMRustDIBuilderCreateOpLLVMFragment() };
+        let mut dwarf_ops = SmallVec::<[_; 8]>::new();
 
         if direct_offset.bytes() > 0 {
-            addr_ops.push(op_plus_uconst());
-            addr_ops.push(direct_offset.bytes() as i64);
+            dwarf_ops.push(op_plus_uconst());
+            dwarf_ops.push(direct_offset.bytes() as i64);
         }
         for &offset in indirect_offsets {
-            addr_ops.push(op_deref());
+            dwarf_ops.push(op_deref());
             if offset.bytes() > 0 {
-                addr_ops.push(op_plus_uconst());
-                addr_ops.push(offset.bytes() as i64);
+                dwarf_ops.push(op_plus_uconst());
+                dwarf_ops.push(offset.bytes() as i64);
             }
         }
 
+        if let Some(fragment) = fragment {
+            // `DW_OP_LLVM_fragment` takes as arguments the fragment's
+            // offset and size, both of them in bits.
+            dwarf_ops.push(op_llvm_fragment());
+            dwarf_ops.push(fragment.start.bits() as i64);
+            dwarf_ops.push((fragment.end - fragment.start).bits() as i64);
+        }
+
         // FIXME(eddyb) maybe this information could be extracted from `var`,
         // to avoid having to pass it down in both places?
         source_loc::set_debug_location(
@@ -191,8 +202,8 @@ impl DebugInfoBuilderMethods for Builder<'a, 'll, 'tcx> {
                 DIB(cx),
                 variable_alloca,
                 dbg_var,
-                addr_ops.as_ptr(),
-                addr_ops.len() as c_uint,
+                dwarf_ops.as_ptr(),
+                dwarf_ops.len() as c_uint,
                 debug_loc,
                 self.llbb(),
             );

src/librustc_codegen_llvm/llvm/ffi.rs

@@ -1809,6 +1809,7 @@ extern "C" {
     ) -> &'a Value;
     pub fn LLVMRustDIBuilderCreateOpDeref() -> i64;
     pub fn LLVMRustDIBuilderCreateOpPlusUconst() -> i64;
+    pub fn LLVMRustDIBuilderCreateOpLLVMFragment() -> i64;
 
     #[allow(improper_ctypes)]
     pub fn LLVMRustWriteTypeToString(Type: &Type, s: &RustString);

src/librustc_codegen_ssa/mir/debuginfo.rs

@@ -5,9 +5,9 @@ use rustc::ty;
 use rustc::ty::layout::{LayoutOf, Size};
 use rustc_hir::def_id::CrateNum;
 use rustc_index::vec::IndexVec;
-
 use rustc_span::symbol::{kw, Symbol};
 use rustc_span::{BytePos, Span};
+use std::ops::Range;
 
 use super::OperandValue;
 use super::{FunctionCx, LocalRef};
@@ -25,14 +25,18 @@ pub enum VariableKind {
 }
 
 /// Like `mir::VarDebugInfo`, but within a `mir::Local`.
-#[derive(Copy, Clone)]
+#[derive(Clone)]
 pub struct PerLocalVarDebugInfo<'tcx, D> {
     pub name: Symbol,
     pub source_info: mir::SourceInfo,
 
     /// `DIVariable` returned by `create_dbg_var`.
     pub dbg_var: Option<D>,
 
+    /// Byte range in the `dbg_var` covered by this fragment,
+    /// if this is a fragment of a composite `VarDebugInfo`.
+    pub fragment: Option<Range<Size>>,
+
     /// `.place.projection` from `mir::VarDebugInfo`.
     pub projection: &'tcx ty::List<mir::PlaceElem<'tcx>>,
 }
@@ -127,7 +131,8 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
             Some(per_local) => &per_local[local],
             None => return,
         };
-        let whole_local_var = vars.iter().find(|var| var.projection.is_empty()).copied();
+        let whole_local_var =
+            vars.iter().find(|var| var.fragment.is_none() && var.projection.is_empty()).cloned();
         let has_proj = || vars.iter().any(|var| !var.projection.is_empty());
 
         let fallback_var = if self.mir.local_kind(local) == mir::LocalKind::Arg {
@@ -173,6 +178,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                     name,
                     source_info: decl.source_info,
                     dbg_var,
+                    fragment: None,
                     projection: ty::List::empty(),
                 })
             }
@@ -183,7 +189,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         let local_ref = &self.locals[local];
 
         if !bx.sess().fewer_names() {
-            let name = match whole_local_var.or(fallback_var) {
+            let name = match whole_local_var.or(fallback_var.clone()) {
                 Some(var) if var.name != kw::Invalid => var.name.to_string(),
                 _ => format!("{:?}", local),
             };
@@ -221,7 +227,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
             _ => return,
         };
 
-        let vars = vars.iter().copied().chain(fallback_var);
+        let vars = vars.iter().cloned().chain(fallback_var);
 
         for var in vars {
             let mut layout = base.layout;
@@ -270,6 +276,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                         base.llval,
                         direct_offset,
                         &indirect_offsets,
+                        var.fragment,
                         span,
                     );
                 }
@@ -302,25 +309,31 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
             } else {
                 (None, var.source_info.span)
             };
+            let (var_ty, var_kind) = match var.contents {
+                mir::VarDebugInfoContents::Compact(place) => (
+                    self.monomorphized_place_ty(place.as_ref()),
+                    if self.mir.local_kind(place.local) == mir::LocalKind::Arg
+                        && place.projection.is_empty()
+                    {
+                        // FIXME(eddyb, #67586) take `var.source_info.scope` into
+                        // account to avoid using `ArgumentVariable` more than once
+                        // per argument local.
+
+                        let arg_index = place.local.index() - 1;
+
+                        // FIXME(eddyb) shouldn't `ArgumentVariable` indices be
+                        // offset in closures to account for the hidden environment?
+                        // Also, is this `+ 1` needed at all?
+                        VariableKind::ArgumentVariable(arg_index + 1)
+                    } else {
+                        VariableKind::LocalVariable
+                    },
+                ),
+                mir::VarDebugInfoContents::Composite { ty, fragments: _ } => {
+                    (self.monomorphize(&ty), VariableKind::LocalVariable)
+                }
+            };
             let dbg_var = scope.map(|scope| {
-                let place = var.place;
-                let var_ty = self.monomorphized_place_ty(place.as_ref());
-                let var_kind = if self.mir.local_kind(place.local) == mir::LocalKind::Arg
-                    && place.projection.is_empty()
-                {
-                    // FIXME(eddyb, #67586) take `var.source_info.scope` into
-                    // account to avoid using `ArgumentVariable` more than once
-                    // per argument local.
-
-                    let arg_index = place.local.index() - 1;
-
-                    // FIXME(eddyb) shouldn't `ArgumentVariable` indices be
-                    // offset in closures to account for the hidden environment?
-                    // Also, is this `+ 1` needed at all?
-                    VariableKind::ArgumentVariable(arg_index + 1)
-                } else {
-                    VariableKind::LocalVariable
-                };
                 self.cx.create_dbg_var(
                     self.debug_context.as_ref().unwrap(),
                     var.name,
@@ -331,12 +344,54 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 )
             });
 
-            per_local[var.place.local].push(PerLocalVarDebugInfo {
-                name: var.name,
-                source_info: var.source_info,
-                dbg_var,
-                projection: var.place.projection,
-            });
+            match &var.contents {
+                mir::VarDebugInfoContents::Compact(place) => {
+                    per_local[place.local].push(PerLocalVarDebugInfo {
+                        name: var.name,
+                        source_info: var.source_info,
+                        dbg_var,
+                        fragment: None,
+                        projection: place.projection,
+                    });
+                }
+                mir::VarDebugInfoContents::Composite { ty: _, fragments } => {
+                    let var_layout = self.cx.layout_of(var_ty);
+                    for fragment in fragments {
+                        let mut fragment_start = Size::ZERO;
+                        let mut fragment_layout = var_layout;
+
+                        for elem in &fragment.projection {
+                            match *elem {
+                                mir::ProjectionElem::Field(field, _) => {
+                                    let i = field.index();
+                                    fragment_start += fragment_layout.fields.offset(i);
+                                    fragment_layout = fragment_layout.field(self.cx, i);
+                                }
+                                _ => span_bug!(
+                                    var.source_info.span,
+                                    "unsupported fragment projection `{:?}`",
+                                    elem,
+                                ),
+                            }
+                        }
+
+                        let place = fragment.contents;
+                        per_local[place.local].push(PerLocalVarDebugInfo {
+                            name: var.name,
+                            source_info: var.source_info,
+                            dbg_var,
+                            fragment: if fragment_layout.size == var_layout.size {
+                                // Fragment covers entire variable, so as far as
+                                // DWARF is concerned, it's not really a fragment.
+                                None
+                            } else {
+                                Some(fragment_start..fragment_start + fragment_layout.size)
+                            },
+                            projection: place.projection,
+                        });
+                    }
+                }
+            }
         }
         Some(per_local)
     }