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/builder.rs

@@ -57,6 +57,7 @@ impl BackendTypes for Builder<'_, 'll, 'tcx> {
     type Funclet = <CodegenCx<'ll, 'tcx> as BackendTypes>::Funclet;
 
     type DIScope = <CodegenCx<'ll, 'tcx> as BackendTypes>::DIScope;
+    type DIVariable = <CodegenCx<'ll, 'tcx> as BackendTypes>::DIVariable;
 }
 
 impl ty::layout::HasDataLayout for Builder<'_, '_, '_> {

src/librustc_codegen_llvm/common.rs

@@ -91,6 +91,7 @@ impl BackendTypes for CodegenCx<'ll, 'tcx> {
     type Funclet = Funclet<'ll>;
 
     type DIScope = &'ll llvm::debuginfo::DIScope;
+    type DIVariable = &'ll llvm::debuginfo::DIVariable;
 }
 
 impl CodegenCx<'ll, 'tcx> {

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

@@ -11,7 +11,7 @@ use self::utils::{create_DIArray, is_node_local_to_unit, span_start, DIB};
 
 use crate::llvm;
 use crate::llvm::debuginfo::{
-    DIArray, DIBuilder, DIFile, DIFlags, DILexicalBlock, DISPFlags, DIScope, DIType,
+    DIArray, DIBuilder, DIFile, DIFlags, DILexicalBlock, DISPFlags, DIScope, DIType, DIVariable,
 };
 use rustc::middle::codegen_fn_attrs::CodegenFnAttrFlags;
 use rustc::ty::subst::{GenericArgKind, SubstsRef};
@@ -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::*;
@@ -143,79 +144,66 @@ pub fn finalize(cx: &CodegenCx<'_, '_>) {
     };
 }
 
-impl DebugInfoBuilderMethods<'tcx> for Builder<'a, 'll, 'tcx> {
-    fn declare_local(
+impl DebugInfoBuilderMethods for Builder<'a, 'll, 'tcx> {
+    // FIXME(eddyb) find a common convention for all of the debuginfo-related
+    // names (choose between `dbg`, `debug`, `debuginfo`, `debug_info` etc.).
+    fn dbg_var_addr(
         &mut self,
         dbg_context: &FunctionDebugContext<&'ll DIScope>,
-        variable_name: ast::Name,
-        variable_type: Ty<'tcx>,
+        dbg_var: &'ll DIVariable,
         scope_metadata: &'ll DIScope,
         variable_alloca: Self::Value,
         direct_offset: Size,
         indirect_offsets: &[Size],
-        variable_kind: VariableKind,
+        fragment: Option<Range<Size>>,
         span: Span,
     ) {
         assert!(!dbg_context.source_locations_enabled);
         let cx = self.cx();
 
-        let file = span_start(cx, span).file;
-        let file_metadata = file_metadata(cx, &file.name, dbg_context.defining_crate);
-
         let loc = span_start(cx, span);
-        let type_metadata = type_metadata(cx, variable_type, span);
 
-        let (argument_index, dwarf_tag) = match variable_kind {
-            ArgumentVariable(index) => (index as c_uint, DW_TAG_arg_variable),
-            LocalVariable => (0, DW_TAG_auto_variable),
-        };
-        let align = cx.align_of(variable_type);
-
-        // 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);
             }
         }
 
-        let name = SmallCStr::new(&variable_name.as_str());
-        let metadata = unsafe {
-            llvm::LLVMRustDIBuilderCreateVariable(
-                DIB(cx),
-                dwarf_tag,
-                scope_metadata,
-                name.as_ptr(),
-                file_metadata,
-                loc.line as c_uint,
-                type_metadata,
-                cx.sess().opts.optimize != config::OptLevel::No,
-                DIFlags::FlagZero,
-                argument_index,
-                align.bytes() as u32,
-            )
-        };
+        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(
             self,
             InternalDebugLocation::new(scope_metadata, loc.line, loc.col.to_usize()),
         );
         unsafe {
             let debug_loc = llvm::LLVMGetCurrentDebugLocation(self.llbuilder);
+            // FIXME(eddyb) replace `llvm.dbg.declare` with `llvm.dbg.addr`.
             let instr = llvm::LLVMRustDIBuilderInsertDeclareAtEnd(
                 DIB(cx),
                 variable_alloca,
-                metadata,
-                addr_ops.as_ptr(),
-                addr_ops.len() as c_uint,
+                dbg_var,
+                dwarf_ops.as_ptr(),
+                dwarf_ops.len() as c_uint,
                 debug_loc,
                 self.llbb(),
             );
@@ -558,4 +546,45 @@ impl DebugInfoMethods<'tcx> for CodegenCx<'ll, 'tcx> {
     fn debuginfo_finalize(&self) {
         finalize(self)
     }
+
+    // FIXME(eddyb) find a common convention for all of the debuginfo-related
+    // names (choose between `dbg`, `debug`, `debuginfo`, `debug_info` etc.).
+    fn create_dbg_var(
+        &self,
+        dbg_context: &FunctionDebugContext<&'ll DIScope>,
+        variable_name: ast::Name,
+        variable_type: Ty<'tcx>,
+        scope_metadata: &'ll DIScope,
+        variable_kind: VariableKind,
+        span: Span,
+    ) -> &'ll DIVariable {
+        let file = span_start(self, span).file;
+        let file_metadata = file_metadata(self, &file.name, dbg_context.defining_crate);
+
+        let loc = span_start(self, span);
+        let type_metadata = type_metadata(self, variable_type, span);
+
+        let (argument_index, dwarf_tag) = match variable_kind {
+            ArgumentVariable(index) => (index as c_uint, DW_TAG_arg_variable),
+            LocalVariable => (0, DW_TAG_auto_variable),
+        };
+        let align = self.align_of(variable_type);
+
+        let name = SmallCStr::new(&variable_name.as_str());
+        unsafe {
+            llvm::LLVMRustDIBuilderCreateVariable(
+                DIB(self),
+                dwarf_tag,
+                scope_metadata,
+                name.as_ptr(),
+                file_metadata,
+                loc.line as c_uint,
+                type_metadata,
+                self.sess().opts.optimize != config::OptLevel::No,
+                DIFlags::FlagZero,
+                argument_index,
+                align.bytes() as u32,
+            )
+        }
+    }
 }

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);