Auto merge of rust-lang#111440 - cjgillot:refprop-debuginfo, r=oli-obk

Allow MIR debuginfo to point to a variable's address

MIR optimizations currently do not to operate on borrowed locals.

When enabling rust-lang#106285, many borrows will be left as-is because they are used in debuginfo. This pass allows to replace this pattern directly in MIR debuginfo:
```rust
a => _1
_1 = &raw? mut? _2
```
becomes
```rust
a => &_2
// No statement to borrow _2.
```

This pass is implemented as a drive-by in ReferencePropagation MIR pass.

This transformation allows following following MIR opts to treat _2 as an unborrowed local, and optimize it as such, even in builds with debuginfo.

In codegen, when encountering `a => &..&_2`, we create a list of allocas:
```llvm
store ptr %_2.dbg.spill, ptr %a.ref0.dbg.spill
store ptr %a.ref0.dbg.spill, ptr %a.ref1.dbg.spill
...
call void `@llvm.dbg.declare(metadata` ptr %a.ref{n}.dbg.spill, /* ... */)
```

Caveat: this transformation looses the exact type, we do not differentiate `a` as a immutable, mutable reference or a raw pointer. Everything is declared to `*mut` to codegen. I'm not convinced this is a blocker.

compiler/rustc_codegen_ssa/src/mir/debuginfo.rs

@@ -8,7 +8,7 @@ use rustc_middle::ty::layout::{HasTyCtxt, LayoutOf};
 use rustc_session::config::DebugInfo;
 use rustc_span::symbol::{kw, Symbol};
 use rustc_span::{BytePos, Span};
-use rustc_target::abi::{Abi, FieldIdx, Size, VariantIdx};
+use rustc_target::abi::{Abi, FieldIdx, FieldsShape, Size, VariantIdx};
 
 use super::operand::{OperandRef, OperandValue};
 use super::place::PlaceRef;
@@ -41,6 +41,9 @@ pub struct PerLocalVarDebugInfo<'tcx, D> {
 
     /// `.place.projection` from `mir::VarDebugInfo`.
     pub projection: &'tcx ty::List<mir::PlaceElem<'tcx>>,
+
+    /// `references` from `mir::VarDebugInfo`.
+    pub references: u8,
 }
 
 #[derive(Clone, Copy, Debug)]
@@ -80,6 +83,7 @@ trait DebugInfoOffsetLocation<'tcx, Bx> {
     fn deref(&self, bx: &mut Bx) -> Self;
     fn layout(&self) -> TyAndLayout<'tcx>;
     fn project_field(&self, bx: &mut Bx, field: FieldIdx) -> Self;
+    fn project_constant_index(&self, bx: &mut Bx, offset: u64) -> Self;
     fn downcast(&self, bx: &mut Bx, variant: VariantIdx) -> Self;
 }
 
@@ -98,6 +102,11 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> DebugInfoOffsetLocation<'tcx, Bx>
         PlaceRef::project_field(*self, bx, field.index())
     }
 
+    fn project_constant_index(&self, bx: &mut Bx, offset: u64) -> Self {
+        let lloffset = bx.cx().const_usize(offset);
+        self.project_index(bx, lloffset)
+    }
+
     fn downcast(&self, bx: &mut Bx, variant: VariantIdx) -> Self {
         self.project_downcast(bx, variant)
     }
@@ -120,6 +129,10 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> DebugInfoOffsetLocation<'tcx, Bx>
         self.field(bx.cx(), field.index())
     }
 
+    fn project_constant_index(&self, bx: &mut Bx, index: u64) -> Self {
+        self.field(bx.cx(), index as usize)
+    }
+
     fn downcast(&self, bx: &mut Bx, variant: VariantIdx) -> Self {
         self.for_variant(bx.cx(), variant)
     }
@@ -165,6 +178,18 @@ fn calculate_debuginfo_offset<
             mir::ProjectionElem::Downcast(_, variant) => {
                 place = place.downcast(bx, variant);
             }
+            mir::ProjectionElem::ConstantIndex {
+                offset: index,
+                min_length: _,
+                from_end: false,
+            } => {
+                let offset = indirect_offsets.last_mut().unwrap_or(&mut direct_offset);
+                let FieldsShape::Array { stride, count: _ } = place.layout().fields else {
+                    span_bug!(var.source_info.span, "ConstantIndex on non-array type {:?}", place.layout())
+                };
+                *offset += stride * index;
+                place = place.project_constant_index(bx, index);
+            }
             _ => {
                 // Sanity check for `can_use_in_debuginfo`.
                 debug_assert!(!elem.can_use_in_debuginfo());
@@ -293,6 +318,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                     dbg_var,
                     fragment: None,
                     projection: ty::List::empty(),
+                    references: 0,
                 })
             }
         } else {
@@ -358,55 +384,74 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         let vars = vars.iter().cloned().chain(fallback_var);
 
         for var in vars {
-            let Some(dbg_var) = var.dbg_var else { continue };
-            let Some(dbg_loc) = self.dbg_loc(var.source_info) else { continue };
-
-            let DebugInfoOffset { direct_offset, indirect_offsets, result: _ } =
-                calculate_debuginfo_offset(bx, local, &var, base.layout);
-
-            // When targeting MSVC, create extra allocas for arguments instead of pointing multiple
-            // dbg_var_addr() calls into the same alloca with offsets. MSVC uses CodeView records
-            // not DWARF and LLVM doesn't support translating the resulting
-            // [DW_OP_deref, DW_OP_plus_uconst, offset, DW_OP_deref] debug info to CodeView.
-            // Creating extra allocas on the stack makes the resulting debug info simple enough
-            // that LLVM can generate correct CodeView records and thus the values appear in the
-            // debugger. (#83709)
-            let should_create_individual_allocas = bx.cx().sess().target.is_like_msvc
-                && self.mir.local_kind(local) == mir::LocalKind::Arg
-                // LLVM can handle simple things but anything more complex than just a direct
-                // offset or one indirect offset of 0 is too complex for it to generate CV records
-                // correctly.
-                && (direct_offset != Size::ZERO
-                    || !matches!(&indirect_offsets[..], [Size::ZERO] | []));
-
-            if should_create_individual_allocas {
-                let DebugInfoOffset { direct_offset: _, indirect_offsets: _, result: place } =
-                    calculate_debuginfo_offset(bx, local, &var, base);
-
-                // Create a variable which will be a pointer to the actual value
-                let ptr_ty = bx
-                    .tcx()
-                    .mk_ptr(ty::TypeAndMut { mutbl: mir::Mutability::Mut, ty: place.layout.ty });
-                let ptr_layout = bx.layout_of(ptr_ty);
-                let alloca = PlaceRef::alloca(bx, ptr_layout);
-                bx.set_var_name(alloca.llval, &(var.name.to_string() + ".dbg.spill"));
-
-                // Write the pointer to the variable
-                bx.store(place.llval, alloca.llval, alloca.align);
-
-                // Point the debug info to `*alloca` for the current variable
-                bx.dbg_var_addr(dbg_var, dbg_loc, alloca.llval, Size::ZERO, &[Size::ZERO], None);
-            } else {
-                bx.dbg_var_addr(
-                    dbg_var,
-                    dbg_loc,
-                    base.llval,
-                    direct_offset,
-                    &indirect_offsets,
-                    None,
-                );
+            self.debug_introduce_local_as_var(bx, local, base, var);
+        }
+    }
+
+    fn debug_introduce_local_as_var(
+        &self,
+        bx: &mut Bx,
+        local: mir::Local,
+        mut base: PlaceRef<'tcx, Bx::Value>,
+        var: PerLocalVarDebugInfo<'tcx, Bx::DIVariable>,
+    ) {
+        let Some(dbg_var) = var.dbg_var else { return };
+        let Some(dbg_loc) = self.dbg_loc(var.source_info) else { return };
+
+        let DebugInfoOffset { mut direct_offset, indirect_offsets, result: _ } =
+            calculate_debuginfo_offset(bx, local, &var, base.layout);
+        let mut indirect_offsets = &indirect_offsets[..];
+
+        // When targeting MSVC, create extra allocas for arguments instead of pointing multiple
+        // dbg_var_addr() calls into the same alloca with offsets. MSVC uses CodeView records
+        // not DWARF and LLVM doesn't support translating the resulting
+        // [DW_OP_deref, DW_OP_plus_uconst, offset, DW_OP_deref] debug info to CodeView.
+        // Creating extra allocas on the stack makes the resulting debug info simple enough
+        // that LLVM can generate correct CodeView records and thus the values appear in the
+        // debugger. (#83709)
+        let should_create_individual_allocas = bx.cx().sess().target.is_like_msvc
+            && self.mir.local_kind(local) == mir::LocalKind::Arg
+            // LLVM can handle simple things but anything more complex than just a direct
+            // offset or one indirect offset of 0 is too complex for it to generate CV records
+            // correctly.
+            && (direct_offset != Size::ZERO || !matches!(indirect_offsets, [Size::ZERO] | []));
+
+        let create_alloca = |bx: &mut Bx, place: PlaceRef<'tcx, Bx::Value>, refcount| {
+            // Create a variable which will be a pointer to the actual value
+            let ptr_ty = bx
+                .tcx()
+                .mk_ptr(ty::TypeAndMut { mutbl: mir::Mutability::Mut, ty: place.layout.ty });
+            let ptr_layout = bx.layout_of(ptr_ty);
+            let alloca = PlaceRef::alloca(bx, ptr_layout);
+            bx.set_var_name(alloca.llval, &format!("{}.ref{}.dbg.spill", var.name, refcount));
+
+            // Write the pointer to the variable
+            bx.store(place.llval, alloca.llval, alloca.align);
+
+            // Point the debug info to `*alloca` for the current variable
+            alloca
+        };
+
+        if var.references > 0 {
+            base = calculate_debuginfo_offset(bx, local, &var, base).result;
+
+            // Point the debug info to `&...&base == alloca` for the current variable
+            for refcount in 0..var.references {
+                base = create_alloca(bx, base, refcount);
             }
+
+            direct_offset = Size::ZERO;
+            indirect_offsets = &[];
+        } else if should_create_individual_allocas {
+            let place = calculate_debuginfo_offset(bx, local, &var, base).result;
+
+            // Point the debug info to `*alloca` for the current variable
+            base = create_alloca(bx, place, 0);
+            direct_offset = Size::ZERO;
+            indirect_offsets = &[Size::ZERO];
         }
+
+        bx.dbg_var_addr(dbg_var, dbg_loc, base.llval, direct_offset, indirect_offsets, None);
     }
 
     pub fn debug_introduce_locals(&self, bx: &mut Bx) {
@@ -439,7 +484,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
             };
 
             let dbg_var = dbg_scope_and_span.map(|(dbg_scope, _, span)| {
-                let (var_ty, var_kind) = match var.value {
+                let (mut var_ty, var_kind) = match var.value {
                     mir::VarDebugInfoContents::Place(place) => {
                         let var_ty = self.monomorphized_place_ty(place.as_ref());
                         let var_kind = if let Some(arg_index) = var.argument_index
@@ -476,6 +521,11 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                     }
                 };
 
+                for _ in 0..var.references {
+                    var_ty =
+                        bx.tcx().mk_ptr(ty::TypeAndMut { mutbl: mir::Mutability::Mut, ty: var_ty });
+                }
+
                 self.cx.create_dbg_var(var.name, var_ty, dbg_scope, var_kind, span)
             });
 
@@ -487,6 +537,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                         dbg_var,
                         fragment: None,
                         projection: place.projection,
+                        references: var.references,
                     });
                 }
                 mir::VarDebugInfoContents::Const(c) => {
@@ -540,6 +591,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                                 Some(fragment_start..fragment_start + fragment_layout.size)
                             },
                             projection: place.projection,
+                            references: var.references,
                         });
                     }
                 }

compiler/rustc_const_eval/src/transform/validate.rs

@@ -448,7 +448,15 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
         };
         match debuginfo.value {
             VarDebugInfoContents::Const(_) => {}
-            VarDebugInfoContents::Place(place) => check_place(place),
+            VarDebugInfoContents::Place(place) => {
+                check_place(place);
+                if debuginfo.references != 0 && place.projection.last() == Some(&PlaceElem::Deref) {
+                    self.fail(
+                        START_BLOCK.start_location(),
+                        format!("debuginfo {:?}, has both ref and deref", debuginfo),
+                    );
+                }
+            }
             VarDebugInfoContents::Composite { ty, ref fragments } => {
                 for f in fragments {
                     check_place(f.contents);

compiler/rustc_middle/src/mir/mod.rs

@@ -1111,6 +1111,10 @@ pub struct VarDebugInfo<'tcx> {
     /// originated from (starting from 1). Note, if MIR inlining is enabled, then this is the
     /// argument number in the original function before it was inlined.
     pub argument_index: Option<u16>,
+
+    /// The data represents `name` dereferenced `references` times,
+    /// and not the direct value.
+    pub references: u8,
 }
 
 ///////////////////////////////////////////////////////////////////////////
@@ -1550,8 +1554,11 @@ impl<V, T> ProjectionElem<V, T> {
     /// Returns `true` if this is accepted inside `VarDebugInfoContents::Place`.
     pub fn can_use_in_debuginfo(&self) -> bool {
         match self {
-            Self::Deref | Self::Downcast(_, _) | Self::Field(_, _) => true,
-            Self::ConstantIndex { .. }
+            Self::ConstantIndex { from_end: false, .. }
+            | Self::Deref
+            | Self::Downcast(_, _)
+            | Self::Field(_, _) => true,
+            Self::ConstantIndex { from_end: true, .. }
             | Self::Index(_)
             | Self::OpaqueCast(_)
             | Self::Subslice { .. } => false,
@@ -1639,18 +1646,7 @@ impl<'tcx> Place<'tcx> {
             return self;
         }
 
-        let mut v: Vec<PlaceElem<'tcx>>;
-
-        let new_projections = if self.projection.is_empty() {
-            more_projections
-        } else {
-            v = Vec::with_capacity(self.projection.len() + more_projections.len());
-            v.extend(self.projection);
-            v.extend(more_projections);
-            &v
-        };
-
-        Place { local: self.local, projection: tcx.mk_place_elems(new_projections) }
+        self.as_ref().project_deeper(more_projections, tcx)
     }
 }
 
@@ -1721,6 +1717,27 @@ impl<'tcx> PlaceRef<'tcx> {
             (base, *proj)
         })
     }
+
+    /// Generates a new place by appending `more_projections` to the existing ones
+    /// and interning the result.
+    pub fn project_deeper(
+        self,
+        more_projections: &[PlaceElem<'tcx>],
+        tcx: TyCtxt<'tcx>,
+    ) -> Place<'tcx> {
+        let mut v: Vec<PlaceElem<'tcx>>;
+
+        let new_projections = if self.projection.is_empty() {
+            more_projections
+        } else {
+            v = Vec::with_capacity(self.projection.len() + more_projections.len());
+            v.extend(self.projection);
+            v.extend(more_projections);
+            &v
+        };
+
+        Place { local: self.local, projection: tcx.mk_place_elems(new_projections) }
+    }
 }
 
 impl Debug for Place<'_> {

compiler/rustc_middle/src/mir/pretty.rs

@@ -551,8 +551,13 @@ fn write_scope_tree(
         }
 
         let indented_debug_info = format!(
-            "{0:1$}debug {2} => {3:?};",
-            INDENT, indent, var_debug_info.name, var_debug_info.value,
+            "{0:1$}debug {2} => {3:&<4$}{5:?};",
+            INDENT,
+            indent,
+            var_debug_info.name,
+            "",
+            var_debug_info.references as usize,
+            var_debug_info.value,
         );
 
         writeln!(

compiler/rustc_middle/src/mir/visit.rs

@@ -842,6 +842,7 @@ macro_rules! make_mir_visitor {
                     source_info,
                     value,
                     argument_index: _,
+                    references: _,
                 } = var_debug_info;
 
                 self.visit_source_info(source_info);

compiler/rustc_middle/src/ty/structural_impls.rs

@@ -204,6 +204,7 @@ CloneLiftImpls! {
     (),
     bool,
     usize,
+    u8,
     u16,
     u32,
     u64,

compiler/rustc_mir_build/src/build/matches/mod.rs

@@ -2241,6 +2241,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         self.var_debug_info.push(VarDebugInfo {
             name,
             source_info: debug_source_info,
+            references: 0,
             value: VarDebugInfoContents::Place(for_arm_body.into()),
             argument_index: None,
         });
@@ -2260,6 +2261,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             self.var_debug_info.push(VarDebugInfo {
                 name,
                 source_info: debug_source_info,
+                references: 0,
                 value: VarDebugInfoContents::Place(ref_for_guard.into()),
                 argument_index: None,
             });

compiler/rustc_mir_build/src/build/mod.rs

@@ -798,6 +798,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 };
                 self.var_debug_info.push(VarDebugInfo {
                     name,
+                    references: 0,
                     source_info: SourceInfo::outermost(captured_place.var_ident.span),
                     value: VarDebugInfoContents::Place(use_place),
                     argument_index: None,
@@ -828,6 +829,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 self.var_debug_info.push(VarDebugInfo {
                     name,
                     source_info,
+                    references: 0,
                     value: VarDebugInfoContents::Place(arg_local.into()),
                     argument_index: Some(argument_index as u16 + 1),
                 });