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

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Allow MIR debuginfo to point to a variable's address

MIR optimizations currently do not to operate on borrowed locals.

When enabling #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.
This commit is contained in:
bors 2023-05-14 05:31:10 +00:00
commit bc888958c9
26 changed files with 1537 additions and 653 deletions
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152
compiler/rustc_codegen_ssa/src/mir/debuginfo.rs
View file

@ -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 {
@ -358,57 +384,76 @@ 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) {
if bx.sess().opts.debuginfo == DebugInfo::Full || !bx.sess().fewer_names() {
for local in self.locals.indices() {
@ -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,
});
}
}