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Fix closed over variables not available in debuginfo for Windows MSVC
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The issue was that the resulting debuginfo was too complex for LLVM to
translate into CodeView records correctly. As a result, it simply
ignored the debuginfo which meant Windows debuggers could not display
any closed over variables when stepping inside a closure.

This fixes that by spilling additional variables to the stack so that
the resulting debuginfo is simple (just `*my_variable.dbg.spill`) and
LLVM can generate the correct CV records.
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@wesleywiser
wesleywiser committed Apr 8, 2021
1 parent e9cdccc commit 533002d
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Showing 4 changed files with 142 additions and 15 deletions.
54 changes: 39 additions & 15 deletions compiler/rustc_codegen_ssa/src/mir/debuginfo.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -6,7 +6,7 @@ use rustc_middle::ty;
use rustc_session::config::DebugInfo;
use rustc_span::symbol::{kw, Symbol};
use rustc_span::{BytePos, Span};
use rustc_target::abi::{LayoutOf, Size};
use rustc_target::abi::Size;

use super::operand::{OperandRef, OperandValue};
use super::place::PlaceRef;
Expand Down Expand Up @@ -265,33 +265,25 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
None => continue,
};

let mut layout = base.layout;
let mut direct_offset = Size::ZERO;
// FIXME(eddyb) use smallvec here.
let mut indirect_offsets = vec![];
let mut place = base;

for elem in &var.projection[..] {
match *elem {
mir::ProjectionElem::Deref => {
indirect_offsets.push(Size::ZERO);
layout = bx.cx().layout_of(
layout
.ty
.builtin_deref(true)
.unwrap_or_else(|| {
span_bug!(var.source_info.span, "cannot deref `{}`", layout.ty)
})
.ty,
);
place = place.project_deref(bx);
}
mir::ProjectionElem::Field(field, _) => {
let i = field.index();
let offset = indirect_offsets.last_mut().unwrap_or(&mut direct_offset);
*offset += layout.fields.offset(i);
layout = layout.field(bx.cx(), i);
*offset += place.layout.fields.offset(i);
place = place.project_field(bx, i);
}
mir::ProjectionElem::Downcast(_, variant) => {
layout = layout.for_variant(bx.cx(), variant);
place = place.project_downcast(bx, variant);
}
_ => span_bug!(
var.source_info.span,
Expand All @@ -301,7 +293,39 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
}
}

bx.dbg_var_addr(dbg_var, dbg_loc, base.llval, direct_offset, &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] | []));

if should_create_individual_allocas {
// Create a variable which will be a pointer to the actual value
let ptr_ty = bx.tcx().mk_ty(ty::RawPtr(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]);
} else {
bx.dbg_var_addr(dbg_var, dbg_loc, base.llval, direct_offset, &indirect_offsets);
}
}
}

Expand Down
12 changes: 12 additions & 0 deletions compiler/rustc_codegen_ssa/src/mir/place.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -402,6 +402,18 @@ impl<'a, 'tcx, V: CodegenObject> PlaceRef<'tcx, V> {
downcast
}

pub fn project_deref<Bx: BuilderMethods<'a, 'tcx, Value = V>>(&self, bx: &mut Bx) -> Self {
let target_ty = self.layout.ty.builtin_deref(true).expect("failed to deref");
let layout = bx.layout_of(target_ty.ty);

PlaceRef {
llval: bx.load(self.llval, self.align),
llextra: None,
layout,
align: layout.align.abi,
}
}

pub fn storage_live<Bx: BuilderMethods<'a, 'tcx, Value = V>>(&self, bx: &mut Bx) {
bx.lifetime_start(self.llval, self.layout.size);
}
Expand Down
49 changes: 49 additions & 0 deletions src/test/debuginfo/var-captured-in-nested-closure.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -83,6 +83,55 @@
// lldbr-check:(isize) closure_local = 8
// lldb-command:continue


// === CDB TESTS ===================================================================================

// cdb-command: g

// cdb-command: dx variable
// cdb-check:variable : 1 [Type: [...]]
// cdb-command: dx constant
// cdb-check:constant : 2 [Type: [...]]
// cdb-command: dx a_struct
// cdb-check:a_struct [Type: var_captured_in_nested_closure::Struct]
// cdb-check: [+0x[...]] a : -3 [Type: [...]]
// cdb-check: [+0x[...]] b : 4.500000 [Type: [...]]
// cdb-check: [+0x[...]] c : 0x5 [Type: unsigned [...]]
// cdb-command: dx struct_ref
// cdb-check:struct_ref : 0x[...] [Type: var_captured_in_nested_closure::Struct *]
// cdb-check: [+0x[...]] a : -3 [Type: [...]]
// cdb-check: [+0x[...]] b : 4.500000 [Type: [...]]
// cdb-check: [+0x[...]] c : 0x5 [Type: unsigned [...]]
// cdb-command: dx owned
// cdb-check:owned : 0x[...] : 6 [Type: [...] *]
// cdb-check: 6 [Type: [...]]
// cdb-command: dx closure_local
// cdb-check:closure_local : 8 [Type: [...]]
// cdb-command: dx nested_closure
// cdb-check:nested_closure [Type: var_captured_in_nested_closure::main::{{closure}}::closure-0]

// cdb-command: g

// cdb-command: dx variable
// cdb-check:variable : 1 [Type: [...]]
// cdb-command: dx constant
// cdb-check:constant : 2 [Type: [...]]
// cdb-command: dx a_struct
// cdb-check:a_struct [Type: var_captured_in_nested_closure::Struct]
// cdb-check: [+0x[...]] a : -3 [Type: [...]]
// cdb-check: [+0x[...]] b : 4.500000 [Type: [...]]
// cdb-check: [+0x[...]] c : 0x5 [Type: unsigned [...]]
// cdb-command: dx struct_ref
// cdb-check:struct_ref : 0x[...] [Type: var_captured_in_nested_closure::Struct *]
// cdb-check: [+0x[...]] a : -3 [Type: [...]]
// cdb-check: [+0x[...]] b : 4.500000 [Type: [...]]
// cdb-check: [+0x[...]] c : 0x5 [Type: unsigned [...]]
// cdb-command: dx owned
// cdb-check:owned : 0x[...] : 6 [Type: [...] *]
// cdb-check: 6 [Type: [...]]
// cdb-command: dx closure_local
// cdb-check:closure_local : 8 [Type: [...]]

#![allow(unused_variables)]
#![feature(box_syntax)]
#![feature(omit_gdb_pretty_printer_section)]
Expand Down
42 changes: 42 additions & 0 deletions src/test/debuginfo/var-captured-in-stack-closure.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -73,6 +73,48 @@
// lldbg-check:[...]$9 = 6
// lldbr-check:(isize) *owned = 6


// === CDB TESTS ===================================================================================

// cdb-command: g

// cdb-command: dx variable
// cdb-check:variable : 1 [Type: [...]]
// cdb-command: dx constant
// cdb-check:constant : 2 [Type: [...]]
// cdb-command: dx a_struct
// cdb-check:a_struct [Type: var_captured_in_stack_closure::Struct]
// cdb-check: [+0x[...]] a : -3 [Type: [...]]
// cdb-check: [+0x[...]] b : 4.500000 [Type: [...]]
// cdb-check: [+0x[...]] c : 0x5 [Type: unsigned [...]]
// cdb-command: dx struct_ref
// cdb-check:struct_ref : 0x[...] [Type: var_captured_in_stack_closure::Struct *]
// cdb-check: [+0x[...]] a : -3 [Type: [...]]
// cdb-check: [+0x[...]] b : 4.500000 [Type: [...]]
// cdb-check: [+0x[...]] c : 0x5 [Type: unsigned [...]]
// cdb-command: dx owned
// cdb-check:owned : 0x[...] : 6 [Type: [...] *]


// cdb-command: g

// cdb-command: dx variable
// cdb-check:variable : 2 [Type: [...]]
// cdb-command: dx constant
// cdb-check:constant : 2 [Type: [...]]
// cdb-command: dx a_struct
// cdb-check:a_struct [Type: var_captured_in_stack_closure::Struct]
// cdb-check: [+0x[...]] a : -3 [Type: [...]]
// cdb-check: [+0x[...]] b : 4.500000 [Type: [...]]
// cdb-check: [+0x[...]] c : 0x5 [Type: unsigned [...]]
// cdb-command: dx struct_ref
// cdb-check:struct_ref : 0x[...] [Type: var_captured_in_stack_closure::Struct *]
// cdb-check: [+0x[...]] a : -3 [Type: [...]]
// cdb-check: [+0x[...]] b : 4.500000 [Type: [...]]
// cdb-check: [+0x[...]] c : 0x5 [Type: unsigned [...]]
// cdb-command: dx owned
// cdb-check:owned : 0x[...] : 6 [Type: [...] *]

#![feature(box_syntax)]
#![allow(unused_variables)]
#![feature(omit_gdb_pretty_printer_section)]
Expand Down

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