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Auto merge of #68961 - eddyb:dbg-stack-dunk, r=nagisa

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rustc_codegen_ssa: only "spill" SSA-like values to the stack for debuginfo.

This is an implementation of the idea described in https://github.com/rust-lang/rust/issues/68817#issuecomment-583719182.

In short, instead of debuginfo forcing otherwise-SSA-like MIR locals into `alloca`s, and requiring a `load` for each use (or two, for scalar pairs), the `alloca` is now *only* used for attaching debuginfo with `llvm.dbg.declare`: the `OperandRef` is stored to the `alloca`, but *never loaded* from it.

Outside of `debug_introduce_local`, nothing cares about the debuginfo-only `alloca`, and instead works with `OperandRef` the same as MIR locals without debuginfo before this PR.

This should have some of the benefits of `llvm.dbg.value`, while working today.

cc @nagisa @nikomatsakis
This commit is contained in:
bors 2020-02-11 07:36:59 +00:00
commit b6690a8c35
5 changed files with 41 additions and 38 deletions
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19
src/librustc_codegen_ssa/mir/analyze.rs
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@ -8,7 +8,6 @@ use rustc::mir::visit::{
MutatingUseContext, NonMutatingUseContext, NonUseContext, PlaceContext, Visitor,
};
use rustc::mir::{self, Location, TerminatorKind};
use rustc::session::config::DebugInfo;
use rustc::ty;
use rustc::ty::layout::{HasTyCtxt, LayoutOf};
use rustc_data_structures::graph::dominators::Dominators;
@ -24,15 +23,6 @@ pub fn non_ssa_locals<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
analyzer.visit_body(mir);
for (local, decl) in mir.local_decls.iter_enumerated() {
// FIXME(eddyb): We should figure out how to use llvm.dbg.value instead
// of putting everything in allocas just so we can use llvm.dbg.declare.
if fx.cx.sess().opts.debuginfo == DebugInfo::Full {
if fx.mir.local_kind(local) == mir::LocalKind::Arg {
analyzer.not_ssa(local);
continue;
}
}
let ty = fx.monomorphize(&decl.ty);
debug!("local {:?} has type `{}`", local, ty);
let layout = fx.cx.spanned_layout_of(ty, decl.source_info.span);
@ -281,15 +271,6 @@ impl<'mir, 'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> Visitor<'tcx>
self.assign(local, location);
}
PlaceContext::NonUse(NonUseContext::VarDebugInfo) => {
// We need to keep locals in `alloca`s for debuginfo.
// FIXME(eddyb): We should figure out how to use `llvm.dbg.value` instead
// of putting everything in allocas just so we can use `llvm.dbg.declare`.
if self.fx.cx.sess().opts.debuginfo == DebugInfo::Full {
self.not_ssa(local);
}
}
PlaceContext::NonUse(_) | PlaceContext::MutatingUse(MutatingUseContext::Retag) => {}
PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy)

3
src/librustc_codegen_ssa/mir/block.rs
View File

@ -1145,6 +1145,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
let op = bx.load_operand(place);
place.storage_dead(bx);
self.locals[index] = LocalRef::Operand(Some(op));
self.debug_introduce_local(bx, index);
}
LocalRef::Operand(Some(op)) => {
assert!(op.layout.is_zst(), "assigning to initialized SSAtemp");
@ -1186,6 +1187,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
let op = bx.load_operand(tmp);
tmp.storage_dead(bx);
self.locals[index] = LocalRef::Operand(Some(op));
self.debug_introduce_local(bx, index);
}
DirectOperand(index) => {
// If there is a cast, we have to store and reload.
@ -1200,6 +1202,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
OperandRef::from_immediate_or_packed_pair(bx, llval, ret_abi.layout)
};
self.locals[index] = LocalRef::Operand(Some(op));
self.debug_introduce_local(bx, index);
}
}
}

47
src/librustc_codegen_ssa/mir/debuginfo.rs
View File

@ -9,7 +9,8 @@ use rustc_index::vec::IndexVec;
use rustc_span::symbol::{kw, Symbol};
use rustc_span::{BytePos, Span};
use super::OperandValue;
use super::operand::OperandValue;
use super::place::PlaceRef;
use super::{FunctionCx, LocalRef};
pub struct FunctionDebugContext<D> {
@ -111,8 +112,6 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
/// Apply debuginfo and/or name, after creating the `alloca` for a local,
/// or initializing the local with an operand (whichever applies).
// FIXME(eddyb) use `llvm.dbg.value` (which would work for operands),
// not just `llvm.dbg.declare` (which requires `alloca`).
pub fn debug_introduce_local(&self, bx: &mut Bx, local: mir::Local) {
let full_debug_info = bx.sess().opts.debuginfo == DebugInfo::Full;
@ -180,38 +179,60 @@ 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 = if bx.sess().fewer_names() {
None
} else {
Some(match whole_local_var.or(fallback_var) {
Some(var) if var.name != kw::Invalid => var.name.to_string(),
_ => format!("{:?}", local),
};
})
};
if let Some(name) = &name {
match local_ref {
LocalRef::Place(place) | LocalRef::UnsizedPlace(place) => {
bx.set_var_name(place.llval, &name);
bx.set_var_name(place.llval, name);
}
LocalRef::Operand(Some(operand)) => match operand.val {
OperandValue::Ref(x, ..) | OperandValue::Immediate(x) => {
bx.set_var_name(x, &name);
bx.set_var_name(x, name);
}
OperandValue::Pair(a, b) => {
// FIXME(eddyb) these are scalar components,
// maybe extract the high-level fields?
bx.set_var_name(a, &(name.clone() + ".0"));
bx.set_var_name(b, &(name + ".1"));
bx.set_var_name(b, &(name.clone() + ".1"));
}
},
LocalRef::Operand(None) => {}
}
}
if !full_debug_info {
if !full_debug_info || vars.is_empty() && fallback_var.is_none() {
return;
}
// FIXME(eddyb) add debuginfo for unsized places too.
let base = match local_ref {
LocalRef::Place(place) => place,
_ => return,
LocalRef::Operand(None) => return,
LocalRef::Operand(Some(operand)) => {
// "Spill" the value onto the stack, for debuginfo,
// without forcing non-debuginfo uses of the local
// to also load from the stack every single time.
// FIXME(#68817) use `llvm.dbg.value` instead,
// at least for the cases which LLVM handles correctly.
let spill_slot = PlaceRef::alloca(bx, operand.layout);
if let Some(name) = name {
bx.set_var_name(spill_slot.llval, &(name + ".dbg.spill"));
}
operand.val.store(bx, spill_slot);
spill_slot
}
LocalRef::Place(place) => *place,
// FIXME(eddyb) add debuginfo for unsized places too.
LocalRef::UnsizedPlace(_) => return,
};
let vars = vars.iter().copied().chain(fallback_var);

8
src/test/codegen/naked-functions.rs
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@ -1,5 +1,3 @@
// ignore-tidy-linelength
// compile-flags: -C no-prepopulate-passes
#![crate_type = "lib"]
@ -61,19 +59,19 @@ pub fn naked_recursive() {
naked_empty();
// CHECK-NEXT: %{{[0-9]+}} = call i{{[0-9]+}} @naked_with_return()
// CHECK-NEXT: %_4 = call i{{[0-9]+}} @naked_with_return()
// FIXME(#39685) Avoid one block per call.
// CHECK-NEXT: br label %bb2
// CHECK: bb2:
// CHECK-NEXT: %{{[0-9]+}} = call i{{[0-9]+}} @naked_with_args_and_return(i{{[0-9]+}} %{{[0-9]+}})
// CHECK-NEXT: %_3 = call i{{[0-9]+}} @naked_with_args_and_return(i{{[0-9]+}} %_4)
// FIXME(#39685) Avoid one block per call.
// CHECK-NEXT: br label %bb3
// CHECK: bb3:
// CHECK-NEXT: call void @naked_with_args(i{{[0-9]+}} %{{[0-9]+}})
// CHECK-NEXT: call void @naked_with_args(i{{[0-9]+}} %_3)
// FIXME(#39685) Avoid one block per call.
// CHECK-NEXT: br label %bb4

2
src/test/debuginfo/issue-12886.rs
View File

@ -6,7 +6,7 @@
// gdb-command:run
// gdb-command:next
// gdb-check:[...]25[...]s
// gdb-check:[...]24[...]let s = Some(5).unwrap(); // #break
// gdb-command:continue
#![feature(omit_gdb_pretty_printer_section)]