gccrs: Add straight-line BIR drop style analysis by Lishin1215 · Pull Request #4730 · Rust-GCC/gccrs

Lishin1215 · 2026-07-30T14:44:51Z

This patch adds an initial BIR Drop state analysis based on rustc's MIR
drop scheduling and Drop style classification.

It contains two commits:

commit 1
Fixes an incorrect BIR IndexVec size that could cause an out-of-bounds
access when dumping BIR.

commit 2
Adds basic Drop state analysis for whole local variables in straight-line control flow.
It adds Drop statements at scope exits and marks them as Static or Dead.

It covers these straight-line cases:

static local:

let x = Droppable;

Drop(x): Static

whole-local move:

let x = Droppable;
let y = x;

Drop(y): Static
Drop(x): Dead

Copy assignment:

let x = 1;
let y = x;

Drop(y): Static
Drop(x): Static

Function argument:

fn f(x: Droppable) {}

Drop(x): Static

Current scope

This patch supports:

straight-line functions without branches or loops
whole-local moves
copy versus move assignments
function arguments
Static and Dead Drop styles

The following are left for follow-up patches:

branches and conditional Drops
loops
partial moves
needs_drop filtering
backend integration

BIR Drop statements are currently emitted for all locals as analysis points.
Filtering them using needs_drop will be added separately.

Create the requested number of elements in the IndexVec constructor instead of only reserving space. This fixes an out-of-bounds access when dumping BIR. gcc/rust/ChangeLog: * checks/errors/borrowck/rust-bir-place.h (IndexVec::IndexVec): Create the requested number of elements. Signed-off-by: Lishin <lishin1008@gmail.com>

Add BIR Drop statements at scope exits and classify them as static or dead by tracking whether each local is initialized or moved. Treat function arguments as initialized at function entry and schedule their value drops before returning. Arguments do not receive StorageLive or StorageDead statements. Dump the classification and add tests for static local, whole-local move, a copy, and a function argument. gcc/rust/ChangeLog: * Make-lang.in: Add rust-bir-drop-analysis.o. * checks/errors/borrowck/rust-bir.h: (Statement::DropStyle): New enum. (Statement::Kind): Add DROP. (Statement::make_drop): New function. (Statement::get_drop_style): Likewise. (Statement::set_drop_style): Likewise. * checks/errors/borrowck/rust-bir-builder-internal.h: (AbstractBuilder::declare_argument): New function. (AbstractBuilder::push_drop): Likewise. (AbstractBuilder::push_function_argument_drops): Likewise. (AbstractBuilder::pop_scope): Schedule Drop statements. (AbstractBuilder::unwind_until): Likewise. (AbstractBuilder::push_return): Schedule function argument drops. * checks/errors/borrowck/rust-bir-builder.h (Builder::handle_param): Use declare_argument for function parameters. * checks/errors/borrowck/rust-bir-dump.cc (Dump::visit): Dump Drop classifications. * checks/errors/borrowck/rust-bir-fact-collector.h (FactCollector::visit): Handle Drop statements. * checks/errors/borrowck/rust-borrow-checker.cc (BorrowChecker::go): Run BIR Drop analysis. * checks/errors/borrowck/rust-bir-drop-analysis.cc: New file. * checks/errors/borrowck/rust-bir-drop-analysis.h: New file. gcc/testsuite/ChangeLog: * rust/borrowck/drop_analysis_whole_move.rs: New test. Signed-off-by: Lishin <lishin1008@gmail.com>

Lishin1215 · 2026-07-30T15:19:16Z

+      if (block.successors.empty ())
+	break;
+
+      if (block.successors.size () != 1)


I limit this first version to straight-line control flow.
If the CFG contains branches or loops, the analysis returns and leaves the Drops unclassified.

Lishin1215 · 2026-07-30T15:28:49Z

+		if (expr.get_kind () == ExprKind::ASSIGNMENT)
+		  {
+		    PlaceId rhs = static_cast<Assignment &> (expr).get_rhs ();
+		    const Place &rhs_place = function.place_db[rhs];
+
+		    if (rhs_place.kind == Place::VARIABLE
+			&& rhs_place.should_be_moved ())
+		      initialized[rhs.value] = false;
+		  }
+
+		initialized[lhs.value] = true;


I mark the lhs as initialized after an assignment.
I also mark a non-Copy variable rhs as uninitialized because its value was moved, and leave a copy assignment rhs initialized.

This first version only handles direct whole-local moves.

Lishin1215 · 2026-07-30T15:35:01Z

 public:
  IndexVec () = default;
-  IndexVec (size_t size) { internal_vector.reserve (size); }
+  IndexVec (size_t size) : internal_vector (size) {}


I changed the sized constructor to create the requested number of elements.
The old constructor only reserved capacity, so direct indexing during the BIR dump could access an empty vector.

I would add a comment on this line. This vector ctor is a bit misleading/tricky (many people think this only reserves the space) and it will either turn a warning in reader's mind or be misused. We need to be explicit about the elements default construction.

Thanks! I added a comment to make that clear.

Lishin1215 · 2026-07-30T15:55:32Z

+  for (PlaceId argument : function.arguments)
+    initialized[argument.value] = true;


I handle function argument Drops separately because arguments are already
initialized at function entry and do not use the normal local StorageLive
and StorageDead handling.
I emit their Drops in reverse declaration order before the function returns.

P-E-P

I'm not entirely familiar with BIR but this looks great. I've put a few suggestions.

P-E-P · 2026-08-11T14:33:27Z

 public:
  IndexVec () = default;
-  IndexVec (size_t size) { internal_vector.reserve (size); }
+  IndexVec (size_t size) : internal_vector (size) {}


I would add a comment on this line. This vector ctor is a bit misleading/tricky (many people think this only reserves the space) and it will either turn a warning in reader's mind or be misused. We need to be explicit about the elements default construction.

gcc/rust/ChangeLog: * checks/errors/borrowck/rust-bir-drop-analysis.cc (BasicBlockIdHash): New helper. (DropAnalysis::analyze): Use unordered_set for visited block tracking. * checks/errors/borrowck/rust-bir-drop-analysis.h: Include rust-bir.h directly and remove Function forward declaration. * checks/errors/borrowck/rust-bir-place.h (IndexVec::IndexVec): Add comment for sized constructor. Signed-off-by: Lishin <lishin1008@gmail.com>

P-E-P

LGTM

Lishin1215 added 2 commits July 25, 2026 16:30

Lishin1215 commented Jul 30, 2026

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Lishin1215 changed the title ~~gccrs: Add straight-line BIR drop state analysis~~ gccrs: Add straight-line BIR drop style analysis Aug 1, 2026

P-E-P self-requested a review August 11, 2026 12:18

P-E-P requested changes Aug 11, 2026

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P-E-P approved these changes Aug 13, 2026

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P-E-P added this pull request to the merge queue Aug 13, 2026

Merged via the queue into Rust-GCC:master with commit 7a3c1de Aug 13, 2026
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gccrs: Add straight-line BIR drop style analysis - #4730

gccrs: Add straight-line BIR drop style analysis#4730
P-E-P merged 3 commits into
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Lishin1215:bir-whole-local-drop-analysis

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		for (PlaceId argument : function.arguments)
		initialized[argument.value] = true;

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