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Auto merge of #72205 - ecstatic-morse:nrvo, r=oli-obk
Dumb NRVO This is a very simple version of an NRVO pass, which scans backwards from the `return` terminator to see if there is an an assignment like `_0 = _1`. If a basic block with two or more predecessors is encountered during this scan without first seeing an assignment to the return place, we bail out. This avoids running a full "reaching definitions" dataflow analysis. I wanted to see how much `rustc` would benefit from even a very limited version of this optimization. We should be able to use this as a point of comparison for more advanced versions that are based on live ranges. r? @ghost
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,238 @@ | ||
| use rustc_hir::Mutability; | ||
| use rustc_index::bit_set::HybridBitSet; | ||
| use rustc_middle::mir::visit::{MutVisitor, PlaceContext, Visitor}; | ||
| use rustc_middle::mir::{self, BasicBlock, Local, Location}; | ||
| use rustc_middle::ty::TyCtxt; | ||
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| use crate::transform::{MirPass, MirSource}; | ||
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| /// This pass looks for MIR that always copies the same local into the return place and eliminates | ||
| /// the copy by renaming all uses of that local to `_0`. | ||
| /// | ||
| /// This allows LLVM to perform an optimization similar to the named return value optimization | ||
| /// (NRVO) that is guaranteed in C++. This avoids a stack allocation and `memcpy` for the | ||
| /// relatively common pattern of allocating a buffer on the stack, mutating it, and returning it by | ||
| /// value like so: | ||
| /// | ||
| /// ```rust | ||
| /// fn foo(init: fn(&mut [u8; 1024])) -> [u8; 1024] { | ||
| /// let mut buf = [0; 1024]; | ||
| /// init(&mut buf); | ||
| /// buf | ||
| /// } | ||
| /// ``` | ||
| /// | ||
| /// For now, this pass is very simple and only capable of eliminating a single copy. A more general | ||
| /// version of copy propagation, such as the one based on non-overlapping live ranges in [#47954] and | ||
| /// [#71003], could yield even more benefits. | ||
| /// | ||
| /// [#47954]: https://github.com/rust-lang/rust/pull/47954 | ||
| /// [#71003]: https://github.com/rust-lang/rust/pull/71003 | ||
| pub struct RenameReturnPlace; | ||
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| impl<'tcx> MirPass<'tcx> for RenameReturnPlace { | ||
| fn run_pass(&self, tcx: TyCtxt<'tcx>, src: MirSource<'tcx>, body: &mut mir::Body<'tcx>) { | ||
| if tcx.sess.opts.debugging_opts.mir_opt_level == 0 { | ||
| return; | ||
| } | ||
|
|
||
| let returned_local = match local_eligible_for_nrvo(body) { | ||
| Some(l) => l, | ||
| None => { | ||
| debug!("`{:?}` was ineligible for NRVO", src.def_id()); | ||
| return; | ||
| } | ||
| }; | ||
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| // Sometimes, the return place is assigned a local of a different but coercable type, for | ||
| // example `&T` instead of `&mut T`. Overwriting the `LocalInfo` for the return place would | ||
| // result in it having an incorrect type. Although this doesn't seem to cause a problem in | ||
| // codegen, bail out anyways since it happens so rarely. | ||
| let ret_ty = body.local_decls[mir::RETURN_PLACE].ty; | ||
| let assigned_ty = body.local_decls[returned_local].ty; | ||
| if ret_ty != assigned_ty { | ||
| debug!("`{:?}` was eligible for NRVO but for type mismatch", src.def_id()); | ||
| debug!("typeof(_0) != typeof({:?}); {:?} != {:?}", returned_local, ret_ty, assigned_ty); | ||
| return; | ||
| } | ||
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||
| debug!( | ||
| "`{:?}` was eligible for NRVO, making {:?} the return place", | ||
| src.def_id(), | ||
| returned_local | ||
| ); | ||
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| RenameToReturnPlace { tcx, to_rename: returned_local }.visit_body(body); | ||
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| // Clean up the `NOP`s we inserted for statements made useless by our renaming. | ||
| for block_data in body.basic_blocks_mut() { | ||
| block_data.statements.retain(|stmt| stmt.kind != mir::StatementKind::Nop); | ||
| } | ||
|
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| // Overwrite the debuginfo of `_0` with that of the renamed local. | ||
| let (renamed_decl, ret_decl) = | ||
| body.local_decls.pick2_mut(returned_local, mir::RETURN_PLACE); | ||
| ret_decl.clone_from(renamed_decl); | ||
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| // The return place is always mutable. | ||
| ret_decl.mutability = Mutability::Mut; | ||
| } | ||
| } | ||
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| /// MIR that is eligible for the NRVO must fulfill two conditions: | ||
| /// 1. The return place must not be read prior to the `Return` terminator. | ||
| /// 2. A simple assignment of a whole local to the return place (e.g., `_0 = _1`) must be the | ||
| /// only definition of the return place reaching the `Return` terminator. | ||
| /// | ||
| /// If the MIR fulfills both these conditions, this function returns the `Local` that is assigned | ||
| /// to the return place along all possible paths through the control-flow graph. | ||
| fn local_eligible_for_nrvo(body: &mut mir::Body<'_>) -> Option<Local> { | ||
| if IsReturnPlaceRead::run(body) { | ||
| return None; | ||
| } | ||
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| let mut copied_to_return_place = None; | ||
| for block in body.basic_blocks().indices() { | ||
| // Look for blocks with a `Return` terminator. | ||
| if !matches!(body[block].terminator().kind, mir::TerminatorKind::Return) { | ||
| continue; | ||
| } | ||
|
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| // Look for an assignment of a single local to the return place prior to the `Return`. | ||
| let returned_local = find_local_assigned_to_return_place(block, body)?; | ||
| match body.local_kind(returned_local) { | ||
| // FIXME: Can we do this for arguments as well? | ||
| mir::LocalKind::Arg => return None, | ||
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| mir::LocalKind::ReturnPointer => bug!("Return place was assigned to itself?"), | ||
| mir::LocalKind::Var | mir::LocalKind::Temp => {} | ||
| } | ||
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| // If multiple different locals are copied to the return place. We can't pick a | ||
| // single one to rename. | ||
| if copied_to_return_place.map_or(false, |old| old != returned_local) { | ||
| return None; | ||
| } | ||
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| copied_to_return_place = Some(returned_local); | ||
| } | ||
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| return copied_to_return_place; | ||
| } | ||
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| fn find_local_assigned_to_return_place( | ||
| start: BasicBlock, | ||
| body: &mut mir::Body<'_>, | ||
| ) -> Option<Local> { | ||
| let mut block = start; | ||
| let mut seen = HybridBitSet::new_empty(body.basic_blocks().len()); | ||
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| // Iterate as long as `block` has exactly one predecessor that we have not yet visited. | ||
| while seen.insert(block) { | ||
| trace!("Looking for assignments to `_0` in {:?}", block); | ||
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| let local = body[block].statements.iter().rev().find_map(as_local_assigned_to_return_place); | ||
| if local.is_some() { | ||
| return local; | ||
| } | ||
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| match body.predecessors()[block].as_slice() { | ||
| &[pred] => block = pred, | ||
| _ => return None, | ||
| } | ||
| } | ||
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| return None; | ||
| } | ||
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| // If this statement is an assignment of an unprojected local to the return place, | ||
| // return that local. | ||
| fn as_local_assigned_to_return_place(stmt: &mir::Statement<'_>) -> Option<Local> { | ||
| if let mir::StatementKind::Assign(box (lhs, rhs)) = &stmt.kind { | ||
| if lhs.as_local() == Some(mir::RETURN_PLACE) { | ||
| if let mir::Rvalue::Use(mir::Operand::Copy(rhs) | mir::Operand::Move(rhs)) = rhs { | ||
| return rhs.as_local(); | ||
| } | ||
| } | ||
| } | ||
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| None | ||
| } | ||
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| struct RenameToReturnPlace<'tcx> { | ||
| to_rename: Local, | ||
| tcx: TyCtxt<'tcx>, | ||
| } | ||
|
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| /// Replaces all uses of `self.to_rename` with `_0`. | ||
| impl MutVisitor<'tcx> for RenameToReturnPlace<'tcx> { | ||
| fn tcx(&self) -> TyCtxt<'tcx> { | ||
| self.tcx | ||
| } | ||
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| fn visit_statement(&mut self, stmt: &mut mir::Statement<'tcx>, loc: Location) { | ||
| // Remove assignments of the local being replaced to the return place, since it is now the | ||
| // return place: | ||
| // _0 = _1 | ||
| if as_local_assigned_to_return_place(stmt) == Some(self.to_rename) { | ||
| stmt.kind = mir::StatementKind::Nop; | ||
| return; | ||
| } | ||
|
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| // Remove storage annotations for the local being replaced: | ||
| // StorageLive(_1) | ||
| if let mir::StatementKind::StorageLive(local) | mir::StatementKind::StorageDead(local) = | ||
| stmt.kind | ||
| { | ||
| if local == self.to_rename { | ||
| stmt.kind = mir::StatementKind::Nop; | ||
| return; | ||
| } | ||
| } | ||
|
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| self.super_statement(stmt, loc) | ||
| } | ||
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| fn visit_terminator(&mut self, terminator: &mut mir::Terminator<'tcx>, loc: Location) { | ||
| // Ignore the implicit "use" of the return place in a `Return` statement. | ||
| if let mir::TerminatorKind::Return = terminator.kind { | ||
| return; | ||
| } | ||
|
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| self.super_terminator(terminator, loc); | ||
| } | ||
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| fn visit_local(&mut self, l: &mut Local, _: PlaceContext, _: Location) { | ||
| assert_ne!(*l, mir::RETURN_PLACE); | ||
| if *l == self.to_rename { | ||
| *l = mir::RETURN_PLACE; | ||
| } | ||
| } | ||
| } | ||
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| struct IsReturnPlaceRead(bool); | ||
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| impl IsReturnPlaceRead { | ||
| fn run(body: &mir::Body<'_>) -> bool { | ||
| let mut vis = IsReturnPlaceRead(false); | ||
| vis.visit_body(body); | ||
| vis.0 | ||
| } | ||
| } | ||
|
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| impl Visitor<'tcx> for IsReturnPlaceRead { | ||
| fn visit_local(&mut self, &l: &Local, ctxt: PlaceContext, _: Location) { | ||
| if l == mir::RETURN_PLACE && ctxt.is_use() && !ctxt.is_place_assignment() { | ||
| self.0 = true; | ||
| } | ||
| } | ||
|
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| fn visit_terminator(&mut self, terminator: &mir::Terminator<'tcx>, loc: Location) { | ||
| // Ignore the implicit "use" of the return place in a `Return` statement. | ||
| if let mir::TerminatorKind::Return = terminator.kind { | ||
| return; | ||
| } | ||
|
|
||
| self.super_terminator(terminator, loc); | ||
| } | ||
| } |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,17 @@ | ||
| // compile-flags: -O | ||
|
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| #![crate_type = "lib"] | ||
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| // Ensure that we do not call `memcpy` for the following function. | ||
| // `memset` and `init` should be called directly on the return pointer. | ||
| #[no_mangle] | ||
| pub fn nrvo(init: fn(&mut [u8; 4096])) -> [u8; 4096] { | ||
| // CHECK-LABEL: nrvo | ||
| // CHECK: @llvm.memset | ||
| // CHECK-NOT: @llvm.memcpy | ||
| // CHECK: ret | ||
| // CHECK-EMPTY | ||
| let mut buf = [0; 4096]; | ||
| init(&mut buf); | ||
| buf | ||
| } |
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