Various fixes and improvements to the MIR Dataflow framework #1
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -24,12 +24,13 @@ | ||
| //! For all of them we will be using a lattice of Hashmap from Lvalue to | ||
| //! WTop<Either<Lvalue, Constant>> | ||
| //! | ||
| //! My personal belief is that it should be possible to make a way to compose two hashmap lattices | ||
| //! into one, but I can’t seem to get it just right yet, so we do the composing and decomposing | ||
| //! manually here. | ||
| use self::AcsLattice::*; | ||
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| use rustc_data_structures::fnv::FnvHashMap; | ||
| use rustc::mir::repr::*; | ||
| use rustc::mir::visit::{MutVisitor, LvalueContext}; | ||
| use rustc::mir::transform::lattice::Lattice; | ||
| @@ -40,64 +41,140 @@ use rustc::middle::const_val::ConstVal; | ||
| use pretty; | ||
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| #[derive(PartialEq, Debug, Eq, Clone)] | ||
| enum Either<'tcx> { | ||
| Lvalue(Lvalue<'tcx>), | ||
| Const(Constant<'tcx>), | ||
| } | ||
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| #[derive(Debug, Clone)] | ||
| enum AcsLattice<'tcx> { | ||
| Bottom, | ||
|
There was a problem hiding this comment. The bottom element in this lattice is an empty HashMap. It is fine to have it this way, because Similarly, I’m confused as to why the /me shrugs There was a problem hiding this comment. I changed the lattice so that fn foo(mut x: u32) {
if /* random unoptimizable condition */ {
x = 5;
}
println!("{}", x);
}Previously, since the value stored for fn foo(mut x: u32) {
if /* random unoptimizable condition */ {
x = 5;
}
println!("{}", 5);
}which is just wrong. Note that, in terms of inspiration, while the original paper used a union-based lattice for constant propogation, GHC actually uses an intersection based lattice (see here). There was a problem hiding this comment. Also, I realized after I wrote that code that |
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| Wrap(FnvHashMap<Lvalue<'tcx>, Either<'tcx>>) | ||
| } | ||
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| impl<'tcx> Lattice for AcsLattice<'tcx> { | ||
| fn bottom() -> Self { Bottom } | ||
| fn join(&mut self, other: &Self) -> bool { | ||
| let other_map = match *other { | ||
| Bottom => return false, | ||
| Wrap(ref map) => map | ||
| }; | ||
| let self_map = match *self { | ||
| Bottom => { | ||
| *self = Wrap(other_map.clone()); | ||
| return true; | ||
| }, | ||
| Wrap(ref mut map) => map | ||
| }; | ||
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| let mut changed = false; | ||
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| for (k, v) in other_map { | ||
| let should_remove = if let Some(cur_v) = self_map.get(k) { | ||
| cur_v != v | ||
| } else { | ||
| false | ||
| }; | ||
| if should_remove { | ||
| self_map.remove(k); | ||
| changed = true; | ||
| } | ||
| } | ||
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| changed | ||
| } | ||
| } | ||
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| pub struct AcsPropagate; | ||
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| impl Pass for AcsPropagate {} | ||
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| impl<'tcx> MirPass<'tcx> for AcsPropagate { | ||
| fn run_pass<'a>(&mut self, tcx: TyCtxt<'a, 'tcx, 'tcx>, src: MirSource, mir: &mut Mir<'tcx>) { | ||
| let ret = ar_forward( | ||
| &mut mir.cfg, | ||
| Facts::new(), | ||
| AcsPropagateTransfer, | ||
| AliasRewrite.and_then(ConstRewrite).and_then(SimplifyRewrite) | ||
| ); | ||
| mir.cfg = ret.0; | ||
| pretty::dump_mir(tcx, "acs_propagate", &0, src, mir, None); | ||
| } | ||
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| } | ||
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| struct AcsPropagateTransfer; | ||
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| fn base_lvalue<'a, 'tcx>(mut lval: &'a Lvalue<'tcx>) -> &'a Lvalue<'tcx> { | ||
| while let &Lvalue::Projection(ref proj) = lval { | ||
| lval = &proj.base; | ||
| } | ||
| lval | ||
| } | ||
|
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| fn invalidate<'tcx>(map: &mut FnvHashMap<Lvalue<'tcx>, Either<'tcx>>, lval: &Lvalue<'tcx>) { | ||
| map.remove(lval); | ||
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| let mut repl = None; | ||
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| for (k, v) in &mut *map { | ||
| if let Either::Lvalue(ref mut nlval) = *v { | ||
| if nlval == lval { | ||
| match repl { | ||
| None => { | ||
| repl = Some(k.clone()) | ||
| }, | ||
| Some(ref r) => { | ||
| *nlval = r.clone(); | ||
| } | ||
| } | ||
| } | ||
| } | ||
| } | ||
|
|
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| if let Some(repl) = repl { | ||
| map.remove(&repl); | ||
| } | ||
| } | ||
|
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| impl<'tcx> Transfer<'tcx> for AcsPropagateTransfer { | ||
| type Lattice = AcsLattice<'tcx>; | ||
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| fn stmt(&self, s: &Statement<'tcx>, lat: AcsLattice<'tcx>) -> AcsLattice<'tcx> { | ||
| let mut lat_map = match lat { | ||
| Bottom => FnvHashMap::default(), | ||
| Wrap(map) => map | ||
| }; | ||
|
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| let StatementKind::Assign(ref lval, ref rval) = s.kind; | ||
| invalidate(&mut lat_map, base_lvalue(lval)); | ||
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| if let &Lvalue::Projection(_) = lval { | ||
| return Wrap(lat_map); | ||
| } | ||
|
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| match *rval { | ||
| Rvalue::Use(Operand::Consume(ref nlval)) => { | ||
| lat_map.insert(lval.clone(), Either::Lvalue(nlval.clone())); | ||
| }, | ||
| Rvalue::Use(Operand::Constant(ref c)) => { | ||
| lat_map.insert(lval.clone(), Either::Const(c.clone())); | ||
| }, | ||
| _ => { } | ||
| }; | ||
| Wrap(lat_map) | ||
| } | ||
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| fn term(&self, t: &Terminator<'tcx>, mut lat: AcsLattice<'tcx>) -> Vec<AcsLattice<'tcx>> { | ||
| match t.kind { | ||
| TerminatorKind::Call { .. } | | ||
| TerminatorKind::Drop { .. } => { | ||
| // FIXME: Be smarter here by using an alias analysis | ||
| lat = Wrap(FnvHashMap::default()); | ||
| }, | ||
| _ => { } | ||
| } | ||
|
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| // FIXME: this should inspect the terminators and set their known values to constants. Esp. | ||
| // for the if: in the truthy branch the operand is known to be true and in the falsy branch | ||
| // the operand is known to be false. Now we just ignore the potential here. | ||
| @@ -107,67 +184,86 @@ impl<'tcx> Transfer<'tcx, ACSLattice<'tcx>> for ACSPropagateTransfer { | ||
| } | ||
| } | ||
|
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| struct AliasRewrite; | ||
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| impl<'tcx> Rewrite<'tcx, AcsLattice<'tcx>> for AliasRewrite { | ||
| fn stmt(&self, s: &Statement<'tcx>, l: &AcsLattice<'tcx>, _: &mut CFG<'tcx>) | ||
| -> StatementChange<'tcx> { | ||
| if let Wrap(ref map) = *l { | ||
| let mut ns = s.clone(); | ||
| let mut vis = RewriteAliasVisitor(map, false); | ||
| vis.visit_statement(START_BLOCK, &mut ns); | ||
| if vis.1 { | ||
| return StatementChange::Statement(ns); | ||
| } | ||
| } | ||
| StatementChange::None | ||
| } | ||
|
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| fn term(&self, t: &Terminator<'tcx>, l: &AcsLattice<'tcx>, _: &mut CFG<'tcx>) | ||
| -> TerminatorChange<'tcx> { | ||
| if let Wrap(ref map) = *l { | ||
| let mut nt = t.clone(); | ||
| let mut vis = RewriteAliasVisitor(map, false); | ||
| vis.visit_terminator(START_BLOCK, &mut nt); | ||
| if vis.1 { | ||
| return TerminatorChange::Terminator(nt); | ||
| } | ||
| } | ||
| TerminatorChange::None | ||
| } | ||
| } | ||
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| struct RewriteAliasVisitor<'a, 'tcx: 'a>(&'a FnvHashMap<Lvalue<'tcx>, Either<'tcx>>, bool); | ||
| impl<'a, 'tcx> MutVisitor<'tcx> for RewriteAliasVisitor<'a, 'tcx> { | ||
| fn visit_lvalue(&mut self, lvalue: &mut Lvalue<'tcx>, context: LvalueContext) { | ||
| match context { | ||
| LvalueContext::Consume => { | ||
| if let Some(&Either::Lvalue(ref nlval)) = self.0.get(lvalue) { | ||
| self.1 = true; | ||
| *lvalue = nlval.clone(); | ||
| } | ||
| }, | ||
| _ => { } | ||
| } | ||
| self.super_lvalue(lvalue, context); | ||
| } | ||
| } | ||
|
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| struct ConstRewrite; | ||
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| impl<'tcx> Rewrite<'tcx, AcsLattice<'tcx>> for ConstRewrite { | ||
| fn stmt(&self, s: &Statement<'tcx>, l: &AcsLattice<'tcx>, _: &mut CFG<'tcx>) | ||
| -> StatementChange<'tcx> { | ||
| if let Wrap(ref map) = *l { | ||
| let mut ns = s.clone(); | ||
| let mut vis = RewriteConstVisitor(map, false); | ||
| vis.visit_statement(START_BLOCK, &mut ns); | ||
| if vis.1 { | ||
| return StatementChange::Statement(ns); | ||
| } | ||
| } | ||
| StatementChange::None | ||
| } | ||
|
|
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| fn term(&self, t: &Terminator<'tcx>, l: &AcsLattice<'tcx>, _: &mut CFG<'tcx>) | ||
| -> TerminatorChange<'tcx> { | ||
| if let Wrap(ref map) = *l { | ||
| let mut nt = t.clone(); | ||
| let mut vis = RewriteConstVisitor(map, false); | ||
| vis.visit_terminator(START_BLOCK, &mut nt); | ||
| if vis.1 { | ||
| return TerminatorChange::Terminator(nt); | ||
| } | ||
| } | ||
| TerminatorChange::None | ||
| } | ||
| } | ||
|
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| struct RewriteConstVisitor<'a, 'tcx: 'a>(&'a FnvHashMap<Lvalue<'tcx>, Either<'tcx>>, bool); | ||
| impl<'a, 'tcx> MutVisitor<'tcx> for RewriteConstVisitor<'a, 'tcx> { | ||
| fn visit_operand(&mut self, op: &mut Operand<'tcx>) { | ||
| // To satisy borrow checker, modify `op` after inspecting it | ||
| let repl = if let Operand::Consume(ref lval) = *op { | ||
| if let Some(&Either::Const(ref c)) = self.0.get(lval) { | ||
| Some(c.clone()) | ||
| @@ -178,21 +274,24 @@ impl<'a, 'tcx> MutVisitor<'tcx> for RewriteConstVisitor<'a, 'tcx> { | ||
| None | ||
| }; | ||
| if let Some(c) = repl { | ||
| self.1 = true; | ||
| *op = Operand::Constant(c); | ||
| } | ||
|
|
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| self.super_operand(op); | ||
| } | ||
| } | ||
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| struct SimplifyRewrite; | ||
|
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| impl<'tcx, L: Lattice> Rewrite<'tcx, L> for SimplifyRewrite { | ||
| fn stmt(&self, _: &Statement<'tcx>, _: &L, _: &mut CFG<'tcx>) | ||
| -> StatementChange<'tcx> { | ||
| StatementChange::None | ||
| } | ||
|
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| fn term(&self, t: &Terminator<'tcx>, _: &L, _: &mut CFG<'tcx>) | ||
| -> TerminatorChange<'tcx> { | ||
| match t.kind { | ||
| TerminatorKind::If { ref targets, .. } if targets.0 == targets.1 => { | ||
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This change makes the trait not very reusable for backward analysis, where terminators only have a single edge regardless of how many edges the terminator has in the forward direction. That is the primary motivation for TerminatorOut associated type.
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True, but I'm not sure that the same trait should be reused for the forward and backwards cases. Regardless, even if the two directions share a trait, I'd prefer a clearer way of denoting which direction a pass goes than using
Vec<Lattice>for forward andLatticefor backwards.Transfercould have another argument (Direction) which itself has an associated type denoting what outputtermshould have.I got rid of the associated
TerminatorOutbecause it felt clumsy, it cause recursive trait bounds when I moved in the associated lattice type, and because it wasn't necessary yet. When backwards transformation gets implemented, I'd be happy for this to be generalized.