WIP.

Cargo.toml

@@ -15,3 +15,4 @@ log = "0.4"
 env_logger = "0.9"
 fxhash = "0.2"
 smallvec = "1.7"
+regalloc2 = { git = 'https://github.com/bytecodealliance/regalloc2', rev = 'c7bc6c941cd81bbd30b95969009b7e61539f2b4c' }

src/dataflow.rs

@@ -1,153 +0,0 @@
-//! Dataflow analysis.
-
-use fxhash::{FxHashMap, FxHashSet};
-use std::collections::VecDeque;
-use std::{fmt::Debug, hash::Hash};
-
-use crate::{BlockId, FunctionBody, InstId};
-
-pub trait Lattice: Clone + Debug + PartialEq + Eq {
-    fn top() -> Self;
-    fn bottom() -> Self;
-    fn meet(a: &Self, b: &Self) -> Self;
-}
-
-pub trait AnalysisKey: Clone + Debug + PartialEq + Eq + Hash {}
-
-impl AnalysisKey for u32 {}
-
-#[derive(Clone, Debug)]
-pub struct AnalysisValue<K: AnalysisKey, L: Lattice> {
-    pub values: FxHashMap<K, L>,
-}
-
-impl<K: AnalysisKey, L: Lattice> std::default::Default for AnalysisValue<K, L> {
-    fn default() -> Self {
-        Self {
-            values: FxHashMap::default(),
-        }
-    }
-}
-
-impl<K: AnalysisKey, L: Lattice> AnalysisValue<K, L> {
-    fn meet_with(&mut self, other: &Self, meet_mode: MapMeetMode) -> bool {
-        let mut changed = false;
-        let mut to_remove = vec![];
-        for (key, value) in &mut self.values {
-            if let Some(other_value) = other.values.get(key) {
-                let met = L::meet(value, other_value);
-                if met != *value {
-                    changed = true;
-                    *value = met;
-                }
-            } else {
-                if meet_mode == MapMeetMode::Intersection {
-                    to_remove.push(key.clone());
-                    changed = true;
-                }
-            }
-        }
-        for k in to_remove {
-            self.values.remove(&k);
-        }
-        if meet_mode == MapMeetMode::Union {
-            for (key, value) in &other.values {
-                if !self.values.contains_key(key) {
-                    self.values.insert(key.clone(), value.clone());
-                    changed = true;
-                }
-            }
-        }
-
-        changed
-    }
-}
-
-pub trait AnalysisFunction {
-    type K: AnalysisKey;
-    type L: Lattice;
-
-    fn instruction(
-        &self,
-        _input: &mut AnalysisValue<Self::K, Self::L>,
-        _func: &FunctionBody,
-        _block: BlockId,
-        _inst: InstId,
-    ) -> bool {
-        false
-    }
-
-    fn terminator(
-        &self,
-        _input: &mut AnalysisValue<Self::K, Self::L>,
-        _func: &FunctionBody,
-        _block: BlockId,
-        _index: usize,
-        _next: BlockId,
-    ) -> bool {
-        false
-    }
-
-    fn meet_mode(&self) -> MapMeetMode {
-        MapMeetMode::Union
-    }
-}
-
-#[derive(Clone, Copy, Debug, PartialEq, Eq)]
-pub enum MapMeetMode {
-    Union,
-    Intersection,
-}
-
-#[derive(Clone, Debug, Default)]
-pub struct ForwardDataflow<F: AnalysisFunction> {
-    block_in: Vec<AnalysisValue<F::K, F::L>>,
-    workqueue: VecDeque<BlockId>,
-    workqueue_set: FxHashSet<BlockId>,
-}
-
-impl<F: AnalysisFunction> ForwardDataflow<F> {
-    pub fn new(analysis: &F, func: &FunctionBody) -> Self {
-        let mut ret = ForwardDataflow {
-            block_in: vec![AnalysisValue::default(); func.blocks.len()],
-            workqueue: vec![0].into(),
-            workqueue_set: vec![0].into_iter().collect(),
-        };
-        ret.compute(analysis, func);
-        ret
-    }
-
-    fn compute(&mut self, analysis: &F, func: &FunctionBody) {
-        while let Some(block) = self.workqueue.pop_front() {
-            self.workqueue_set.remove(&block);
-            self.update_block(analysis, func, block);
-        }
-    }
-
-    fn update_block(&mut self, analysis: &F, func: &FunctionBody, block: BlockId) {
-        let mut value = self.block_in[block].clone();
-        let mut changed = false;
-        for i in 0..func.blocks[block].insts.len() {
-            changed |= analysis.instruction(&mut value, func, block, i);
-        }
-
-        for (i, succ) in func.blocks[block]
-            .terminator
-            .successors()
-            .into_iter()
-            .enumerate()
-        {
-            let mut term_changed = changed;
-            let mut value = value.clone();
-            term_changed |= analysis.terminator(&mut value, func, block, i, succ);
-            if term_changed {
-                if self.block_in[succ].meet_with(&value, analysis.meet_mode()) {
-                    if !self.workqueue_set.contains(&succ) {
-                        self.workqueue.push_back(succ);
-                        self.workqueue_set.insert(succ);
-                    }
-                }
-            }
-        }
-    }
-}

src/frontend.rs

@@ -143,11 +143,8 @@ fn parse_body<'a, 'b>(
 
     for (arg_idx, &arg_ty) in module.signatures[my_sig].params.iter().enumerate() {
         let local_idx = arg_idx as LocalId;
-        let value = builder.body.values.len() as ValueId;
+        let value = Value::arg(arg_idx);
-        builder.body.values.push(ValueDef {
+        builder.body.types.insert(value, arg_ty);
-            kind: ValueKind::Arg(arg_idx),
-            ty: arg_ty,
-        });
         trace!("defining local {} to value {}", local_idx, value);
         builder.locals.insert(local_idx, (arg_ty, value));
     }
@@ -174,8 +171,8 @@ struct FunctionBodyBuilder<'a, 'b> {
     body: &'b mut FunctionBody<'a>,
     cur_block: Option<BlockId>,
     ctrl_stack: Vec<Frame>,
-    op_stack: Vec<(Type, ValueId)>,
+    op_stack: Vec<(Type, Value)>,
-    locals: FxHashMap<LocalId, (Type, ValueId)>,
+    locals: FxHashMap<LocalId, (Type, Value)>,
     block_param_locals: FxHashMap<BlockId, Vec<LocalId>>,
 }
 
@@ -198,7 +195,7 @@ enum Frame {
         start_depth: usize,
         out: BlockId,
         el: BlockId,
-        param_values: Vec<(Type, ValueId)>,
+        param_values: Vec<(Type, Value)>,
         params: Vec<Type>,
         results: Vec<Type>,
     },
@@ -278,7 +275,7 @@ impl<'a, 'b> FunctionBodyBuilder<'a, 'b> {
         self.body.blocks[block].params.extend_from_slice(tys);
     }
 
-    fn pop_n(&mut self, n: usize) -> Vec<ValueId> {
+    fn pop_n(&mut self, n: usize) -> Vec<Value> {
         let new_top = self.op_stack.len() - n;
         let ret = self.op_stack[new_top..]
             .iter()
@@ -288,7 +285,7 @@ impl<'a, 'b> FunctionBodyBuilder<'a, 'b> {
         ret
     }
 
-    fn pop_1(&mut self) -> ValueId {
+    fn pop_1(&mut self) -> Value {
         self.op_stack.pop().unwrap().1
     }
 
@@ -327,14 +324,9 @@ impl<'a, 'b> FunctionBodyBuilder<'a, 'b> {
                             })
                             .unwrap();
                             self.op_stack.pop();
-                            let value = self.body.values.len() as ValueId;
+                            Value::inst(block, inst, 0)
-                            self.body.values.push(ValueDef {
-                                ty,
-                                kind: ValueKind::Inst(block, inst, 0),
-                            });
-                            value
                         } else {
-                            NO_VALUE
+                            Value::undef()
                         }
                     });
                 self.op_stack.push((ty, value));
@@ -786,7 +778,7 @@ impl<'a, 'b> FunctionBodyBuilder<'a, 'b> {
         &self.ctrl_stack[self.ctrl_stack.len() - 1 - relative_depth as usize]
     }
 
-    fn fill_block_params_with_locals(&mut self, target: BlockId, args: &mut Vec<Operand>) {
+    fn fill_block_params_with_locals(&mut self, target: BlockId, args: &mut Vec<Value>) {
         if !self.block_param_locals.contains_key(&target) {
             let mut keys: Vec<LocalId> = self.locals.keys().cloned().collect();
             keys.sort();
@@ -799,13 +791,13 @@ impl<'a, 'b> FunctionBodyBuilder<'a, 'b> {
         let block_param_locals = self.block_param_locals.get(&target).unwrap();
         for local in block_param_locals {
             let local_value = self.locals.get(local).unwrap();
-            args.push(Operand::value(local_value.1));
+            args.push(local_value.1);
         }
     }
 
-    fn emit_branch(&mut self, target: BlockId, args: &[ValueId]) {
+    fn emit_branch(&mut self, target: BlockId, args: &[Value]) {
         if let Some(block) = self.cur_block {
-            let mut args: Vec<Operand> = args.iter().map(|&val| Operand::value(val)).collect();
+            let mut args = args.to_vec();
             self.fill_block_params_with_locals(target, &mut args);
             let target = BlockTarget {
                 block: target,
@@ -817,25 +809,19 @@ impl<'a, 'b> FunctionBodyBuilder<'a, 'b> {
 
     fn emit_cond_branch(
         &mut self,
-        cond: ValueId,
+        cond: Value,
         if_true: BlockId,
-        if_true_args: &[ValueId],
+        if_true_args: &[Value],
         if_false: BlockId,
-        if_false_args: &[ValueId],
+        if_false_args: &[Value],
     ) {
         if let Some(block) = self.cur_block {
-            let mut if_true_args = if_true_args
+            let mut if_true_args = if_true_args.to_vec();
-                .iter()
+            let mut if_false_args = if_false_args.to_vec();
-                .map(|&val| Operand::value(val))
-                .collect();
-            let mut if_false_args = if_false_args
-                .iter()
-                .map(|&val| Operand::value(val))
-                .collect();
             self.fill_block_params_with_locals(if_true, &mut if_true_args);
             self.fill_block_params_with_locals(if_false, &mut if_false_args);
             self.body.blocks[block].terminator = Terminator::CondBr {
-                cond: Operand::value(cond),
+                cond,
                 if_true: BlockTarget {
                     block: if_true,
                     args: if_true_args,
@@ -850,13 +836,13 @@ impl<'a, 'b> FunctionBodyBuilder<'a, 'b> {
 
     fn emit_br_table(
         &mut self,
-        index: ValueId,
+        index: Value,
         default_target: BlockId,
         indexed_targets: &[BlockId],
-        args: &[ValueId],
+        args: &[Value],
     ) {
         if let Some(block) = self.cur_block {
-            let args: Vec<Operand> = args.iter().map(|&arg| Operand::value(arg)).collect();
+            let args = args.to_vec();
             let targets = indexed_targets
                 .iter()
                 .map(|&block| {
@@ -873,16 +859,16 @@ impl<'a, 'b> FunctionBodyBuilder<'a, 'b> {
                 args: default_args,
             };
             self.body.blocks[block].terminator = Terminator::Select {
-                value: Operand::value(index),
+                value: index,
                 targets,
                 default,
             };
         }
     }
 
-    fn emit_ret(&mut self, vals: &[ValueId]) {
+    fn emit_ret(&mut self, values: &[Value]) {
         if let Some(block) = self.cur_block {
-            let values = vals.iter().map(|&value| Operand::value(value)).collect();
+            let values = values.to_vec();
             self.body.blocks[block].terminator = Terminator::Return { values };
         }
     }
@@ -899,12 +885,8 @@ impl<'a, 'b> FunctionBodyBuilder<'a, 'b> {
 
         let mut block_param_num = 0;
         for &ty in wasm_stack_val_tys.iter() {
-            let value_id = self.body.values.len() as ValueId;
+            let value = Value::blockparam(block, block_param_num);
-            self.body.values.push(ValueDef {
+            self.op_stack.push((ty, value));
-                kind: ValueKind::BlockParam(block, block_param_num),
-                ty,
-            });
-            self.op_stack.push((ty, value_id));
             block_param_num += 1;
         }
 
@@ -913,13 +895,9 @@ impl<'a, 'b> FunctionBodyBuilder<'a, 'b> {
                 .iter()
                 .zip(block_param_locals.iter())
             {
-                let value_id = self.body.values.len() as ValueId;
+                let value = Value::blockparam(block, block_param_num);
-                self.body.values.push(ValueDef {
-                    kind: ValueKind::BlockParam(block, block_param_num),
-                    ty,
-                });
                 block_param_num += 1;
-                self.locals.insert(local_id, (ty, value_id));
+                self.locals.insert(local_id, (ty, value));
             }
         }
     }
@@ -941,30 +919,24 @@ impl<'a, 'b> FunctionBodyBuilder<'a, 'b> {
             for input in inputs.into_iter().rev() {
                 let (stack_top_ty, stack_top) = self.op_stack.pop().unwrap();
                 assert_eq!(stack_top_ty, input);
-                input_operands.push(Operand::value(stack_top));
+                input_operands.push(stack_top);
             }
             input_operands.reverse();
 
-            let mut output_operands = vec![];
+            let n_outputs = outputs.len();
             for (i, output_ty) in outputs.into_iter().enumerate() {
-                let val = self.body.values.len() as ValueId;
+                self.op_stack.push((output_ty, Value::inst(block, inst, i)));
-                output_operands.push(val);
-                self.body.values.push(ValueDef {
-                    kind: ValueKind::Inst(block, inst, i),
-                    ty: output_ty,
-                });
-                self.op_stack.push((output_ty, val));
             }
 
             self.body.blocks[block].insts.push(Inst {
                 operator: op,
-                outputs: output_operands,
+                n_outputs,
                 inputs: input_operands,
             });
         } else {
             let _ = self.pop_n(inputs.len());
             for ty in outputs {
-                self.op_stack.push((ty, NO_VALUE));
+                self.op_stack.push((ty, Value::undef()));
             }
         }
 

src/ir.rs

@@ -2,16 +2,15 @@
 
 use crate::{backend::Shape, cfg::CFGInfo, frontend};
 use anyhow::Result;
+use fxhash::FxHashMap;
 use wasmparser::{FuncType, Operator, Type};
 
 pub type SignatureId = usize;
 pub type FuncId = usize;
 pub type BlockId = usize;
 pub type InstId = usize;
-pub type ValueId = usize;
 pub type LocalId = u32;
 
-pub const NO_VALUE: ValueId = usize::MAX;
 pub const INVALID_BLOCK: BlockId = usize::MAX;
 
 #[derive(Clone, Debug, Default)]
@@ -40,26 +39,15 @@ impl<'a> FuncDecl<'a> {
 
 #[derive(Clone, Debug, Default)]
 pub struct FunctionBody<'a> {
+    pub arg_values: Vec<Value>,
     pub locals: Vec<Type>,
     pub blocks: Vec<Block<'a>>,
-    pub values: Vec<ValueDef>,
+    pub types: FxHashMap<Value, Type>,
-}
-
-#[derive(Clone, Debug)]
-pub struct ValueDef {
-    pub kind: ValueKind,
-    pub ty: Type,
-}
-
-#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
-pub enum ValueKind {
-    Arg(usize),
-    BlockParam(BlockId, usize),
-    Inst(BlockId, InstId, usize),
 }
 
 #[derive(Clone, Debug, Default)]
 pub struct Block<'a> {
+    pub id: BlockId,
     pub params: Vec<Type>,
     pub insts: Vec<Inst<'a>>,
     pub terminator: Terminator,
@@ -70,14 +58,18 @@ impl<'a> Block<'a> {
         self.terminator.successors()
     }
 
-    pub fn values<'b>(&'b self) -> impl Iterator<Item = ValueId> + 'b {
+    pub fn values<'b>(&'b self) -> impl Iterator<Item = Value> + 'b {
+        let block = self.id;
         self.insts
             .iter()
-            .map(|inst| inst.outputs.iter().cloned())
+            .enumerate()
+            .map(move |(inst_id, inst)| {
+                (0..inst.n_outputs).map(move |i| Value::inst(block, inst_id, i))
+            })
             .flatten()
     }
 
-    pub fn visit_operands<F: Fn(&Operand)>(&self, f: F) {
+    pub fn visit_values<F: Fn(&Value)>(&self, f: F) {
         for inst in &self.insts {
             for input in &inst.inputs {
                 f(input);
@@ -95,7 +87,7 @@ impl<'a> Block<'a> {
         }
     }
 
-    pub fn update_operands<F: Fn(&mut Operand)>(&mut self, f: F) {
+    pub fn update_values<F: Fn(&mut Value)>(&mut self, f: F) {
         for inst in &mut self.insts {
             for input in &mut inst.inputs {
                 f(input);
@@ -117,33 +109,123 @@ impl<'a> Block<'a> {
 #[derive(Clone, Debug)]
 pub struct Inst<'a> {
     pub operator: Operator<'a>,
-    pub outputs: Vec<ValueId>,
+    pub n_outputs: usize,
-    pub inputs: Vec<Operand>,
+    pub inputs: Vec<Value>,
 }
 
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
-pub enum Operand {
+pub struct Value(u64);
-    /// An SSA value.
+
-    Value(ValueId),
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
-    /// Undef values are produced when code is unreachable and thus
+#[repr(u64)]
-    /// removed/never executed.
+enum ValueTag {
+    /// Undefined value. Fields: tag(4) unused(60).
+    Undef = 0,
+    /// Function argument. Fields: tag(4) unused(52) index(8).
+    Arg = 1,
+    /// Block param. Fields: tag(4) unused(2) block(26) param(32).
+    BlockParam = 2,
+    /// Instruction output. Fields: tag(4) block(26) inst(26) output(8).
+    InstOutput = 3,
+}
+
+const VALUE_TAG_SHIFT: usize = 60;
+
+impl Value {
+    pub fn undef() -> Self {
+        Value((ValueTag::Undef as u64) << VALUE_TAG_SHIFT)
+    }
+
+    pub fn arg(index: usize) -> Self {
+        assert!(index < 256);
+        Value(((ValueTag::Arg as u64) << VALUE_TAG_SHIFT) | (index as u64))
+    }
+
+    pub fn blockparam(block: BlockId, index: usize) -> Self {
+        assert!(index < 256);
+        assert!(block < (1 << 26));
+        Value(
+            ((ValueTag::BlockParam as u64) << VALUE_TAG_SHIFT)
+                | ((block as u64) << 32)
+                | (index as u64),
+        )
+    }
+
+    pub fn inst(block: BlockId, inst: InstId, index: usize) -> Self {
+        assert!(index < 256);
+        assert!(block < (1 << 26));
+        assert!(inst < (1 << 26));
+        Value(
+            ((ValueTag::InstOutput as u64) << VALUE_TAG_SHIFT)
+                | ((block as u64) << 34)
+                | ((inst as u64) << 8)
+                | (index as u64),
+        )
+    }
+
+    pub fn unpack(self) -> ValueKind {
+        let tag = self.0 >> VALUE_TAG_SHIFT;
+        match tag {
+            0 => ValueKind::Undef,
+            1 => ValueKind::Arg((self.0 & ((1 << 8) - 1)) as usize),
+            2 => ValueKind::BlockParam(
+                ((self.0 >> 32) & ((1 << 26) - 1)) as usize,
+                (self.0 & 0xff) as usize,
+            ),
+            3 => ValueKind::Inst(
+                ((self.0 >> 34) & ((1 << 26) - 1)) as usize,
+                ((self.0 >> 8) & ((1 << 26) - 1)) as usize,
+                (self.0 & 0xff) as usize,
+            ),
+            _ => unreachable!(),
+        }
+    }
+
+    pub fn as_arg(self) -> Option<usize> {
+        match self.unpack() {
+            ValueKind::Arg(arg) => Some(arg),
+            _ => None,
+        }
+    }
+
+    pub fn as_blockparam(self) -> Option<(BlockId, usize)> {
+        match self.unpack() {
+            ValueKind::BlockParam(block, param) => Some((block, param)),
+            _ => None,
+        }
+    }
+
+    pub fn as_inst(self) -> Option<(BlockId, InstId, usize)> {
+        match self.unpack() {
+            ValueKind::Inst(block, inst, param) => Some((block, inst, param)),
+            _ => None,
+        }
+    }
+}
+
+impl std::fmt::Display for Value {
+    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+        match self.unpack() {
+            ValueKind::Undef => write!(f, "undef"),
+            ValueKind::Arg(i) => write!(f, "arg{}", i),
+            ValueKind::BlockParam(block, i) => write!(f, "block{}_{}", block, i),
+            ValueKind::Inst(block, inst, i) => write!(f, "inst{}_{}_{}", block, inst, i),
+        }
+    }
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub enum ValueKind {
     Undef,
-}
+    Arg(usize),
-
+    BlockParam(BlockId, usize),
-impl Operand {
+    Inst(BlockId, InstId, usize),
-    pub fn value(value: ValueId) -> Self {
-        if value == NO_VALUE {
-            Operand::Undef
-        } else {
-            Operand::Value(value)
-        }
-    }
 }
 
 #[derive(Clone, Debug)]
 pub struct BlockTarget {
     pub block: BlockId,
-    pub args: Vec<Operand>,
+    pub args: Vec<Value>,
 }
 
 #[derive(Clone, Debug)]
@@ -152,17 +234,17 @@ pub enum Terminator {
         target: BlockTarget,
     },
     CondBr {
-        cond: Operand,
+        cond: Value,
         if_true: BlockTarget,
         if_false: BlockTarget,
     },
     Select {
-        value: Operand,
+        value: Value,
         targets: Vec<BlockTarget>,
         default: BlockTarget,
     },
     Return {
-        values: Vec<Operand>,
+        values: Vec<Value>,
     },
     None,
 }
@@ -174,7 +256,7 @@ impl std::default::Default for Terminator {
 }
 
 impl Terminator {
-    pub fn args(&self) -> Vec<Operand> {
+    pub fn args(&self) -> Vec<Value> {
         match self {
             Terminator::Br { target } => target.args.clone(),
             Terminator::CondBr {

src/lib.rs

@@ -9,7 +9,6 @@ pub use wasmparser;
 
 mod backend;
 mod cfg;
-mod dataflow;
 mod frontend;
 mod ir;
 mod op_traits;

src/op_traits.rs

@@ -1,6 +1,6 @@
 //! Metadata on operators.
 
-use crate::ir::{Module, SignatureId, ValueId};
+use crate::ir::{Module, SignatureId, Value};
 use anyhow::{bail, Result};
 use wasmparser::{Operator, Type};
 
@@ -8,7 +8,7 @@ pub fn op_inputs(
     module: &Module,
     my_sig: SignatureId,
     my_locals: &[Type],
-    op_stack: &[(Type, ValueId)],
+    op_stack: &[(Type, Value)],
     op: &Operator<'_>,
 ) -> Result<Vec<Type>> {
     match op {
@@ -228,7 +228,7 @@ pub fn op_inputs(
 pub fn op_outputs(
     module: &Module,
     my_locals: &[Type],
-    op_stack: &[(Type, ValueId)],
+    op_stack: &[(Type, Value)],
     op: &Operator<'_>,
 ) -> Result<Vec<Type>> {
     match op {

src/pass/dataflow.rs

@@ -0,0 +1,180 @@
+//! Iterative dataflow analysis (forward and backward) using lattice
+//! analysis values.
+
+use crate::cfg::CFGInfo;
+use crate::ir::*;
+use crate::pass::Lattice;
+use fxhash::{FxHashMap, FxHashSet};
+use std::collections::hash_map::Entry as HashEntry;
+use std::{collections::VecDeque, default::Default};
+use wasmparser::Type;
+
+impl<'a> FunctionBody<'a> {
+    fn insts(&self) -> impl Iterator<Item = &Inst<'a>> {
+        self.blocks.iter().map(|block| block.insts.iter()).flatten()
+    }
+}
+
+pub trait DataflowFunctions<L: Lattice> {
+    fn start_block(&self, _lattice: &mut L, _block: BlockId, _param_types: &[Type]) -> bool {
+        false
+    }
+    fn end_block(
+        &self,
+        _lattce: &mut L,
+        _block: BlockId,
+        _next: BlockId,
+        _terminator: &Terminator,
+    ) -> bool {
+        false
+    }
+    fn instruction<'a>(
+        &self,
+        _lattice: &mut L,
+        _block: BlockId,
+        _instid: InstId,
+        _inst: &Inst<'a>,
+    ) -> bool {
+        false
+    }
+}
+
+#[derive(Clone, Debug)]
+pub struct ForwardDataflow<L: Lattice> {
+    block_in: FxHashMap<BlockId, L>,
+}
+
+impl<L: Lattice> ForwardDataflow<L> {
+    pub fn new<'a, D: DataflowFunctions<L>>(f: &FunctionBody<'a>, d: &D) -> Self {
+        let mut analysis = Self {
+            block_in: FxHashMap::default(),
+        };
+        analysis.compute(f, d);
+        analysis
+    }
+
+    fn compute<'a, D: DataflowFunctions<L>>(&mut self, f: &FunctionBody<'a>, d: &D) {
+        let mut workqueue = VecDeque::new();
+        let mut workqueue_set = FxHashSet::default();
+
+        workqueue.push_back(0);
+        workqueue_set.insert(0);
+        while let Some(block) = workqueue.pop_front() {
+            workqueue_set.remove(&block);
+
+            let mut value = self
+                .block_in
+                .entry(block)
+                .or_insert_with(|| L::top())
+                .clone();
+
+            d.start_block(&mut value, block, &f.blocks[block].params[..]);
+
+            for (instid, inst) in f.blocks[block].insts.iter().enumerate() {
+                d.instruction(&mut value, block, instid, inst);
+            }
+
+            let succs = f.blocks[block].terminator.successors();
+            for (i, &succ) in succs.iter().enumerate() {
+                let mut value = if i + 1 < succs.len() {
+                    value.clone()
+                } else {
+                    std::mem::replace(&mut value, L::top())
+                };
+
+                d.end_block(&mut value, block, succ, &f.blocks[block].terminator);
+
+                let (succ_in, mut changed) = match self.block_in.entry(succ) {
+                    HashEntry::Vacant(v) => (v.insert(L::top()), true),
+                    HashEntry::Occupied(o) => (o.into_mut(), false),
+                };
+                changed |= succ_in.meet_with(&value);
+
+                if changed && !workqueue_set.contains(&succ) {
+                    workqueue.push_back(succ);
+                    workqueue_set.insert(succ);
+                }
+            }
+        }
+    }
+}
+
+#[derive(Clone, Debug)]
+pub struct BackwardDataflow<L: Lattice> {
+    block_out: FxHashMap<BlockId, L>,
+}
+
+impl<L: Lattice> BackwardDataflow<L> {
+    pub fn new<'a, D: DataflowFunctions<L>>(
+        f: &FunctionBody<'a>,
+        cfginfo: &CFGInfo,
+        d: &D,
+    ) -> Self {
+        let mut analysis = Self {
+            block_out: FxHashMap::default(),
+        };
+        analysis.compute(f, cfginfo, d);
+        analysis
+    }
+
+    fn compute<'a, D: DataflowFunctions<L>>(
+        &mut self,
+        f: &FunctionBody<'a>,
+        cfginfo: &CFGInfo,
+        d: &D,
+    ) {
+        let mut workqueue = VecDeque::new();
+        let mut workqueue_set = FxHashSet::default();
+
+        let returns = f
+            .blocks
+            .iter()
+            .enumerate()
+            .filter(|(_, block)| matches!(&block.terminator, &Terminator::Return { .. }))
+            .map(|(id, _)| id)
+            .collect::<Vec<BlockId>>();
+
+        for ret in returns {
+            workqueue.push_back(ret);
+            workqueue_set.insert(ret);
+        }
+
+        while let Some(block) = workqueue.pop_front() {
+            workqueue_set.remove(&block);
+
+            let mut value = self
+                .block_out
+                .entry(block)
+                .or_insert_with(|| L::top())
+                .clone();
+
+            for (instid, inst) in f.blocks[block].insts.iter().rev().enumerate() {
+                d.instruction(&mut value, block, instid, inst);
+            }
+
+            d.start_block(&mut value, block, &f.blocks[block].params[..]);
+
+            let preds = &cfginfo.block_preds[block];
+            for (i, pred) in preds.iter().cloned().enumerate() {
+                let mut value = if i + 1 < preds.len() {
+                    value.clone()
+                } else {
+                    std::mem::replace(&mut value, L::top())
+                };
+
+                d.end_block(&mut value, pred, block, &f.blocks[pred].terminator);
+
+                let (pred_out, mut changed) = match self.block_out.entry(pred) {
+                    HashEntry::Vacant(v) => (v.insert(L::top()), true),
+                    HashEntry::Occupied(o) => (o.into_mut(), false),
+                };
+                changed |= pred_out.meet_with(&value);
+
+                if changed && !workqueue_set.contains(&pred) {
+                    workqueue.push_back(pred);
+                    workqueue_set.insert(pred);
+                }
+            }
+        }
+    }
+}

src/pass/lattice.rs

@@ -0,0 +1,139 @@
+//! Lattice trait definition and some common implementations.
+
+use crate::ir::*;
+use regalloc2::indexset::IndexSet;
+use std::fmt::Debug;
+
+/// A lattice type used for an analysis.
+///
+/// The `meet` operator must compute the greatest lower bound for its
+/// operands (that is, its result must be "less than or equal to" its
+/// operands, according to the lattice's partial order, and must be
+/// the greatest value that satisfies this condition). It must obey
+/// the usual lattice laws:
+///
+/// * a `meet` a == a  (reflexivity)
+/// * a `meet` b == b `meet` a (commutativity)
+/// * a `meet` (b `meet` c) == (a `meet` b) `meet` c (associativity)
+/// * a `meet` top == a
+/// * a `meet` bottom == bottom
+///
+/// Note that while we require that the lattice is a consistent
+/// partial order, we don't actually require the user to implement
+/// `PartialOrd` on the type, because we never make direct ordering
+/// comparisons when we perform a dataflow analysis. Instead the
+/// ordering is only implicitly depended upon, in order to ensure that
+/// the analysis terminates. For this to be true, we also require that
+/// the lattice has only a finite chain length -- that is, there must
+/// not be an infinite ordered sequence in the lattice (or, moving to
+/// "lesser" values will always reach bottom in finite steps).
+pub trait Lattice: Clone + Debug {
+    /// Return the `top` lattice value.
+    fn top() -> Self;
+    /// Return the `bottom` lattice value.
+    fn bottom() -> Self;
+    /// Mutate self to `meet(self, other)`. Returns `true` if any
+    /// changes occurred.
+    fn meet_with(&mut self, other: &Self) -> bool;
+}
+
+/// An analysis-value lattice whose values are sets of `ValueId`
+/// indices. `top` is empty and `bottom` is the universe set; the
+/// `meet` function is a union. This is useful for may-analyses,
+/// i.e. when an analysis computes whether a property *may* be true
+/// about a value in some case.
+#[derive(Clone, Debug)]
+pub struct UnionBitSet {
+    set: IndexSet,
+    /// The set has degenerated to contain "the universe" (all
+    /// possible values).
+    universe: bool,
+}
+
+impl Lattice for UnionBitSet {
+    fn top() -> Self {
+        UnionBitSet {
+            set: IndexSet::new(),
+            universe: false,
+        }
+    }
+
+    fn bottom() -> Self {
+        UnionBitSet {
+            set: IndexSet::new(),
+            universe: true,
+        }
+    }
+
+    fn meet_with(&mut self, other: &UnionBitSet) -> bool {
+        if !self.universe && other.universe {
+            self.universe = true;
+            return true;
+        }
+        self.set.union_with(&other.set)
+    }
+}
+
+impl UnionBitSet {
+    pub fn contains(&self, index: usize) -> bool {
+        self.universe || self.set.get(index)
+    }
+
+    pub fn add(&mut self, index: usize) {
+        if !self.universe {
+            self.set.set(index, true);
+        }
+    }
+
+    pub fn remove(&mut self, index: usize) {
+        if !self.universe {
+            self.set.set(index, false);
+        }
+    }
+}
+
+/// An analysis-value lattice whose values are sets of `ValueId`
+/// indices. `top` is the universe set and `bottom` is the empty set;
+/// the `meet` function is an intersection. This is useful for
+/// must-analyses, i.e. when an analysis computes whether a property
+/// *must* be true about a value in all cases.
+#[derive(Clone, Debug)]
+pub struct IntersectionBitSet {
+    /// We store the dual to the actual set, i.e., elements that are
+    /// *not* included.
+    not_set: UnionBitSet,
+}
+
+impl Lattice for IntersectionBitSet {
+    fn top() -> Self {
+        // `top` here is the universe-set; the dual of this set is the
+        // empty-set, which is UnionBitSet's `top()`.
+        Self {
+            not_set: UnionBitSet::top(),
+        }
+    }
+
+    fn bottom() -> Self {
+        Self {
+            not_set: UnionBitSet::bottom(),
+        }
+    }
+
+    fn meet_with(&mut self, other: &IntersectionBitSet) -> bool {
+        self.not_set.meet_with(&other.not_set)
+    }
+}
+
+impl IntersectionBitSet {
+    pub fn contains(&self, index: usize) -> bool {
+        !self.not_set.contains(index)
+    }
+
+    pub fn add(&mut self, index: usize) {
+        self.not_set.remove(index);
+    }
+
+    pub fn remove(&mut self, index: usize) {
+        self.not_set.add(index);
+    }
+}

src/pass/mod.rs

@@ -0,0 +1,11 @@
+//! Pass framework: skeletons for common kinds of passes over code.
+//!
+//! Terminology note: a "pass" is a readonly analysis of a function
+//! body. It does not mutate code; it only traverses the code in a
+//! certain order, possibly multiple times (to converge), in order to
+//! compute some derived information.
+
+pub mod dataflow;
+pub use dataflow::*;
+pub mod lattice;
+pub use lattice::*;