cranelift_codegen/
inst_predicates.rs

1//! Instruction predicates/properties, shared by various analyses.
2use crate::ir::immediates::Offset32;
3use crate::ir::{self, Block, Function, Inst, InstructionData, Opcode, Type, Value};
4
5/// Test whether the given opcode is unsafe to even consider as side-effect-free.
6#[inline(always)]
7fn trivially_has_side_effects(opcode: Opcode) -> bool {
8    opcode.is_call()
9        || opcode.is_branch()
10        || opcode.is_terminator()
11        || opcode.is_return()
12        || opcode.can_trap()
13        || opcode.other_side_effects()
14        || opcode.can_store()
15}
16
17/// Load instructions without the `notrap` flag are defined to trap when
18/// operating on inaccessible memory, so we can't treat them as side-effect-free even if the loaded
19/// value is unused.
20#[inline(always)]
21fn is_load_with_defined_trapping(opcode: Opcode, data: &InstructionData) -> bool {
22    if !opcode.can_load() {
23        return false;
24    }
25    match *data {
26        InstructionData::StackLoad { .. } => false,
27        InstructionData::Load { flags, .. } => !flags.notrap(),
28        _ => true,
29    }
30}
31
32/// Does the given instruction have any side-effect that would preclude it from being removed when
33/// its value is unused?
34#[inline(always)]
35fn has_side_effect(func: &Function, inst: Inst) -> bool {
36    let data = &func.dfg.insts[inst];
37    let opcode = data.opcode();
38    trivially_has_side_effects(opcode) || is_load_with_defined_trapping(opcode, data)
39}
40
41/// Does the given instruction behave as a "pure" node with respect to
42/// aegraph semantics?
43///
44/// - Trivially pure nodes (bitwise arithmetic, etc)
45/// - Loads with the `readonly`, `notrap`, and `can_move` flags set
46pub fn is_pure_for_egraph(func: &Function, inst: Inst) -> bool {
47    let is_pure_load = match func.dfg.insts[inst] {
48        InstructionData::Load {
49            opcode: Opcode::Load,
50            flags,
51            ..
52        } => flags.readonly() && flags.notrap() && flags.can_move(),
53        _ => false,
54    };
55
56    // Multi-value results do not play nicely with much of the egraph
57    // infrastructure. They are in practice used only for multi-return
58    // calls and some other odd instructions (e.g. uadd_overflow) which,
59    // for now, we can afford to leave in place as opaque
60    // side-effecting ops. So if more than one result, then the inst
61    // is "not pure". Similarly, ops with zero results can be used
62    // only for their side-effects, so are never pure. (Or if they
63    // are, we can always trivially eliminate them with no effect.)
64    let has_one_result = func.dfg.inst_results(inst).len() == 1;
65
66    let op = func.dfg.insts[inst].opcode();
67
68    has_one_result && (is_pure_load || (!op.can_load() && !trivially_has_side_effects(op)))
69}
70
71/// Can the given instruction be merged into another copy of itself?
72/// These instructions may have side-effects, but as long as we retain
73/// the first instance of the instruction, the second and further
74/// instances are redundant if they would produce the same trap or
75/// result.
76pub fn is_mergeable_for_egraph(func: &Function, inst: Inst) -> bool {
77    let op = func.dfg.insts[inst].opcode();
78    // We can only merge zero- and one-result operators due to the way that GVN
79    // is structured in the egraph implementation.
80    func.dfg.inst_results(inst).len() <= 1
81        // Loads/stores are handled by alias analysis and not
82        // otherwise mergeable.
83        && !op.can_load()
84        && !op.can_store()
85        // Can only have idempotent side-effects.
86        && (!has_side_effect(func, inst) || op.side_effects_idempotent())
87}
88
89/// Does the given instruction have any side-effect as per [has_side_effect], or else is a load,
90/// but not the get_pinned_reg opcode?
91pub fn has_lowering_side_effect(func: &Function, inst: Inst) -> bool {
92    let op = func.dfg.insts[inst].opcode();
93    op != Opcode::GetPinnedReg && (has_side_effect(func, inst) || op.can_load())
94}
95
96/// Is the given instruction a constant value (`iconst`, `fconst`) that can be
97/// represented in 64 bits?
98pub fn is_constant_64bit(func: &Function, inst: Inst) -> Option<u64> {
99    match &func.dfg.insts[inst] {
100        &InstructionData::UnaryImm { imm, .. } => Some(imm.bits() as u64),
101        &InstructionData::UnaryIeee16 { imm, .. } => Some(imm.bits() as u64),
102        &InstructionData::UnaryIeee32 { imm, .. } => Some(imm.bits() as u64),
103        &InstructionData::UnaryIeee64 { imm, .. } => Some(imm.bits()),
104        _ => None,
105    }
106}
107
108/// Get the address, offset, and access type from the given instruction, if any.
109pub fn inst_addr_offset_type(func: &Function, inst: Inst) -> Option<(Value, Offset32, Type)> {
110    match &func.dfg.insts[inst] {
111        InstructionData::Load { arg, offset, .. } => {
112            let ty = func.dfg.value_type(func.dfg.inst_results(inst)[0]);
113            Some((*arg, *offset, ty))
114        }
115        InstructionData::LoadNoOffset { arg, .. } => {
116            let ty = func.dfg.value_type(func.dfg.inst_results(inst)[0]);
117            Some((*arg, 0.into(), ty))
118        }
119        InstructionData::Store { args, offset, .. } => {
120            let ty = func.dfg.value_type(args[0]);
121            Some((args[1], *offset, ty))
122        }
123        InstructionData::StoreNoOffset { args, .. } => {
124            let ty = func.dfg.value_type(args[0]);
125            Some((args[1], 0.into(), ty))
126        }
127        _ => None,
128    }
129}
130
131/// Get the store data, if any, from an instruction.
132pub fn inst_store_data(func: &Function, inst: Inst) -> Option<Value> {
133    match &func.dfg.insts[inst] {
134        InstructionData::Store { args, .. } | InstructionData::StoreNoOffset { args, .. } => {
135            Some(args[0])
136        }
137        _ => None,
138    }
139}
140
141/// Determine whether this opcode behaves as a memory fence, i.e.,
142/// prohibits any moving of memory accesses across it.
143pub fn has_memory_fence_semantics(op: Opcode) -> bool {
144    match op {
145        Opcode::AtomicRmw
146        | Opcode::AtomicCas
147        | Opcode::AtomicLoad
148        | Opcode::AtomicStore
149        | Opcode::Fence
150        | Opcode::Debugtrap => true,
151        Opcode::Call | Opcode::CallIndirect => true,
152        op if op.can_trap() => true,
153        _ => false,
154    }
155}
156
157/// Visit all successors of a block with a given visitor closure. The closure
158/// arguments are the branch instruction that is used to reach the successor,
159/// the successor block itself, and a flag indicating whether the block is
160/// branched to via a table entry.
161pub(crate) fn visit_block_succs<F: FnMut(Inst, Block, bool)>(
162    f: &Function,
163    block: Block,
164    mut visit: F,
165) {
166    if let Some(inst) = f.layout.last_inst(block) {
167        match &f.dfg.insts[inst] {
168            ir::InstructionData::Jump {
169                destination: dest, ..
170            } => {
171                visit(inst, dest.block(&f.dfg.value_lists), false);
172            }
173
174            ir::InstructionData::Brif {
175                blocks: [block_then, block_else],
176                ..
177            } => {
178                visit(inst, block_then.block(&f.dfg.value_lists), false);
179                visit(inst, block_else.block(&f.dfg.value_lists), false);
180            }
181
182            ir::InstructionData::BranchTable { table, .. } => {
183                let pool = &f.dfg.value_lists;
184                let table = &f.stencil.dfg.jump_tables[*table];
185
186                // The default block is reached via a direct conditional branch,
187                // so it is not part of the table. We visit the default block
188                // first explicitly, to mirror the traversal order of
189                // `JumpTableData::all_branches`, and transitively the order of
190                // `InstructionData::branch_destination`.
191                //
192                // Additionally, this case is why we are unable to replace this
193                // whole function with a loop over `branch_destination`: we need
194                // to report which branch targets come from the table vs the
195                // default.
196                visit(inst, table.default_block().block(pool), false);
197
198                for dest in table.as_slice() {
199                    visit(inst, dest.block(pool), true);
200                }
201            }
202
203            inst => debug_assert!(!inst.opcode().is_branch()),
204        }
205    }
206}
cranelift_codegen/inst_predicates.rs

cranelift_codegen/
inst_predicates.rs
