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Normalize fuel costs to roughly 1 fuel per executed instruction #705

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Mar 3, 2023
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Normalize fuel costs to roughly 1 fuel per executed instruction #705

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11 changes: 1 addition & 10 deletions crates/wasmi/src/engine/bytecode/utils.rs
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Original file line number Diff line number Diff line change
@@ -1,4 +1,4 @@
use crate::engine::{config::FuelCosts, Instr};
use crate::engine::Instr;
use core::fmt::Display;

/// Defines how many stack values are going to be dropped and kept after branching.
Expand Down Expand Up @@ -62,15 +62,6 @@ impl DropKeep {
pub fn keep(self) -> usize {
self.keep as usize
}

/// Returns the fuel consumption required for this [`DropKeep`].
pub fn fuel_consumption(self, costs: &FuelCosts) -> u64 {
if self.drop == 0 {
// If nothing is dropped no copy is required.
return 0;
}
u64::from(self.keep) * costs.branch_per_kept
}
}

/// A function index.
Expand Down
94 changes: 74 additions & 20 deletions crates/wasmi/src/engine/config.rs
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Original file line number Diff line number Diff line change
@@ -1,4 +1,6 @@
use super::stack::StackLimits;
use super::{stack::StackLimits, DropKeep};
use core::{mem::size_of, num::NonZeroU64};
use wasmi_core::UntypedValue;
use wasmparser::WasmFeatures;

/// The default amount of stacks kept in the cache at most.
Expand Down Expand Up @@ -54,33 +56,85 @@ pub struct FuelCosts {
pub store: u64,
/// The fuel cost offset for `call` and `call_indirect` instructions.
pub call: u64,
/// The fuel cost offset per local variable for `call` and `call_indirect` instruction.
/// Determines how many moved stack values consume one fuel upon a branch or return instruction.
///
/// # Note
///
/// This is also applied to all function parameters since
/// they are translated to local variable slots.
pub call_per_local: u64,
/// The fuel cost offset per kept value for branches that need to copy values on the stack.
pub branch_per_kept: u64,
/// The fuel cost offset per byte for `bulk-memory` instructions.
pub memory_per_byte: u64,
/// The fuel cost offset per element for `bulk-table` instructions.
pub table_per_element: u64,
/// If this is zero then processing [`DropKeep`] costs nothing.
branch_kept_per_fuel: u64,
/// Determines how many function locals consume one fuel per function call.
///
/// # Note
///
/// - This is also applied to all function parameters since
/// they are translated to local variable slots.
/// - If this is zero then processing function locals costs nothing.
func_locals_per_fuel: u64,
/// How many memory bytes can be processed per fuel in a `bulk-memory` instruction.
///
/// # Note
///
/// If this is zero then processing memory bytes costs nothing.
memory_bytes_per_fuel: u64,
/// How many table elements can be processed per fuel in a `bulk-table` instruction.
///
/// # Note
///
/// If this is zero then processing table elements costs nothing.
table_elements_per_fuel: u64,
}

impl FuelCosts {
/// Returns the fuel consumption of the amount of items with costs per items.
fn costs_per(len_items: u64, items_per_fuel: u64) -> u64 {
NonZeroU64::new(items_per_fuel)
.map(|items_per_fuel| len_items / items_per_fuel)
.unwrap_or(0)
}

/// Returns the fuel consumption for branches and returns using the given [`DropKeep`].
pub fn fuel_for_drop_keep(&self, drop_keep: DropKeep) -> u64 {
if drop_keep.drop() == 0 {
return 0;
}
Self::costs_per(drop_keep.keep() as u64, self.branch_kept_per_fuel)
}

/// Returns the fuel consumption for calling a function with the amount of local variables.
///
/// # Note
///
/// Function parameters are also treated as local variables.
pub fn fuel_for_locals(&self, locals: u64) -> u64 {
Self::costs_per(locals, self.func_locals_per_fuel)
}

/// Returns the fuel consumption for processing the amount of memory bytes.
pub fn fuel_for_bytes(&self, bytes: u64) -> u64 {
Self::costs_per(bytes, self.memory_bytes_per_fuel)
}

/// Returns the fuel consumption for processing the amount of table elements.
pub fn fuel_for_elements(&self, elements: u64) -> u64 {
Self::costs_per(elements, self.table_elements_per_fuel)
}
}

impl Default for FuelCosts {
fn default() -> Self {
let memory_bytes_per_fuel = 64;
let bytes_per_register = size_of::<UntypedValue>() as u64;
let registers_per_fuel = memory_bytes_per_fuel / bytes_per_register;
Self {
base: 100,
entity: 500,
load: 650,
store: 450,
call: 1500,
call_per_local: 10,
branch_per_kept: 10,
memory_per_byte: 2,
table_per_element: 10,
base: 1,
entity: 1,
load: 1,
store: 1,
call: 1,
func_locals_per_fuel: registers_per_fuel,
branch_kept_per_fuel: registers_per_fuel,
memory_bytes_per_fuel,
table_elements_per_fuel: registers_per_fuel,
}
}
}
Expand Down
16 changes: 8 additions & 8 deletions crates/wasmi/src/engine/executor.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -895,7 +895,7 @@ impl<'ctx, 'engine> Executor<'ctx, 'engine> {
let result = self.consume_fuel_on_success(
|costs| {
let delta_in_bytes = delta.to_bytes().unwrap_or(0) as u64;
delta_in_bytes * costs.memory_per_byte
costs.fuel_for_bytes(delta_in_bytes)
},
|this| {
let memory = this.cache.default_memory(this.ctx);
Expand Down Expand Up @@ -929,7 +929,7 @@ impl<'ctx, 'engine> Executor<'ctx, 'engine> {
let offset = i32::from(d) as usize;
let byte = u8::from(val);
self.consume_fuel_on_success(
|costs| n as u64 * costs.memory_per_byte,
|costs| costs.fuel_for_bytes(n as u64),
|this| {
let memory = this
.cache
Expand All @@ -952,7 +952,7 @@ impl<'ctx, 'engine> Executor<'ctx, 'engine> {
let src_offset = i32::from(s) as usize;
let dst_offset = i32::from(d) as usize;
self.consume_fuel_on_success(
|costs| n as u64 * costs.memory_per_byte,
|costs| costs.fuel_for_bytes(n as u64),
|this| {
let data = this.cache.default_memory_bytes(this.ctx);
// These accesses just perform the bounds checks required by the Wasm spec.
Expand All @@ -977,7 +977,7 @@ impl<'ctx, 'engine> Executor<'ctx, 'engine> {
let src_offset = i32::from(s) as usize;
let dst_offset = i32::from(d) as usize;
self.consume_fuel_on_success(
|costs| n as u64 * costs.memory_per_byte,
|costs| costs.fuel_for_bytes(n as u64),
|this| {
let (memory, data) = this
.cache
Expand Down Expand Up @@ -1019,7 +1019,7 @@ impl<'ctx, 'engine> Executor<'ctx, 'engine> {
let (init, delta) = self.sp.pop2();
let delta: u32 = delta.into();
let result = self.consume_fuel_on_success(
|costs| u64::from(delta) * costs.table_per_element,
|costs| costs.fuel_for_elements(u64::from(delta)),
|this| {
let table = this.cache.get_table(this.ctx, table_index);
this.ctx
Expand All @@ -1044,7 +1044,7 @@ impl<'ctx, 'engine> Executor<'ctx, 'engine> {
let dst: u32 = i.into();
let len: u32 = n.into();
self.consume_fuel_on_success(
|costs| u64::from(len) * costs.table_per_element,
|costs| costs.fuel_for_elements(u64::from(len)),
|this| {
let table = this.cache.get_table(this.ctx, table_index);
this.ctx
Expand Down Expand Up @@ -1089,7 +1089,7 @@ impl<'ctx, 'engine> Executor<'ctx, 'engine> {
let src_index = u32::from(s);
let dst_index = u32::from(d);
self.consume_fuel_on_success(
|costs| u64::from(len) * costs.table_per_element,
|costs| costs.fuel_for_elements(u64::from(len)),
|this| {
// Query both tables and check if they are the same:
let dst = this.cache.get_table(this.ctx, dst);
Expand Down Expand Up @@ -1121,7 +1121,7 @@ impl<'ctx, 'engine> Executor<'ctx, 'engine> {
let src_index = u32::from(s);
let dst_index = u32::from(d);
self.consume_fuel_on_success(
|costs| u64::from(len) * costs.table_per_element,
|costs| costs.fuel_for_elements(u64::from(len)),
|this| {
let (instance, table, element) = this
.cache
Expand Down
11 changes: 11 additions & 0 deletions crates/wasmi/src/engine/func_builder/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -65,6 +65,17 @@ impl<'parser> FuncBuilder<'parser> {
Ok(())
}

/// This informs the [`FuncBuilder`] that the function header translation is finished.
///
/// # Note
///
/// This was introduced to properly calculate the fuel costs for all local variables
/// and function parameters. After this function call no more locals and parameters may
/// be added to this function translation.
pub fn finish_translate_locals(&mut self) {
self.translator.finish_translate_locals()
}

/// Updates the current position within the Wasm binary while parsing operators.
pub fn update_pos(&mut self, pos: usize) {
self.pos = pos;
Expand Down
33 changes: 23 additions & 10 deletions crates/wasmi/src/engine/func_builder/translator.rs
View file
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Original file line number Diff line number Diff line change
Expand Up @@ -153,7 +153,6 @@ impl<'parser> FuncTranslator<'parser> {
/// Registers the function parameters in the emulated value stack.
fn init_func_params(&mut self) {
for _param_type in self.func_type().params() {
self.bump_fuel_consumption(self.fuel_costs().call_per_local);
self.locals.register_locals(1);
}
}
Expand All @@ -164,11 +163,23 @@ impl<'parser> FuncTranslator<'parser> {
///
/// If too many local variables have been registered.
pub fn register_locals(&mut self, amount: u32) {
let fuel_costs = u64::from(amount) * self.fuel_costs().call_per_local;
self.bump_fuel_consumption(fuel_costs);
self.locals.register_locals(amount);
}

/// This informs the [`FuncTranslator`] that the function header translation is finished.
///
/// # Note
///
/// This was introduced to properly calculate the fuel costs for all local variables
/// and function parameters. After this function call no more locals and parameters may
/// be added to this function translation.
pub fn finish_translate_locals(&mut self) {
self.bump_fuel_consumption(
self.fuel_costs()
.fuel_for_locals(u64::from(self.locals.len_registered())),
)
}

/// Finishes constructing the function and returns its [`FuncBody`].
pub fn finish(&mut self) -> Result<FuncBody, TranslationError> {
let func_body = self.alloc.inst_builder.finish(
Expand Down Expand Up @@ -996,7 +1007,8 @@ impl<'a> VisitOperator<'a> for FuncTranslator<'a> {
match builder.acquire_target(relative_depth)? {
AcquiredTarget::Branch(end_label, drop_keep) => {
builder.bump_fuel_consumption(builder.fuel_costs().base);
builder.bump_fuel_consumption(drop_keep.fuel_consumption(builder.fuel_costs()));
builder
.bump_fuel_consumption(builder.fuel_costs().fuel_for_drop_keep(drop_keep));
let params = builder.branch_params(end_label, drop_keep);
builder
.alloc
Expand All @@ -1019,7 +1031,8 @@ impl<'a> VisitOperator<'a> for FuncTranslator<'a> {
match builder.acquire_target(relative_depth)? {
AcquiredTarget::Branch(end_label, drop_keep) => {
builder.bump_fuel_consumption(builder.fuel_costs().base);
builder.bump_fuel_consumption(drop_keep.fuel_consumption(builder.fuel_costs()));
builder
.bump_fuel_consumption(builder.fuel_costs().fuel_for_drop_keep(drop_keep));
let params = builder.branch_params(end_label, drop_keep);
builder
.alloc
Expand Down Expand Up @@ -1052,7 +1065,7 @@ impl<'a> VisitOperator<'a> for FuncTranslator<'a> {
match builder.acquire_target(depth.into_u32())? {
AcquiredTarget::Branch(label, drop_keep) => {
*max_drop_keep_fuel = (*max_drop_keep_fuel)
.max(drop_keep.fuel_consumption(builder.fuel_costs()));
.max(builder.fuel_costs().fuel_for_drop_keep(drop_keep));
let base = builder.alloc.inst_builder.current_pc();
let instr = offset_instr(base, n + 1);
let offset = builder
Expand All @@ -1065,7 +1078,7 @@ impl<'a> VisitOperator<'a> for FuncTranslator<'a> {
}
AcquiredTarget::Return(drop_keep) => {
*max_drop_keep_fuel = (*max_drop_keep_fuel)
.max(drop_keep.fuel_consumption(builder.fuel_costs()));
.max(builder.fuel_costs().fuel_for_drop_keep(drop_keep));
Ok(Instruction::Return(drop_keep))
}
}
Expand Down Expand Up @@ -1118,7 +1131,7 @@ impl<'a> VisitOperator<'a> for FuncTranslator<'a> {
self.translate_if_reachable(|builder| {
let drop_keep = builder.drop_keep_return()?;
builder.bump_fuel_consumption(builder.fuel_costs().base);
builder.bump_fuel_consumption(drop_keep.fuel_consumption(builder.fuel_costs()));
builder.bump_fuel_consumption(builder.fuel_costs().fuel_for_drop_keep(drop_keep));
builder
.alloc
.inst_builder
Expand All @@ -1134,7 +1147,7 @@ impl<'a> VisitOperator<'a> for FuncTranslator<'a> {
let func_type = builder.func_type_of(func_idx.into());
let drop_keep = builder.drop_keep_return_call(&func_type)?;
builder.bump_fuel_consumption(builder.fuel_costs().call);
builder.bump_fuel_consumption(drop_keep.fuel_consumption(builder.fuel_costs()));
builder.bump_fuel_consumption(builder.fuel_costs().fuel_for_drop_keep(drop_keep));
builder
.alloc
.inst_builder
Expand All @@ -1156,7 +1169,7 @@ impl<'a> VisitOperator<'a> for FuncTranslator<'a> {
builder.stack_height.pop1();
let drop_keep = builder.drop_keep_return_call(&func_type)?;
builder.bump_fuel_consumption(builder.fuel_costs().call);
builder.bump_fuel_consumption(drop_keep.fuel_consumption(builder.fuel_costs()));
builder.bump_fuel_consumption(builder.fuel_costs().fuel_for_drop_keep(drop_keep));
builder
.alloc
.inst_builder
Expand Down
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