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Merged
pyk merged 1 commit into from
Dec 8, 2025
Merged

fix(run): prevent constant folding #10

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108 changes: 73 additions & 35 deletions src/root.zig
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Original file line number Diff line number Diff line change
Expand Up @@ -45,9 +45,12 @@ pub const ReportOptions = struct {
pub fn run(allocator: Allocator, name: []const u8, function: anytype, args: anytype, options: Options) !Metrics {
assertFunctionDef(function, args);

// ref: https://pyk.sh/blog/2025-12-08-bench-fixing-constant-folding
var runtime_args = createRuntimeArgs(function, args);
const volatile_args_ptr: *volatile @TypeOf(runtime_args) = &runtime_args;

for (0..options.warmup_iters) |_| {
std.mem.doNotOptimizeAway(args);
try execute(function, args);
try execute(function, volatile_args_ptr.*);
}

// We need to determine a batch_size such that the total execution time of the batch
Expand All @@ -60,8 +63,7 @@ pub fn run(allocator: Allocator, name: []const u8, function: anytype, args: anyt
while (true) {
timer.reset();
for (0..batch_size) |_| {
std.mem.doNotOptimizeAway(args);
try execute(function, args);
try execute(function, volatile_args_ptr.*);
}
const duration = timer.read();

Expand All @@ -87,8 +89,7 @@ pub fn run(allocator: Allocator, name: []const u8, function: anytype, args: anyt
for (0..options.sample_size) |i| {
timer.reset();
for (0..batch_size) |_| {
std.mem.doNotOptimizeAway(args);
try execute(function, args);
try execute(function, volatile_args_ptr.*);
}
const total_ns = timer.read();
// Average time per operation for this batch
Expand Down Expand Up @@ -141,8 +142,7 @@ pub fn run(allocator: Allocator, name: []const u8, function: anytype, args: anyt
try perf.capture();
for (0..options.sample_size) |_| {
for (0..batch_size) |_| {
std.mem.doNotOptimizeAway(args);
try execute(function, args);
try execute(function, volatile_args_ptr.*);
}
}
try perf.stop();
Expand All @@ -165,48 +165,86 @@ pub fn run(allocator: Allocator, name: []const u8, function: anytype, args: anyt
return metrics;
}

inline fn execute(function: anytype, args: anytype) !void {
const FnType = unwrapFnType(@TypeOf(function));
const return_type = @typeInfo(FnType).@"fn".return_type.?;
// Conditional execution based on whether the function can fail
if (@typeInfo(return_type) == .error_union) {
const result = try @call(.auto, function, args);
std.mem.doNotOptimizeAway(result);
} else {
const result = @call(.auto, function, args);
std.mem.doNotOptimizeAway(result);
}
}

/// Returns the underlying Function type, unwrapping it if it is a pointer.
fn unwrapFnType(comptime T: type) type {
if (@typeInfo(T) == .pointer) return @typeInfo(T).pointer.child;
return T;
}

////////////////////////////////////////////////////////////////////////////////
// Function definition checker

fn assertFunctionDef(function: anytype, args: anytype) void {
// Verify args is a tuple
const ArgsType = @TypeOf(args);
const args_info = @typeInfo(ArgsType);
if (args_info != .@"struct" or !args_info.@"struct".is_tuple) {
@compileError("Expected 'args' to be a tuple, found " ++ @typeName(ArgsType));
@compileError("Expected 'args' to be a tuple, found '" ++ @typeName(ArgsType) ++ "'");
}

// Unwrap function type
const FnType = @TypeOf(function);
const UnwrappedFnType = if (@typeInfo(FnType) == .pointer)
@typeInfo(FnType).pointer.child
else
FnType;
const fn_info = @typeInfo(UnwrappedFnType);
if (fn_info != .@"fn") {
@compileError("Expected 'function' to be a function or function pointer, found " ++ @typeName(@TypeOf(function)));
const FnType = unwrapFnType(@TypeOf(function));
if (@typeInfo(FnType) != .@"fn") {
@compileError("Expected 'function' to be a function or function pointer, found '" ++ @typeName(@TypeOf(function)) ++ "'");
}

// Verify argument count matches
if (fn_info.@"fn".params.len != args_info.@"struct".fields.len) {
const params_len = @typeInfo(FnType).@"fn".params.len;
const args_len = @typeInfo(ArgsType).@"struct".fields.len;

if (params_len != args_len) {
@compileError(std.fmt.comptimePrint(
"Function expects {d} arguments, but args tuple has {d}",
.{ fn_info.@"fn".params.len, args_info.@"struct".fields.len },
.{ params_len, args_len },
));
}
}

inline fn execute(function: anytype, args: anytype) !void {
const FnType = @TypeOf(function);
const UnwrappedFnType = if (@typeInfo(FnType) == .pointer)
@typeInfo(FnType).pointer.child
else
FnType;
const return_type = @typeInfo(UnwrappedFnType).@"fn".return_type.?;
if (@typeInfo(return_type) == .error_union) {
const result = try @call(.auto, function, args);
std.mem.doNotOptimizeAway(result);
} else {
const result = @call(.auto, function, args);
std.mem.doNotOptimizeAway(result);
////////////////////////////////////////////////////////////////////////////////
// Runtime Arguments Helpers

/// Constructs the runtime argument tuple based on function parameters and input args.
fn createRuntimeArgs(function: anytype, args: anytype) RuntimeArgsType(@TypeOf(function), @TypeOf(args)) {
const TupleType = RuntimeArgsType(@TypeOf(function), @TypeOf(args));
var runtime_args: TupleType = undefined;

// We only need the length here to iterate
const fn_params = getFnParams(@TypeOf(function));

inline for (0..fn_params.len) |i| {
runtime_args[i] = args[i];
}
return runtime_args;
}

/// Computes the precise Tuple type required to hold the arguments.
fn RuntimeArgsType(comptime FnType: type, comptime ArgsType: type) type {
const fn_params = getFnParams(FnType);
const args_fields = @typeInfo(ArgsType).@"struct".fields;
comptime var types: [fn_params.len]type = undefined;
inline for (fn_params, 0..) |p, i| {
if (p.type) |t| {
types[i] = t;
} else {
types[i] = args_fields[i].type;
}
}
return std.meta.Tuple(&types);
}

/// Helper to unwrap function pointers and retrieve parameter info
fn getFnParams(comptime FnType: type) []const std.builtin.Type.Fn.Param {
return @typeInfo(unwrapFnType(FnType)).@"fn".params;
}

////////////////////////////////////////////////////////////////////////////////
Expand Down
34 changes: 34 additions & 0 deletions src/test.zig
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Original file line number Diff line number Diff line change
Expand Up @@ -215,3 +215,37 @@ test "run: suppported signatures" {
_ = try bench.run(allocator, "functionReturnValue", functionReturnValue, .{}, .{});
_ = try bench.run(allocator, "functionReturnValueError", functionReturnValueError, .{}, .{});
}

///////////////////////////////////////////////////////////////////////////////
// Fibonacci

fn fibNaive(n: u64) u64 {
if (n <= 1) return n;
return fibNaive(n - 1) + fibNaive(n - 2);
}

fn fibIterative(n: u64) u64 {
if (n == 0) return 0;

var a: u64 = 0;
var b: u64 = 1;
for (2..n + 1) |_| {
const c = a + b;
a = b;
b = c;
}

return b;
}

test "run: fibonacci" {
const allocator = std.heap.smp_allocator;
const opts = bench.Options{
.sample_size = 100,
.warmup_iters = 3,
};
const m_naive = try bench.run(allocator, "fibNaive", fibNaive, .{@as(u64, 30)}, opts);
const m_iter = try bench.run(allocator, "fibIterative", fibIterative, .{@as(u64, 30)}, opts);

try testing.expect(m_naive.mean_ns > m_iter.mean_ns * 100);
}