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WIP #23151

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a58e4c0
force heap allocators
dylan-conway Oct 1, 2025
302b1e2
update
dylan-conway Oct 1, 2025
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121 changes: 121 additions & 0 deletions src/allocators.zig
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Original file line number Diff line number Diff line change
Expand Up @@ -928,6 +928,127 @@ const basic = if (bun.use_mimalloc)
else
@import("./allocators/fallback.zig");

pub fn stackFallback(comptime size: usize, fallback_allocator: std.mem.Allocator) StackFallbackAllocator(size) {
return StackFallbackAllocator(size){
.buffer = undefined,
.fallback_allocator = fallback_allocator,
.fixed_buffer_allocator = undefined,
.force_heap = if (comptime Environment.ci_assert)
!bun.getRuntimeFeatureFlag(.BUN_DEBUG_FORCE_HEAP_FALLBACK_ALLOCATORS)
else {},
};
Comment on lines +936 to +939

@coderabbitai coderabbitai Bot Oct 1, 2025

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⚠️ Potential issue | 🔴 Critical

force_heap flag is inverted (breaks CI “force heap” behavior).

The PR description says the runtime feature flag forces heap fallback in CI. Current code negates it, so enabling BUN_DEBUG_FORCE_HEAP_FALLBACK_ALLOCATORS disables forcing.

Apply this diff:

-        .force_heap = if (comptime Environment.ci_assert)
-            !bun.getRuntimeFeatureFlag(.BUN_DEBUG_FORCE_HEAP_FALLBACK_ALLOCATORS)
-        else {},
+        .force_heap = if (comptime Environment.ci_assert)
+            bun.getRuntimeFeatureFlag(.BUN_DEBUG_FORCE_HEAP_FALLBACK_ALLOCATORS)
+        else {},
📝 Committable suggestion

‼️ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.

Suggested change
.force_heap = if (comptime Environment.ci_assert)
!bun.getRuntimeFeatureFlag(.BUN_DEBUG_FORCE_HEAP_FALLBACK_ALLOCATORS)
else {},
};
.force_heap = if (comptime Environment.ci_assert)
bun.getRuntimeFeatureFlag(.BUN_DEBUG_FORCE_HEAP_FALLBACK_ALLOCATORS)
else {},
};
🤖 Prompt for AI Agents 
In src/allocators.zig around lines 936 to 939, the .force_heap assignment
inverts the BUN_DEBUG_FORCE_HEAP_FALLBACK_ALLOCATORS flag (using !) so enabling
the flag disables the CI "force heap" behavior; remove the negation so
.force_heap uses
bun.getRuntimeFeatureFlag(.BUN_DEBUG_FORCE_HEAP_FALLBACK_ALLOCATORS) directly
when Environment.ci_assert is true, leaving the else branch unchanged.

}

/// An allocator that attempts to allocate using a
/// `FixedBufferAllocator` using an array of size `size`. If the
/// allocation fails, it will fall back to using
/// `fallback_allocator`. Easily created with `stackFallback`.
pub fn StackFallbackAllocator(comptime size: usize) type {
return struct {
const Self = @This();

buffer: [size]u8,
fallback_allocator: std.mem.Allocator,
fixed_buffer_allocator: std.heap.FixedBufferAllocator,
get_called: if (Environment.ci_assert) bool else void = if (Environment.ci_assert) false,
force_heap: if (Environment.ci_assert) bool else void,

/// This function both fetches a `Allocator` interface to this
/// allocator *and* resets the internal buffer allocator.
pub fn get(self: *Self) std.mem.Allocator {
if (comptime Environment.ci_assert) {
bun.assert(!self.get_called); // `get` called multiple times; instead use `const allocator = stackFallback(N).get();`
self.get_called = true;
}
self.fixed_buffer_allocator = std.heap.FixedBufferAllocator.init(self.buffer[0..]);
return .{
.ptr = self,
.vtable = &.{
.alloc = alloc,
.resize = resize,
.remap = remap,
.free = free,
},
};
}

fn alloc(
ctx: *anyopaque,
len: usize,
alignment: std.mem.Alignment,
ra: usize,
) ?[*]u8 {
const self: *Self = @ptrCast(@alignCast(ctx));
if (comptime Environment.ci_assert) {
if (self.force_heap) {
return self.fallback_allocator.rawAlloc(len, alignment, ra);
}
}
return std.heap.FixedBufferAllocator.alloc(&self.fixed_buffer_allocator, len, alignment, ra) orelse
return self.fallback_allocator.rawAlloc(len, alignment, ra);
}

fn resize(
ctx: *anyopaque,
buf: []u8,
alignment: std.mem.Alignment,
new_len: usize,
ra: usize,
) bool {
const self: *Self = @ptrCast(@alignCast(ctx));
if (comptime Environment.ci_assert) {
if (self.force_heap) {
return self.fallback_allocator.rawResize(buf, alignment, new_len, ra);
}
}
if (self.fixed_buffer_allocator.ownsPtr(buf.ptr)) {
return std.heap.FixedBufferAllocator.resize(&self.fixed_buffer_allocator, buf, alignment, new_len, ra);
} else {
return self.fallback_allocator.rawResize(buf, alignment, new_len, ra);
}
}

fn remap(
context: *anyopaque,
memory: []u8,
alignment: std.mem.Alignment,
new_len: usize,
return_address: usize,
) ?[*]u8 {
const self: *Self = @ptrCast(@alignCast(context));
if (comptime Environment.ci_assert) {
if (self.force_heap) {
return self.fallback_allocator.rawRemap(memory, alignment, new_len, return_address);
}
}
if (self.fixed_buffer_allocator.ownsPtr(memory.ptr)) {
return std.heap.FixedBufferAllocator.remap(&self.fixed_buffer_allocator, memory, alignment, new_len, return_address);
} else {
return self.fallback_allocator.rawRemap(memory, alignment, new_len, return_address);
}
}

fn free(
ctx: *anyopaque,
buf: []u8,
alignment: std.mem.Alignment,
ra: usize,
) void {
const self: *Self = @ptrCast(@alignCast(ctx));
if (comptime Environment.ci_assert) {
if (self.force_heap) {
return self.fallback_allocator.rawFree(buf, alignment, ra);
}
}
if (self.fixed_buffer_allocator.ownsPtr(buf.ptr)) {
return std.heap.FixedBufferAllocator.free(&self.fixed_buffer_allocator, buf, alignment, ra);
} else {
return self.fallback_allocator.rawFree(buf, alignment, ra);
}
}
};
}

const Environment = @import("./env.zig");
const std = @import("std");

Expand Down
2 changes: 1 addition & 1 deletion src/ast/P.zig
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Expand Up @@ -3537,7 +3537,7 @@ pub fn NewParser_(
// Insert any relocated variable statements now
if (p.relocated_top_level_vars.items.len > 0) {
var already_declared = RefMap{};
var already_declared_allocator_stack = std.heap.stackFallback(1024, allocator);
var already_declared_allocator_stack = bun.allocators.stackFallback(1024, allocator);
const already_declared_allocator = already_declared_allocator_stack.get();
defer if (already_declared_allocator_stack.fixed_buffer_allocator.end_index >= 1023) already_declared.deinit(already_declared_allocator);

Expand Down
28 changes: 14 additions & 14 deletions src/bake/DevServer.zig
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Expand Up @@ -1109,7 +1109,7 @@ fn ensureRouteIsBundled(
}

// Prepare a bundle with just this route.
var sfa = std.heap.stackFallback(4096, dev.allocator());
var sfa = bun.allocators.stackFallback(4096, dev.allocator());
const temp_alloc = sfa.get();

var entry_points: EntryPointList = .empty;
Expand Down Expand Up @@ -1242,7 +1242,7 @@ fn checkRouteFailures(
route_bundle_index: RouteBundle.Index,
resp: DevResponse,
) !enum { stop, ok, rebuild } {
var sfa_state = std.heap.stackFallback(65536, dev.allocator());
var sfa_state = bun.allocators.stackFallback(65536, dev.allocator());
const sfa = sfa_state.get();
var gts = try dev.initGraphTraceState(sfa, 0);
defer gts.deinit(sfa);
Expand Down Expand Up @@ -1535,7 +1535,7 @@ fn generateHTMLPayload(dev: *DevServer, route_bundle_index: RouteBundle.Index, r
defer dev.graph_safety_lock.unlock();

// Prepare bitsets for tracing
var sfa_state = std.heap.stackFallback(65536, dev.allocator());
var sfa_state = bun.allocators.stackFallback(65536, dev.allocator());
const sfa = sfa_state.get();
var gts = try dev.initGraphTraceState(sfa, 0);
defer gts.deinit(sfa);
Expand Down Expand Up @@ -1587,7 +1587,7 @@ fn generateJavaScriptCodeForHTMLFile(
input_file_sources: []bun.logger.Source,
loaders: []bun.options.Loader,
) bun.OOM![]const u8 {
var sfa_state = std.heap.stackFallback(65536, dev.allocator());
var sfa_state = bun.allocators.stackFallback(65536, dev.allocator());
const sfa = sfa_state.get();
var array = bun.handleOom(std.ArrayListUnmanaged(u8).initCapacity(sfa, 65536));
defer array.deinit(sfa);
Expand Down Expand Up @@ -1836,7 +1836,7 @@ pub fn startAsyncBundle(

// Notify inspector about bundle start
if (dev.inspector()) |agent| {
var sfa_state = std.heap.stackFallback(256, dev.allocator());
var sfa_state = bun.allocators.stackFallback(256, dev.allocator());
const sfa = sfa_state.get();
var trigger_files = try std.ArrayList(bun.String).initCapacity(sfa, entry_points.set.count());
defer trigger_files.deinit();
Expand Down Expand Up @@ -1931,7 +1931,7 @@ pub fn prepareAndLogResolutionFailures(dev: *DevServer) !void {

fn indexFailures(dev: *DevServer) !void {
// After inserting failures into the IncrementalGraphs, they are traced to their routes.
var sfa_state = std.heap.stackFallback(65536, dev.allocator());
var sfa_state = bun.allocators.stackFallback(65536, dev.allocator());
const sfa = sfa_state.get();

if (dev.incremental_result.failures_added.items.len > 0) {
Expand Down Expand Up @@ -2014,7 +2014,7 @@ fn generateClientBundle(dev: *DevServer, route_bundle: *RouteBundle) bun.OOM![]u
defer dev.graph_safety_lock.unlock();

// Prepare bitsets
var sfa_state = std.heap.stackFallback(65536, dev.allocator());
var sfa_state = bun.allocators.stackFallback(65536, dev.allocator());
const sfa = sfa_state.get();
var gts = try dev.initGraphTraceState(sfa, 0);
defer gts.deinit(sfa);
Expand Down Expand Up @@ -2078,7 +2078,7 @@ fn generateCssJSArray(dev: *DevServer, route_bundle: *RouteBundle) bun.JSError!j
defer dev.graph_safety_lock.unlock();

// Prepare bitsets
var sfa_state = std.heap.stackFallback(65536, dev.allocator());
var sfa_state = bun.allocators.stackFallback(65536, dev.allocator());

const sfa = sfa_state.get();
var gts = try dev.initGraphTraceState(sfa, 0);
Expand Down Expand Up @@ -2252,7 +2252,7 @@ pub fn finalizeBundle(
const targets = bv2.graph.ast.items(.target);
const scbs = bv2.graph.server_component_boundaries.slice();

var sfa = std.heap.stackFallback(65536, bv2.allocator());
var sfa = bun.allocators.stackFallback(65536, bv2.allocator());
const stack_alloc = sfa.get();
var scb_bitset = try bun.bit_set.DynamicBitSetUnmanaged.initEmpty(stack_alloc, input_file_sources.len);
for (
Expand Down Expand Up @@ -2569,7 +2569,7 @@ pub fn finalizeBundle(

var has_route_bits_set = false;

var hot_update_payload_sfa = std.heap.stackFallback(65536, dev.allocator());
var hot_update_payload_sfa = bun.allocators.stackFallback(65536, dev.allocator());
var hot_update_payload = std.ArrayList(u8).initCapacity(hot_update_payload_sfa.get(), 65536) catch
unreachable; // enough space
defer hot_update_payload.deinit();
Expand Down Expand Up @@ -2976,7 +2976,7 @@ fn startNextBundleIfPresent(dev: *DevServer) void {

// If there were pending requests, begin another bundle.
if (dev.next_bundle.reload_event != null or dev.next_bundle.requests.first != null or dev.next_bundle.promise.strong.hasValue()) {
var sfb = std.heap.stackFallback(4096, dev.allocator());
var sfb = bun.allocators.stackFallback(4096, dev.allocator());
const temp_alloc = sfb.get();
var entry_points: EntryPointList = .empty;
defer entry_points.deinit(temp_alloc);
Expand Down Expand Up @@ -3610,7 +3610,7 @@ pub fn emitVisualizerMessageIfNeeded(dev: *DevServer) void {
defer dev.emitMemoryVisualizerMessageIfNeeded();
if (dev.emit_incremental_visualizer_events == 0) return;

var sfb = std.heap.stackFallback(65536, dev.allocator());
var sfb = bun.allocators.stackFallback(65536, dev.allocator());
var payload = std.ArrayList(u8).initCapacity(sfb.get(), 65536) catch
unreachable; // enough capacity on the stack
defer payload.deinit();
Expand Down Expand Up @@ -3640,7 +3640,7 @@ pub fn emitMemoryVisualizerMessage(dev: *DevServer) void {
comptime assert(bun.FeatureFlags.bake_debugging_features);
bun.debugAssert(dev.emit_memory_visualizer_events > 0);

var sfb = std.heap.stackFallback(65536, dev.allocator());
var sfb = bun.allocators.stackFallback(65536, dev.allocator());
var payload = std.ArrayList(u8).initCapacity(sfb.get(), 65536) catch
unreachable; // enough capacity on the stack
defer payload.deinit();
Expand Down Expand Up @@ -4215,7 +4215,7 @@ fn dumpStateDueToCrash(dev: *DevServer) !void {
try file.writeAll(start);
try file.writeAll("\nlet inlinedData = Uint8Array.from(atob(\"");

var sfb = std.heap.stackFallback(4096, dev.allocator());
var sfb = bun.allocators.stackFallback(4096, dev.allocator());
var payload = try std.ArrayList(u8).initCapacity(sfb.get(), 4096);
defer payload.deinit();
try dev.writeVisualizerMessage(&payload);
Expand Down
4 changes: 2 additions & 2 deletions src/bake/DevServer/ErrorReportRequest.zig
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Expand Up @@ -41,8 +41,8 @@ pub fn runWithBody(ctx: *ErrorReportRequest, body: []const u8, r: AnyResponse) !
var s = std.io.fixedBufferStream(body);
const reader = s.reader();

var sfa_general = std.heap.stackFallback(65536, ctx.dev.allocator());
var sfa_sourcemap = std.heap.stackFallback(65536, ctx.dev.allocator());
var sfa_general = bun.allocators.stackFallback(65536, ctx.dev.allocator());
var sfa_sourcemap = bun.allocators.stackFallback(65536, ctx.dev.allocator());
const temp_alloc = sfa_general.get();
var arena = std.heap.ArenaAllocator.init(temp_alloc);
defer arena.deinit();
Expand Down
2 changes: 1 addition & 1 deletion src/bake/DevServer/HotReloadEvent.zig
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Expand Up @@ -187,7 +187,7 @@ pub fn run(first: *HotReloadEvent) void {
return;
}

var sfb = std.heap.stackFallback(4096, dev.allocator());
var sfb = bun.allocators.stackFallback(4096, dev.allocator());
const temp_alloc = sfb.get();
var entry_points: EntryPointList = .empty;
defer entry_points.deinit(temp_alloc);
Expand Down
4 changes: 2 additions & 2 deletions src/bake/DevServer/IncrementalGraph.zig
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Expand Up @@ -878,7 +878,7 @@ pub fn IncrementalGraph(comptime side: bake.Side) type {
bun.assert(bundler_index.isValid());
bun.assert(ctx.loaders[bundler_index.get()].isCSS());

var sfb = std.heap.stackFallback(@sizeOf(bun.ast.Index) * 64, temp_alloc);
var sfb = bun.allocators.stackFallback(@sizeOf(bun.ast.Index) * 64, temp_alloc);
const queue_alloc = sfb.get();

// This queue avoids stack overflow.
Expand Down Expand Up @@ -1728,7 +1728,7 @@ pub fn IncrementalGraph(comptime side: bake.Side) type {
// to inform the HMR runtime some crucial entry-point info. The
// exact upper bound of this can be calculated, but is not to
// avoid worrying about windows paths.
var end_sfa = std.heap.stackFallback(65536, g.allocator());
var end_sfa = bun.allocators.stackFallback(65536, g.allocator());
var end_list = std.ArrayList(u8).initCapacity(end_sfa.get(), 65536) catch unreachable;
defer end_list.deinit();
const end = end: {
Expand Down
4 changes: 2 additions & 2 deletions src/bake/DevServer/SerializedFailure.zig
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Expand Up @@ -110,7 +110,7 @@ pub fn initFromJs(dev: *DevServer, owner: Owner, value: JSValue) !SerializedFail
@panic("TODO");
}
// Avoid small re-allocations without requesting so much from the heap
var sfb = std.heap.stackFallback(65536, dev.allocator());
var sfb = bun.allocators.stackFallback(65536, dev.allocator());
var payload = std.ArrayList(u8).initCapacity(sfb.get(), 65536) catch
unreachable; // enough space
const w = payload.writer();
Expand All @@ -137,7 +137,7 @@ pub fn initFromLog(
assert(messages.len > 0);

// Avoid small re-allocations without requesting so much from the heap
var sfb = std.heap.stackFallback(65536, dev.allocator());
var sfb = bun.allocators.stackFallback(65536, dev.allocator());
var payload = std.ArrayList(u8).initCapacity(sfb.get(), 65536) catch
unreachable; // enough space
const w = payload.writer();
Expand Down
6 changes: 3 additions & 3 deletions src/bake/FrameworkRouter.zig
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Expand Up @@ -473,7 +473,7 @@ pub const Style = union(enum) {
pub fn fromJS(value: JSValue, global: *jsc.JSGlobalObject) !Style {
if (value.isString()) {
const bun_string = try value.toBunString(global);
var sfa = std.heap.stackFallback(4096, bun.default_allocator);
var sfa = bun.allocators.stackFallback(4096, bun.default_allocator);
const utf8 = bun_string.toUTF8(sfa.get());
defer utf8.deinit();
if (map.get(utf8.slice())) |style| {
Expand Down Expand Up @@ -1241,7 +1241,7 @@ pub const JSFrameworkRouter = struct {

var params_out: MatchedParams = undefined;
if (jsfr.router.matchSlow(path_slice.slice(), &params_out)) |index| {
var sfb = std.heap.stackFallback(4096, bun.default_allocator);
var sfb = bun.allocators.stackFallback(4096, bun.default_allocator);
const alloc = sfb.get();

return (try jsc.JSObject.create(.{
Expand All @@ -1264,7 +1264,7 @@ pub const JSFrameworkRouter = struct {
pub fn toJSON(jsfr: *JSFrameworkRouter, global: *JSGlobalObject, callframe: *jsc.CallFrame) bun.JSError!JSValue {
_ = callframe;

var sfb = std.heap.stackFallback(4096, bun.default_allocator);
var sfb = bun.allocators.stackFallback(4096, bun.default_allocator);
const alloc = sfb.get();

return jsfr.routeToJson(global, Route.Index.init(0), alloc);
Expand Down
8 changes: 4 additions & 4 deletions src/bake/production.zig
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Expand Up @@ -795,7 +795,7 @@ pub export fn BakeToWindowsPath(input: bun.String) callconv(.C) bun.String {
if (comptime bun.Environment.isPosix) {
@panic("This code should not be called on POSIX systems.");
}
var sfa = std.heap.stackFallback(1024, bun.default_allocator);
var sfa = bun.allocators.stackFallback(1024, bun.default_allocator);
const alloc = sfa.get();
const input_utf8 = input.toUTF8(alloc);
defer input_utf8.deinit();
Expand All @@ -807,7 +807,7 @@ pub export fn BakeToWindowsPath(input: bun.String) callconv(.C) bun.String {
}

pub export fn BakeProdResolve(global: *jsc.JSGlobalObject, a_str: bun.String, specifier_str: bun.String) callconv(.C) bun.String {
var sfa = std.heap.stackFallback(@sizeOf(bun.PathBuffer) * 2, bun.default_allocator);
var sfa = bun.allocators.stackFallback(@sizeOf(bun.PathBuffer) * 2, bun.default_allocator);
const alloc = sfa.get();

const specifier = specifier_str.toUTF8(alloc);
Expand Down Expand Up @@ -1020,7 +1020,7 @@ pub const PerThread = struct {

/// Given a key, returns the source code to load.
pub export fn BakeProdLoad(pt: *PerThread, key: bun.String) bun.String {
var sfa = std.heap.stackFallback(4096, bun.default_allocator);
var sfa = bun.allocators.stackFallback(4096, bun.default_allocator);
const allocator = sfa.get();
const utf8 = key.toUTF8(allocator);
defer utf8.deinit();
Expand All @@ -1033,7 +1033,7 @@ pub export fn BakeProdLoad(pt: *PerThread, key: bun.String) bun.String {
}

pub export fn BakeProdSourceMap(pt: *PerThread, key: bun.String) bun.String {
var sfa = std.heap.stackFallback(4096, bun.default_allocator);
var sfa = bun.allocators.stackFallback(4096, bun.default_allocator);
const allocator = sfa.get();
const utf8 = key.toUTF8(allocator);
defer utf8.deinit();
Expand Down
2 changes: 1 addition & 1 deletion src/bun.js/ConsoleObject.zig
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Expand Up @@ -499,7 +499,7 @@ pub const TablePrinter = struct {
) !void {
const globalObject = this.globalObject;

var stack_fallback = std.heap.stackFallback(@sizeOf(Column) * 16, this.globalObject.allocator());
var stack_fallback = bun.allocators.stackFallback(@sizeOf(Column) * 16, this.globalObject.allocator());
var columns = try std.ArrayList(Column).initCapacity(stack_fallback.get(), 16);
defer {
for (columns.items) |*col| {
Expand Down
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