mirror of
https://github.com/oven-sh/bun
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646aede0d4
## Summary
Fixes CPU profiler generating invalid timestamps that Chrome DevTools
couldn't parse (though VSCode's profiler viewer accepted them).
## The Problem
CPU profiles generated by `--cpu-prof` had timestamps that were either:
1. Negative (in the original broken profile from the gist)
2. Truncated/corrupted (after initial timestamp calculation fix)
Example from the broken profile:
```json
{
"startTime": -822663297,
"endTime": -804820609
}
```
After initial fix, timestamps were positive but still wrong:
```json
{
"startTime": 1573519100, // Should be ~1761784720948727
"endTime": 1573849434
}
```
## Root Cause
**Primary Issue**: `WTF::JSON::Object::setInteger()` has precision
issues with large values (> 2^31). When setting timestamps like
`1761784720948727` (microseconds since Unix epoch - 16 digits), the
method was truncating/corrupting them.
**Secondary Issue**: The timestamp calculation logic needed
clarification - now explicitly uses the earliest sample's wall clock
time as startTime and calculates a consistent wallClockOffset.
## The Fix
### src/bun.js/bindings/BunCPUProfiler.cpp
Changed from `setInteger()` to `setDouble()` for timestamp
serialization:
```cpp
// Before (broken):
json->setInteger("startTime"_s, static_cast<long long>(startTime));
json->setInteger("endTime"_s, static_cast<long long>(endTime));
// After (fixed):
json->setDouble("startTime"_s, startTime);
json->setDouble("endTime"_s, endTime);
```
JSON `Number` type can precisely represent integers up to 2^53 (~9
quadrillion), which is far more than needed for microsecond timestamps
(~10^15 for current dates).
Also clarified the timestamp calculation to use `wallClockStart`
directly as the profile's `startTime` and calculate a `wallClockOffset`
for converting stopwatch times to wall clock times.
### test/cli/run/cpu-prof.test.ts
Added validation that timestamps are:
- Positive
- In microseconds (> 1000000000000000, < 3000000000000000)
- Within valid Unix epoch range
## Testing
```bash
bun bd test test/cli/run/cpu-prof.test.ts
```
All tests pass ✅
Generated profile now has correct timestamps:
```json
{
"startTime": 1761784720948727.2,
"endTime": 1761784721305814
}
```
## Why VSCode Worked But Chrome DevTools Didn't
- **VSCode**: Only cares about relative timing (duration = endTime -
startTime), doesn't validate absolute timestamp ranges
- **Chrome DevTools**: Expects timestamps in microseconds since Unix
epoch (positive, ~16 digits), fails validation when timestamps are
negative, too small, or out of valid range
## References
- Gist with CPU profile format documentation:
https://gist.github.com/Jarred-Sumner/2c12da481845e20ce6a6175ee8b05a3e
- Chrome DevTools Protocol - Profiler:
https://chromedevtools.github.io/devtools-protocol/tot/Profiler/
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>
---------
Co-authored-by: Claude Bot <claude-bot@bun.sh>
Co-authored-by: Claude <noreply@anthropic.com>