Fix Histogram.AddMetric losing values after single-bucket to array transition

[Copilot]

Histogram.AddMetric incorrectly updated m_singleBucketValue instead of the array after transitioning from single-bucket optimization to array mode, causing subsequent additions to the original bucket to be lost.

Root Cause

The method checked if (m_singleBucketNum == bucket) before checking if (m_buckets == null), so after array creation, calls to the original bucket still matched the single-bucket path but the indexer only read from the array.

Example failure sequence:

histogram.AddMetric(1.0f, 0);  // m_singleBucketValue = 1.0
histogram.AddMetric(2.0f, 1);  // Creates array, m_buckets[0] = 1.0, m_buckets[1] = 2.0
histogram.AddMetric(3.0f, 0);  // BUG: adds to m_singleBucketValue, not m_buckets[0]
// histogram[0] returns 1.0 instead of 4.0

Changes

• Restructured AddMetric logic: Check m_buckets == null first to determine mode before applying single-bucket optimization
• Clear single-bucket tracking on transition: Set m_singleBucketNum = -1 after creating array to prevent reentry into single-bucket path
• Added test coverage: 4 tests covering the bug scenario, multiple bucket patterns, single-bucket mode preservation, and AddScaled interaction

Code Review Note

Review identified a pre-existing bug where AddScaled's weight parameter is not applied when the source histogram uses array mode (only applied in single-bucket mode). This is orthogonal to the reported issue and left unmodified per minimal change principle.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>A bug in Histogram?</issue_title> <issue_description>I was looking around the source code and the following piece of code seems a bit buggy to me: https://github.com/microsoft/perfview/blob/c671d460fdd4abd915a96beb1410c8c7748432fb/src/TraceEvent/Stacks/Histogram.cs#L48-L68 Let's say the following sequence of AddMetric is called:

AddMetric(1.0f, 0);
AddMetric(2.0f, 1);
AddMetric(3.0f, 0);

The second time will cause the array to be constructed, and m_singleBucketValue is populated into the array. But within the third call, if (m_singleBucketNum == bucket) will still be true, and the metric is still added into m_singleBucketValue instead of the array. Inside the indexer: https://github.com/microsoft/perfview/blob/c671d460fdd4abd915a96beb1410c8c7748432fb/src/TraceEvent/Stacks/Histogram.cs#L119-L126 the m_singleBucketValue isn't used when the array is not null, causing the entry for m_singleBucketNum to be returned with incorrected value.

I don't have the whole knowledge of how Histogram is used, so I don't sure if this is really a bug. And this problem might not be a big issue due to the usage pattern, cause I don't see any bucket to be obviously wrong in the GUI.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes microsoft/perfview#2006

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.