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

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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:

perfview/src/TraceEvent/Stacks/Histogram.cs

Lines 48 to 68 in c671d46

	public void AddMetric(float metric, int bucket)
	{
	Debug.Assert(0 <= bucket && bucket < Count);
	if (m_singleBucketNum < 0)
	{
	m_singleBucketNum = bucket;
	}
	if (m_singleBucketNum == bucket)
	{
	m_singleBucketValue += metric;
	return;
	}
	if (m_buckets == null)
	{
	m_buckets = new float[Count];
	m_buckets[m_singleBucketNum] = m_singleBucketValue;
	}
	m_buckets[bucket] += metric;
	}

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:

perfview/src/TraceEvent/Stacks/Histogram.cs

Lines 119 to 126 in c671d46

	if (m_buckets != null)
	{
	return m_buckets[index];
	}
	else if (m_singleBucketNum == index)
	{
	return m_singleBucketValue;
	}

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 A bug in Histogram? #2006

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