[llvm] [llvm-exegesis] Fix snippet value scaling (PR #77226)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-tools-llvm-exegesis

Author: Aiden Grossman (boomanaiden154)

<details>
<summary>Changes</summary>

Currently, BenchmarkRunner scales the per snippet counters by multiplying the raw counter values by the number of instructions (casted to a double) divided by the minimum number of instructions. This is incorrect for the loop repetition mode for snippets that don't fit a whole number of times into the minimum instruction count. For example, with 3 instructions in the snippet and the minimum number of instructions set to 4, the loop repetitor will execute a total of six instructions, but BenchmarkRunner will scale the raw count by 3/4 instead of 3/6=1/2. This will also be incorrect for the duplicate snippet repetitor after #<!-- -->77224.

This patch fixes this behavior by dividing the raw count by the ceiling of the number of repetitions divided by the instruction count.

---
Full diff: https://github.com/llvm/llvm-project/pull/77226.diff


1 Files Affected:

- (modified) llvm/tools/llvm-exegesis/lib/BenchmarkRunner.cpp (+4-3) 


``````````diff
diff --git a/llvm/tools/llvm-exegesis/lib/BenchmarkRunner.cpp b/llvm/tools/llvm-exegesis/lib/BenchmarkRunner.cpp
index dee7af5fd520a4..87e2b768d5e63f 100644
--- a/llvm/tools/llvm-exegesis/lib/BenchmarkRunner.cpp
+++ b/llvm/tools/llvm-exegesis/lib/BenchmarkRunner.cpp
@@ -6,6 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include <cmath>
 #include <memory>
 #include <string>
 
@@ -615,9 +616,9 @@ std::pair<Error, Benchmark> BenchmarkRunner::runConfiguration(
     // Scale the measurements by instruction.
     BM.PerInstructionValue /= BenchmarkResult.NumRepetitions;
     // Scale the measurements by snippet.
-    BM.PerSnippetValue *=
-        static_cast<double>(BenchmarkResult.Key.Instructions.size()) /
-        BenchmarkResult.NumRepetitions;
+    BM.PerSnippetValue /=
+        std::ceil(BenchmarkResult.NumRepetitions /
+                  static_cast<double>(BenchmarkResult.Key.Instructions.size()));
   }
   BenchmarkResult.Measurements = std::move(*NewMeasurements);
 

``````````

</details>


https://github.com/llvm/llvm-project/pull/77226