[llvm] [llvm-exegesis] Align loop MBB in loop repetitor (PR #77264)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

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

Author: Aiden Grossman (boomanaiden154)

<details>
<summary>Changes</summary>

This patch sets the alignment of the loob MBB in the loop repetitor to 16 to avoid instruction fetch/predecoding bottlenecks that can come up with unaligned code. The value of 16 was chosen based on numbers for recent Intel microarchitectures and reccomendations from Agner Fog.

---
Full diff: https://github.com/llvm/llvm-project/pull/77264.diff


1 Files Affected:

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


``````````diff
diff --git a/llvm/tools/llvm-exegesis/lib/SnippetRepetitor.cpp b/llvm/tools/llvm-exegesis/lib/SnippetRepetitor.cpp
index cc5a045a8be5dd..2e23351de8e0dc 100644
--- a/llvm/tools/llvm-exegesis/lib/SnippetRepetitor.cpp
+++ b/llvm/tools/llvm-exegesis/lib/SnippetRepetitor.cpp
@@ -74,6 +74,10 @@ class LoopSnippetRepetitor : public SnippetRepetitor {
       auto Loop = Filler.addBasicBlock();
       auto Exit = Filler.addBasicBlock();
 
+      // Align the loop machine basic block to a sixteen byte boundary
+      // so that instruction fetch on modern x86 platforms works optimally.
+      Loop.MBB->setAlignment(Align(16));
+
       const unsigned LoopUnrollFactor =
           LoopBodySize <= Instructions.size()
               ? 1

``````````

</details>


https://github.com/llvm/llvm-project/pull/77264