[llvm-exegesis] Align loop MBB in loop repetitor

[boomanaiden154]

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.

Fixes #77259.

[llvmbot]

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

Author: Aiden Grossman (boomanaiden154)

Changes

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 --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

[RKSimon]

LGTM - cheers

[phoebewang]

Not quite understand the background, but want to share to see if the recent feature #71026 helps with this.

[boomanaiden154]

Not quite understand the background, but want to share to see if the recent feature #71026 helps with this.

Thank you for pointing this out. We don't lower anything from LLVM IR and generate machine code directly for benchmarks in llvm-exegesis, so this isn't applicable with the way we're doing things currently.