[BOLT][AArch64] Use minimal code alignment for cold functions

[maksfb]

On AArch64, a larger cold code size can result in more veneers, increasing potential overhead for hot code. This change minimizes cold code size when the --use-compact-aligner option (default) is enabled.

[llvmbot]

@llvm/pr-subscribers-bolt

Author: Maksim Panchenko (maksfb)

Changes

On AArch64, a larger cold code size can result in more veneers, increasing potential overhead for hot code. This change minimizes cold code size when the --use-compact-aligner option (default) is enabled.

---

Full diff: https://github.com/llvm/llvm-project/pull/172598.diff

1 Files Affected:

• (modified) bolt/lib/Passes/Aligner.cpp (+7)

diff --git a/bolt/lib/Passes/Aligner.cpp b/bolt/lib/Passes/Aligner.cpp
index 5d21bdb3f154a..c631120e9de09 100644
--- a/bolt/lib/Passes/Aligner.cpp
+++ b/bolt/lib/Passes/Aligner.cpp
@@ -77,6 +77,13 @@ static void alignCompact(BinaryFunction &Function,
   size_t HotSize = 0;
   size_t ColdSize = 0;
 
+  // On AArch64, larger cold code size may lead to more veneers and higher
+  // potential overhead for hot code. Minimize the cold code size.
+  if (!Function.hasProfile() && BC.isAArch64()) {
+    Function.setAlignment(Function.getMinAlignment());
+    return;
+  }
+
   for (const BinaryBasicBlock &BB : Function)
     if (BB.isSplit())
       ColdSize += BC.computeCodeSize(BB.begin(), BB.end(), Emitter);