[AArch64] Sets the preferred function alignment for Cortex-A53/A55

[stelios-arm]

Summary

Setting the preferred function alignment to 16 for Cortex A53/A55 improves performance in a wide range of benchmarks. This brings it in line with the
Cortex-A53/A55 tuning that is used in GCC (gcc/config/aarch64/aarch64.c).

[SjoerdMeijer]

Just for completeness, I checked that GCC indeed sets it 16, here:

https://github.com/gcc-mirror/gcc/blob/71834be5b68e0c9839f0647e1bbf1eec4e4bbf49/gcc/config/aarch64/aarch64.c#L1277

Are we not adding tests for these kind of things?

[dmgreen]

The test that needs updating is in CodeGen/AArch64/preferred-function-alignment.ll

[stelios-arm]

Modified the Cortex-A53 test and added one for Cortex-A55.

[dmgreen]

Sounds good to me.

llvm/lib/Target/AArch64/AArch64Subtarget.cpp

void AArch64Subtarget::initializeProperties() {
   case Others:
     break;
   case Carmel:
     CacheLineSize = 64;
     break;
   case CortexA35:
     break;
   case CortexA53:
-    PrefFunctionLogAlignment = 3;
-    break;
   case CortexA55:
+    PrefFunctionLogAlignment = 4;
     break;
   case CortexA57:
     MaxInterleaveFactor = 4;
     PrefFunctionLogAlignment = 4;
     break;
   case CortexA65:
     PrefFunctionLogAlignment = 3;
     break;