[PATCH] D30391: [X86] Add Experimental option to specify preferable loop alignment

[Serguei Katkov via Phabricator via llvm-commits]

skatkov created this revision.

Loop alignment can cause a significant change of
the perfromance for short loops.
To be able to evaluate the impact of loop alignment this change
introduces the new option x86-experimental-pref-loop-alignment.
The alignment will be 2^Value bytes, the default value is 4.


https://reviews.llvm.org/D30391

Files:
  lib/Target/X86/X86ISelLowering.cpp


Index: lib/Target/X86/X86ISelLowering.cpp
===================================================================
--- lib/Target/X86/X86ISelLowering.cpp
+++ lib/Target/X86/X86ISelLowering.cpp
@@ -70,6 +70,13 @@
              "rather than promotion."),
     cl::Hidden);
 
+static cl::opt<int> ExperimentalPrefLoopAlignment(
+    "x86-experimental-pref-loop-alignment", cl::init(4),
+    cl::desc("Sets the prefereable loop alignment for experiments "
+             "(the last ExperimentalPrefLoopAlignment bits"
+             " of the loop header PC will be 0)."),
+    cl::Hidden);
+
 X86TargetLowering::X86TargetLowering(const X86TargetMachine &TM,
                                      const X86Subtarget &STI)
     : TargetLowering(TM), Subtarget(STI) {
@@ -1716,7 +1723,8 @@
   MaxStoresPerMemcpyOptSize = 4;
   MaxStoresPerMemmove = 8; // For @llvm.memmove -> sequence of stores
   MaxStoresPerMemmoveOptSize = 4;
-  setPrefLoopAlignment(4); // 2^4 bytes.
+  // Set loop alignment to 2^ExperimentalPrefLoopAlignment bytes (default: 2^4).
+  setPrefLoopAlignment(ExperimentalPrefLoopAlignment);
 
   // An out-of-order CPU can speculatively execute past a predictable branch,
   // but a conditional move could be stalled by an expensive earlier operation.


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