[llvm] r226995 - [PM] Use a SmallVector instead of std::vector to avoid heap allocations

[Chandler Carruth]

Author: chandlerc
Date: Sat Jan 24 04:47:13 2015
New Revision: 226995

URL: http://llvm.org/viewvc/llvm-project?rev=226995&view=rev
Log:
[PM] Use a SmallVector instead of std::vector to avoid heap allocations
for small switches, and avoid using a complex loop to set up the
weights.

We know what the baseline weights will be so we can just resize the
vector to contain all that value and clobber the one slot that is
likely. This seems much more direct than the previous code that tested
at every iteration, and started off by zeroing the vector.

Modified:
    llvm/trunk/lib/Transforms/Scalar/LowerExpectIntrinsic.cpp

Modified: llvm/trunk/lib/Transforms/Scalar/LowerExpectIntrinsic.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/LowerExpectIntrinsic.cpp?rev=226995&r1=226994&r2=226995&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/LowerExpectIntrinsic.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/LowerExpectIntrinsic.cpp Sat Jan 24 04:47:13 2015
@@ -12,6 +12,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Transforms/Scalar.h"
+#include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/IR/BasicBlock.h"
 #include "llvm/IR/Constants.h"
@@ -24,7 +25,6 @@
 #include "llvm/Pass.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Debug.h"
-#include <vector>
 
 using namespace llvm;
 
@@ -73,13 +73,12 @@ static bool handleSwitchExpect(SwitchIns
 
   SwitchInst::CaseIt Case = SI.findCaseValue(ExpectedValue);
   unsigned n = SI.getNumCases(); // +1 for default case.
-  std::vector<uint32_t> Weights(n + 1);
+  SmallVector<uint32_t, 16> Weights(n + 1, UnlikelyBranchWeight);
 
-  Weights[0] =
-      Case == SI.case_default() ? LikelyBranchWeight : UnlikelyBranchWeight;
-  for (unsigned i = 0; i != n; ++i)
-    Weights[i + 1] =
-        i == Case.getCaseIndex() ? LikelyBranchWeight : UnlikelyBranchWeight;
+  if (Case == SI.case_default())
+    Weights[0] = LikelyBranchWeight;
+  else
+    Weights[Case.getCaseIndex() + 1] = LikelyBranchWeight;
 
   SI.setMetadata(LLVMContext::MD_prof,
                  MDBuilder(CI->getContext()).createBranchWeights(Weights));