From: Chandler Carruth <chandlerc@gmail.com>
Date: Sat, 24 Jan 2015 10:47:13 +0000 (+0000)
Subject: [PM] Use a SmallVector instead of std::vector to avoid heap allocations
X-Git-Url: http://plrg.eecs.uci.edu/git/?p=oota-llvm.git;a=commitdiff_plain;h=fa50a1f2d5141d518f258e76750f65c7a3529a3f

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226995 91177308-0d34-0410-b5e6-96231b3b80d8
---

diff --git a/lib/Transforms/Scalar/LowerExpectIntrinsic.cpp b/lib/Transforms/Scalar/LowerExpectIntrinsic.cpp
index ac3c9f4ac68..aa991faafea 100644
--- a/lib/Transforms/Scalar/LowerExpectIntrinsic.cpp
+++ b/lib/Transforms/Scalar/LowerExpectIntrinsic.cpp
@@ -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(SwitchInst &SI) {
 
   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));