Do not speculatively execute an instruction by hoisting it to its predecessor BB if any of its operands are defined but not used in BB. The transformation will prevent the operand from being sunk into the use block.

llvm-svn: 52244

1 files changed, 16 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 605855d1769..a993784cd25 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -23,12 +23,15 @@
 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/ADT/Statistic.h"
 #include <algorithm>
 #include <functional>
 #include <set>
 #include <map>
 using namespace llvm;
 
+STATISTIC(NumSpeculations, "Number of speculative executed instructions");
+
 /// SafeToMergeTerminators - Return true if it is safe to merge these two
 /// terminator instructions together.
 ///
@@ -1030,12 +1033,22 @@ static bool SpeculativelyExecuteBB(BranchInst *BI, BasicBlock *BB1) {
   if (!FalseV)  // Can this happen?
     return false;
 
+  // Do not hoist the instruction if any of its operands are defined but not
+  // used in this BB. The transformation will prevent the operand from
+  // being sunk into the use block.
+  for (User::op_iterator i = I->op_begin(), e = I->op_end(); i != e; ++i) {
+    Instruction *OpI = dyn_cast<Instruction>(*i);
+    if (OpI && OpI->getParent() == BIParent &&
+        !OpI->isUsedInBasicBlock(BIParent))
+      return false;
+  }
+
   // If we get here, we can hoist the instruction. Try to place it before the
-  // icmp / fcmp instruction preceeding the conditional branch.
+  // icmp instruction preceeding the conditional branch.
   BasicBlock::iterator InsertPos = BI;
   if (InsertPos != BIParent->begin())
     --InsertPos;
-  if (InsertPos == BrCond)
+  if (InsertPos == BrCond && !isa<PHINode>(BrCond))
     BIParent->getInstList().splice(InsertPos, BB1->getInstList(), I);
   else
     BIParent->getInstList().splice(BI, BB1->getInstList(), I);
@@ -1060,6 +1073,7 @@ static bool SpeculativelyExecuteBB(BranchInst *BI, BasicBlock *BB1) {
         PN->setIncomingValue(j, SI);
   }
 
+  ++NumSpeculations;
   return true;
 }