[Beignet] [PATCH] GBE: use shift for PowerOf2 size when lowering GEP.

[Ruiling Song]

For 64bit address, the multiply would expand to several instructions.
As for most time, the size is PowerOf 2. So we can use left-shift to
do this.

Signed-off-by: Ruiling Song <ruiling.song at intel.com>
---
 backend/src/llvm/llvm_passes.cpp | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/backend/src/llvm/llvm_passes.cpp b/backend/src/llvm/llvm_passes.cpp
index 367a2c3..c5f3ffe 100644
--- a/backend/src/llvm/llvm_passes.cpp
+++ b/backend/src/llvm/llvm_passes.cpp
@@ -276,8 +276,6 @@ namespace gbe
         uint32_t align = getAlignmentByte(unit, elementType);
         size += getPadding(size, align);
 
-        Constant* newConstSize = 
-          ConstantInt::get(IntegerType::get(GEPInst->getContext(), ptrSize), size);
 
         Value *operand = GEPInst->getOperand(op); 
 
@@ -308,13 +306,22 @@ namespace gbe
           }
         }
 #endif
-        Value* tmpMul = operand;
+        Value* tmpOffset = operand;
         if (size != 1) {
-          tmpMul = BinaryOperator::Create(Instruction::Mul, newConstSize, operand,
-                                         "", GEPInst);
+          if (isPowerOf<2>(size)) {
+            Constant* shiftAmnt =
+              ConstantInt::get(IntegerType::get(GEPInst->getContext(), ptrSize), logi2(size));
+            tmpOffset = BinaryOperator::Create(Instruction::Shl, operand, shiftAmnt,
+                                           "", GEPInst);
+          } else{
+            Constant* sizeConst =
+              ConstantInt::get(IntegerType::get(GEPInst->getContext(), ptrSize), size);
+            tmpOffset = BinaryOperator::Create(Instruction::Mul, sizeConst, operand,
+                                           "", GEPInst);
+          }
         }
         currentAddrInst = 
-          BinaryOperator::Create(Instruction::Add, currentAddrInst, tmpMul,
+          BinaryOperator::Create(Instruction::Add, currentAddrInst, tmpOffset,
               "", GEPInst);
       }
 
-- 
2.4.1