[Beignet] [PATCH 1/2] GBE: use shl instead of multiply as possible.

[Yang Rong]

i32 multiply and i64 multiply need several instructions, use the shl
instruction when one source is pow of 2 constant.

Signed-off-by: Yang Rong <rong.r.yang at intel.com>
---
 backend/src/llvm/llvm_gen_backend.cpp | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/backend/src/llvm/llvm_gen_backend.cpp b/backend/src/llvm/llvm_gen_backend.cpp
index 0b2169c..5c5266c 100644
--- a/backend/src/llvm/llvm_gen_backend.cpp
+++ b/backend/src/llvm/llvm_gen_backend.cpp
@@ -3272,6 +3272,25 @@ namespace gbe
       case Instruction::Sub:
       case Instruction::FSub: ctx.SUB(type, dst, src0, src1); break;
       case Instruction::Mul:
+      {
+        //LLVM always put constant to src1, but also add the src0 constant check.
+        ConstantInt *c = dyn_cast<ConstantInt>(I.getOperand(0));
+        int index = 0;
+        if (c == NULL) {
+          c = dyn_cast<ConstantInt>(I.getOperand(0));
+          index = 1;
+        }
+        if (c != NULL && isPowerOf<2>(c->getSExtValue())) {
+          c = ConstantInt::get(c->getType(), logi2(c->getZExtValue()));
+          if(index == 0)
+            ctx.SHL(type, dst, src1, this->getRegister(c));
+          else
+            ctx.SHL(type, dst, src0, this->getRegister(c));
+        } else {
+          ctx.MUL(type, dst, src0, src1);
+        }
+        break;
+      }
       case Instruction::FMul: ctx.MUL(type, dst, src0, src1); break;
       case Instruction::URem: ctx.REM(getUnsignedType(ctx, I.getType()), dst, src0, src1); break;
       case Instruction::SRem:
-- 
2.7.4