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

[Yang, Rong R]

This patch looks good to me.
As discuss, we would better add the similar optimize in GEN selection, because GEP lower is a part of PowerOf 2  multiply. I will add it later.

> -----Original Message-----
> From: Beignet [mailto:beignet-bounces at lists.freedesktop.org] On Behalf Of
> Ruiling Song
> Sent: Monday, January 23, 2017 13:32
> To: beignet at lists.freedesktop.org
> Cc: Song, Ruiling <ruiling.song at intel.com>
> Subject: [Beignet] [PATCH] GBE: use shift for PowerOf2 size when lowering
> GEP.
> 
> 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
> 
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