[Beignet] [PATCH 2/2] GBE: don't put a long live register to a selection vector.

[Zhigang Gong]

If an element has very long interval, we don't want to put it into a
vector as it will add more pressure to the register allocation.

With this patch, it can reduce more than 20% spill registers for luxmark's
median scene benchmark(from 288 to 224).

Signed-off-by: Zhigang Gong <zhigang.gong at intel.com>
---
 backend/src/backend/gen_reg_allocation.cpp | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/backend/src/backend/gen_reg_allocation.cpp b/backend/src/backend/gen_reg_allocation.cpp
index 4e276c3..c282b36 100644
--- a/backend/src/backend/gen_reg_allocation.cpp
+++ b/backend/src/backend/gen_reg_allocation.cpp
@@ -271,9 +271,12 @@ namespace gbe
       // case 1: the register is not already in a vector, so it can stay in this
       // vector. Note that local IDs are *non-scalar* special registers but will
       // require a MOV anyway since pre-allocated in the CURBE
+      // If an element has very long interval, we don't want to put it into a
+      // vector as it will add more pressure to the register allocation.
       if (it == vectorMap.end() &&
           ctx.sel->isScalarOrBool(reg) == false &&
-          ctx.isSpecialReg(reg) == false)
+          ctx.isSpecialReg(reg) == false &&
+          (intervals[reg].maxID - intervals[reg].minID) < 2048)
       {
         const VectorLocation location = std::make_pair(vector, regID);
         this->vectorMap.insert(std::make_pair(reg, location));
-- 
1.8.3.2