Add target hook for whether it is profitable to reduce load widths

Add an option to disable optimization to shrink truncated larger type
loads to smaller type loads. On SI this prevents using scalar load
instructions in some cases, since there are no scalar extloads.

llvm-svn: 224084

1 files changed, 23 insertions, 0 deletions

diff --git a/llvm/lib/Target/R600/AMDGPUISelLowering.cpp b/llvm/lib/Target/R600/AMDGPUISelLowering.cpp
index 35371798c18..0e34b4625c8 100644
--- a/llvm/lib/Target/R600/AMDGPUISelLowering.cpp
+++ b/llvm/lib/Target/R600/AMDGPUISelLowering.cpp
@@ -432,6 +432,29 @@ bool AMDGPUTargetLowering::ShouldShrinkFPConstant(EVT VT) const {
   return (ScalarVT != MVT::f32 && ScalarVT != MVT::f64);
 }
 
+bool AMDGPUTargetLowering::shouldReduceLoadWidth(SDNode *N,
+                                                 ISD::LoadExtType,
+                                                 EVT NewVT) const {
+
+  unsigned NewSize = NewVT.getStoreSizeInBits();
+
+  // If we are reducing to a 32-bit load, this is always better.
+  if (NewSize == 32)
+    return true;
+
+  EVT OldVT = N->getValueType(0);
+  unsigned OldSize = OldVT.getStoreSizeInBits();
+
+  // Don't produce extloads from sub 32-bit types. SI doesn't have scalar
+  // extloads, so doing one requires using a buffer_load. In cases where we
+  // still couldn't use a scalar load, using the wider load shouldn't really
+  // hurt anything.
+
+  // If the old size already had to be an extload, there's no harm in continuing
+  // to reduce the width.
+  return (OldSize < 32);
+}
+
 bool AMDGPUTargetLowering::isLoadBitCastBeneficial(EVT LoadTy,
                                                    EVT CastTy) const {
   if (LoadTy.getSizeInBits() != CastTy.getSizeInBits())