[SelectionDAG] ComputeNumSignBits - support constant pool values from target

As I mentioned on D61887 we don't get many hits on ComputeNumSignBits as we did on computeKnownBits.

The case we do get is interesting though - it allows us to use the 'ConditionalNegate' combine in combineLogicBlendIntoPBLENDV to remove a select.

It comes too late for SSE41 (BLENDV) cases, but SSE2 tests can hit it now. We should probably try to make use of this for SSE41+ targets as well - avoiding variable blends is usually a good idea. I'll investigate as a followup.

Differential Revision: https://reviews.llvm.org/D62777

llvm-svn: 362486

1 files changed, 30 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index 4d854f81de2..224265dfee5 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -3885,6 +3885,36 @@ unsigned SelectionDAG::ComputeNumSignBits(SDValue Op, const APInt &DemandedElts,
       case ISD::ZEXTLOAD: // e.g. i16->i32 = '16' bits known.
         Tmp = LD->getMemoryVT().getScalarSizeInBits();
         return VTBits - Tmp;
+      case ISD::NON_EXTLOAD:
+        if (const Constant *Cst = TLI->getTargetConstantFromLoad(LD)) {
+          // We only need to handle vectors - computeKnownBits should handle
+          // scalar cases.
+          Type *CstTy = Cst->getType();
+          if (CstTy->isVectorTy() &&
+              (NumElts * VTBits) == CstTy->getPrimitiveSizeInBits()) {
+            Tmp = VTBits;
+            for (unsigned i = 0; i != NumElts; ++i) {
+              if (!DemandedElts[i])
+                continue;
+              if (Constant *Elt = Cst->getAggregateElement(i)) {
+                if (auto *CInt = dyn_cast<ConstantInt>(Elt)) {
+                  const APInt &Value = CInt->getValue();
+                  Tmp = std::min(Tmp, Value.getNumSignBits());
+                  continue;
+                }
+                if (auto *CFP = dyn_cast<ConstantFP>(Elt)) {
+                  APInt Value = CFP->getValueAPF().bitcastToAPInt();
+                  Tmp = std::min(Tmp, Value.getNumSignBits());
+                  continue;
+                }
+              }
+              // Unknown type. Conservatively assume no bits match sign bit.
+              return 1;
+            }
+            return Tmp;
+          }
+        }
+        break;
       }
     }
   }