[SelectionDAG] computeKnownBits - support constant pool values from target

This patch adds the overridable TargetLowering::getTargetConstantFromLoad function which allows targets to return any constant value loaded by a LoadSDNode node - only X86 makes use of this so far but everything should be in place for other targets.

computeKnownBits then uses this function to improve codegen, notably vector code after legalization.

A future commit will do the same for ComputeNumSignBits but computeKnownBits sees the bigger benefit.

This required a couple of fixes:
* SimplifyDemandedBits must early-out for getTargetConstantFromLoad cases to prevent infinite loops of constant regeneration (similar to what we already do for BUILD_VECTOR).
* Fix a DAGCombiner::visitTRUNCATE issue as we had trunc(shl(v8i32),v8i16) <-> shl(trunc(v8i16),v8i32) infinite loops after legalization on AVX512 targets.

Differential Revision: https://reviews.llvm.org/D61887

llvm-svn: 361620

1 files changed, 12 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
index 4d950984b29..d636e613363 100644
--- a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -659,6 +659,14 @@ bool TargetLowering::SimplifyDemandedBits(
       Known.Zero &= Known2.Zero;
     }
     return false; // Don't fall through, will infinitely loop.
+  case ISD::LOAD: {
+    LoadSDNode *LD = cast<LoadSDNode>(Op);
+    if (getTargetConstantFromLoad(LD)) {
+      Known = TLO.DAG.computeKnownBits(Op, DemandedElts, Depth);
+      return false; // Don't fall through, will infinitely loop.
+    }
+    break;
+  }
   case ISD::INSERT_VECTOR_ELT: {
     SDValue Vec = Op.getOperand(0);
     SDValue Scl = Op.getOperand(1);
@@ -2314,6 +2322,10 @@ bool TargetLowering::SimplifyDemandedBitsForTargetNode(
   return false;
 }
 
+const Constant *TargetLowering::getTargetConstantFromLoad(LoadSDNode*) const {
+  return nullptr;
+}
+
 bool TargetLowering::isKnownNeverNaNForTargetNode(SDValue Op,
                                                   const SelectionDAG &DAG,
                                                   bool SNaN,