[DAGCombiner] Add vector demanded elements support to ComputeNumSignBits

Currently ComputeNumSignBits returns the minimum number of sign bits for all elements of vector data, when we may only be interested in one/some of the elements.

This patch adds a DemandedElts argument that allows us to specify the elements we actually care about. The original ComputeNumSignBits implementation calls with a DemandedElts demanding all elements to match current behaviour. Scalar types set this to 1.

I've only added support for BUILD_VECTOR and EXTRACT_VECTOR_ELT so far, all others will default to demanding all elements but can be updated in due course.

Followup to D25691.

Differential Revision: https://reviews.llvm.org/D31311

llvm-svn: 299219

2 files changed, 3 insertions, 1 deletions

diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index 4f309b422fe..aa741336f92 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -26745,7 +26745,8 @@ void X86TargetLowering::computeKnownBitsForTargetNode(const SDValue Op,
 }
 
 unsigned X86TargetLowering::ComputeNumSignBitsForTargetNode(
-    SDValue Op, const SelectionDAG &DAG, unsigned Depth) const {
+    SDValue Op, const APInt &DemandedElts, const SelectionDAG &DAG,
+    unsigned Depth) const {
   unsigned VTBits = Op.getScalarValueSizeInBits();
   unsigned Opcode = Op.getOpcode();
   switch (Opcode) {
diff --git a/llvm/lib/Target/X86/X86ISelLowering.h b/llvm/lib/Target/X86/X86ISelLowering.h
index a179adcd929..13642c219e8 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.h
+++ b/llvm/lib/Target/X86/X86ISelLowering.h
@@ -832,6 +832,7 @@ namespace llvm {
 
     /// Determine the number of bits in the operation that are sign bits.
     unsigned ComputeNumSignBitsForTargetNode(SDValue Op,
+                                             const APInt &DemandedElts,
                                              const SelectionDAG &DAG,
                                              unsigned Depth) const override;