[SelectionDAG] Add basic vector support to computeKnownBits and SimplifyDemandedBits

[RKSimon]

Summary

Add the ability to computeKnownBits and SimplifyDemandedBits to extract the known zero/one bits from a BUILD_VECTOR, returning the known bits that are shared by every vector element.

This is an initial step towards determining the sign bits of a vector (PR29079).

[niravd]

Is there a reason why you aren't keeping the known bits of a BUILD_VECTOR precise, i.e. concatenating each elements known bits?

lib/CodeGen/SelectionDAG/SelectionDAG.cpp

2035

It would be clearer if you initialized KnownOne and KnownZero to all ones at the start of the loop and replace the conditional with the else case.

lib/CodeGen/SelectionDAG/TargetLowering.cpp

494

Same as above

[mcrosier]

In addition to the already modified tests, we should add more explicit tests for the new functionality.

[RKSimon]

There aren't that many cases that can use vectors that call SelectionDAG::computeKnownBits - the AMDGPU cases already test the ZEXT_LOAD example.

I've added support to DAGCombiner::visitAND to work with splatted constant vector values as well which has exposed one more test case, if you'd prefer for this to be put up for review separately then just say.

SelectionDAG::computeKnownBits like much of DAGCombiner has been coded to work on individual scalar elements (see all the references to getScalarType().getSizeInBits()), its not something that can be easily reworked so that APInt represents every bit of every vector element.

Some vectors already work with SelectionDAG::computeKnownBits (e.g. zext returns the KnownZero bits as its the same for every element).

I agree we want to have full per-element vector support in the long term but its not something that this patch makes any more difficult.

[RKSimon]

We now set all bits in KnownZero and KnownOne and then mask them off when they are not available in a given build_vector element.

[zvi]

Looks like a good start. Can you please share your plans for continuing this work?

lib/CodeGen/SelectionDAG/SelectionDAG.cpp

void SelectionDAG::computeKnownBits(SDValue Op, APInt &KnownZero,
   APInt KnownZero2, KnownOne2;
 
   switch (Op.getOpcode()) {
   case ISD::Constant:
     // We know all of the bits for a constant!
     KnownOne = cast<ConstantSDNode>(Op)->getAPIntValue();
     KnownZero = ~KnownOne;
     break;
+  case ISD::BUILD_VECTOR:
+    // Collect the known bits that are shared by every vector element.
+    for (unsigned i = 0, e = Op.getNumOperands(); i != e; ++i) {
+      SDValue SrcOp = Op.getOperand(i);
+      computeKnownBits(SrcOp, KnownZero2, KnownOne2, Depth + 1);
+
+      // BUILD_VECTOR can implicitly truncate sources, we must handle this.
+      if (SrcOp.getValueSizeInBits() != BitWidth) {
+        assert(SrcOp.getValueSizeInBits() > BitWidth &&
+               "Expected BUILD_VECTOR implicit truncation");
+        KnownOne2 = KnownOne2.trunc(BitWidth);
+        KnownZero2 = KnownZero2.trunc(BitWidth);
+      }
+
+      // Known bits are the values that are shared by every element.
+      // TODO: support per-element known bits.
+      if (i == 0) {
+        KnownOne = KnownOne2;
+        KnownZero = KnownZero2;
+      } else {
+        KnownOne &= KnownOne2;
+        KnownZero &= KnownZero2;
+      }
+    }
+    break;
   case ISD::AND:
     // If either the LHS or the RHS are Zero, the result is zero.
     computeKnownBits(Op.getOperand(1), KnownZero, KnownOne, Depth+1);
     computeKnownBits(Op.getOperand(0), KnownZero2, KnownOne2, Depth+1);
 
     // Output known-1 bits are only known if set in both the LHS & RHS.
     KnownOne &= KnownOne2;
     // Output known-0 are known to be clear if zero in either the LHS | RHS.

lib/CodeGen/SelectionDAG/TargetLowering.cpp

bool TargetLowering::SimplifyDemandedBits(SDValue Op,
 
   APInt KnownZero2, KnownOne2, KnownZeroOut, KnownOneOut;
   switch (Op.getOpcode()) {
   case ISD::Constant:
     // We know all of the bits for a constant!
     KnownOne = cast<ConstantSDNode>(Op)->getAPIntValue();
     KnownZero = ~KnownOne;
     return false;   // Don't fall through, will infinitely loop.
+  case ISD::BUILD_VECTOR:
+    // Collect the known bits that are shared by every constant vector element.
+    for (unsigned i = 0, e = Op.getNumOperands(); i != e; ++i) {
+      SDValue SrcOp = Op.getOperand(i);
+      if (!isa<ConstantSDNode>(SrcOp)) {
+        // We can only handle all constant values - bail out with no known bits.
+        KnownZero = KnownOne = APInt(BitWidth, 0);
+        return false;
+      }
+      KnownOne2 = cast<ConstantSDNode>(SrcOp)->getAPIntValue();
+      KnownZero2 = ~KnownOne2;
+
+      // BUILD_VECTOR can implicitly truncate sources, we must handle this.
+      if (KnownOne2.getBitWidth() != BitWidth) {
+        assert(KnownOne2.getBitWidth() > BitWidth &&
+               KnownZero2.getBitWidth() > BitWidth &&
+               "Expected BUILD_VECTOR implicit truncation");
+        KnownOne2 = KnownOne2.trunc(BitWidth);
+        KnownZero2 = KnownZero2.trunc(BitWidth);
+      }
+
+      // Known bits are the values that are shared by every element.
+      // TODO: support per-element known bits.
+      if (i == 0) {
+        KnownOne = KnownOne2;
+        KnownZero = KnownZero2;
+      } else {
+        KnownOne &= KnownOne2;
+        KnownZero &= KnownZero2;
+      }
+    }
+    return false;   // Don't fall through, will infinitely loop.
   case ISD::AND:
     // If the RHS is a constant, check to see if the LHS would be zero without
     // using the bits from the RHS.  Below, we use knowledge about the RHS to
     // simplify the LHS, here we're using information from the LHS to simplify
     // the RHS.
     if (ConstantSDNode *RHSC = dyn_cast<ConstantSDNode>(Op.getOperand(1))) {
       APInt LHSZero, LHSOne;
       // Do not increment Depth here; that can cause an infinite loop.

test/CodeGen/AMDGPU/load-constant-i16.ll

 ; FUNC-LABEL: {{^}}constant_zextload_v2i16_to_v2i32:
 ; GCN: s_load_dword s
 ; GCN-DAG: s_and_b32 s{{[0-9]+}}, s{{[0-9]+}}, 0xffff{{$}}
 ; GCN-DAG: s_lshr_b32 s{{[0-9]+}}, s{{[0-9]+}}, 16
 
 ; v2i16 is naturally 4 byte aligned
 ; EG: VTX_READ_32 [[DST:T[0-9]\.[XYZW]]], [[DST]], 0, #1
 ; TODO: This should use DST, but for some there are redundant MOVs
-; EG: LSHR {{[* ]*}}T{{[0-9].[XYZW]}}, {{PV.[XYZW]}}, literal
+; EG: BFE_UINT {{[* ]*}}T{{[0-9].[XYZW]}}, {{PV.[XYZW]}}, literal
 ; EG: 16
 define void @constant_zextload_v2i16_to_v2i32(<2 x i32> addrspace(1)* %out, <2 x i16> addrspace(2)* %in) #0 {
   %load = load <2 x i16>, <2 x i16> addrspace(2)* %in
   %ext = zext <2 x i16> %load to <2 x i32>
   store <2 x i32> %ext, <2 x i32> addrspace(1)* %out
   ret void
 }
 

 ; FUNC-LABEL: {{^}}constant_constant_zextload_v4i16_to_v4i32:
 ; GCN: s_load_dwordx2
 ; GCN-DAG: s_and_b32
 ; GCN-DAG: s_lshr_b32
 
 ; v4i16 is naturally 8 byte aligned
 ; EG: VTX_READ_64 [[DST:T[0-9]\.XY]], {{T[0-9].[XYZW]}}, 0, #1
 ; TODO: These should use DST, but for some there are redundant MOVs
-; EG-DAG: LSHR {{[* ]*}}T{{[0-9].[XYZW]}}, {{PV.[XYZW]}}, literal
-; EG-DAG: LSHR {{[* ]*}}T{{[0-9].[XYZW]}}, {{T[0-9].[XYZW]}}, literal
+; EG-DAG: BFE_UINT {{[* ]*}}T{{[0-9].[XYZW]}}, {{PV.[XYZW]}}, literal
 ; EG-DAG: 16
+; EG-DAG: BFE_UINT {{[* ]*}}T{{[0-9].[XYZW]}}, {{T[0-9].[XYZW]}}, literal
+; EG-DAG: AND_INT {{[* ]*}}T{{[0-9].[XYZW]}}, {{T[0-9].[XYZW]}}, literal
 ; EG-DAG: 16
 define void @constant_constant_zextload_v4i16_to_v4i32(<4 x i32> addrspace(1)* %out, <4 x i16> addrspace(2)* %in) #0 {
   %load = load <4 x i16>, <4 x i16> addrspace(2)* %in
   %ext = zext <4 x i16> %load to <4 x i32>
   store <4 x i32> %ext, <4 x i32> addrspace(1)* %out
   ret void
 }
 

test/CodeGen/AMDGPU/load-global-i16.ll

 }
 
 ; FUNC-LABEL: {{^}}global_zextload_v2i16_to_v2i32:
 ; GCN-NOHSA: buffer_load_dword
 ; GCN-HSA: flat_load_dword
 
 ; EG: VTX_READ_32 [[DST:T[0-9]\.[XYZW]]], [[DST]], 0, #1
 ; TODO: This should use DST, but for some there are redundant MOVs
-; EG: LSHR {{[* ]*}}T{{[0-9].[XYZW]}}, {{PV.[XYZW]}}, literal
+; EG: BFE_UINT {{[* ]*}}T{{[0-9].[XYZW]}}, {{PV.[XYZW]}}, literal
 ; EG: 16
 define void @global_zextload_v2i16_to_v2i32(<2 x i32> addrspace(1)* %out, <2 x i16> addrspace(1)* %in) #0 {
   %load = load <2 x i16>, <2 x i16> addrspace(1)* %in
   %ext = zext <2 x i16> %load to <2 x i32>
   store <2 x i32> %ext, <2 x i32> addrspace(1)* %out
   ret void
 }
 

 
 ; FUNC-LABEL: {{^}}global_global_zextload_v4i16_to_v4i32:
 ; GCN-NOHSA: buffer_load_dwordx2
 
 ; GCN-HSA: flat_load_dwordx2
 
 ; EG: VTX_READ_64 [[DST:T[0-9]\.XY]], {{T[0-9].[XYZW]}}, 0, #1
 ; TODO: These should use DST, but for some there are redundant MOVs
-; EG-DAG: LSHR {{[* ]*}}T{{[0-9].[XYZW]}}, {{PV.[XYZW]}}, literal
-; EG-DAG: LSHR {{[* ]*}}T{{[0-9].[XYZW]}}, {{T[0-9].[XYZW]}}, literal
+; EG-DAG: BFE_UINT {{[* ]*}}T{{[0-9].[XYZW]}}, {{T[0-9].[XYZW]}}, literal
 ; EG-DAG: 16
+; EG-DAG: BFE_UINT {{[* ]*}}T{{[0-9].[XYZW]}}, {{T[0-9].[XYZW]}}, literal
+; EG-DAG: AND_INT {{[* ]*}}T{{[0-9].[XYZW]}}, {{T[0-9].[XYZW]}}, literal
 ; EG-DAG: 16
 define void @global_global_zextload_v4i16_to_v4i32(<4 x i32> addrspace(1)* %out, <4 x i16> addrspace(1)* %in) #0 {
   %load = load <4 x i16>, <4 x i16> addrspace(1)* %in
   %ext = zext <4 x i16> %load to <4 x i32>
   store <4 x i32> %ext, <4 x i32> addrspace(1)* %out
   ret void
 }