[AArch64][SVE] Expand gather index to 32 bits instead of 64 bits

[MattDevereau]

Summary

For gathers which load in 8 and 16 bit data then use that as an index, the index can be extended to 32 bits instead of 64 bits

[DavidTruby]

I think the commit message needs to be more clear: my understanding from the IR is that you're not _always_ narrowing a gather's index if it's 64 bits, just if it was extended from < 32 to 64 and hence could trivially have been extended to 32 instead? I think the commit message needs to reflect that.

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

17858–17859

Could you just have the following and remove one level of nesting?

if (Index.getOpcode() == ISD::ZERO_EXTEND && tryNarrowZExtGatherIndex(N, Index, DAG))
  return true;

[peterwaller-arm]

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

17855

s/.getVectorElementType().getFixedSizeInBits()/.getScalarSizeInBits()/

[paulwalker-arm]

Please can you also add the equivalent SIGN_EXTEND case.

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

17852

I'll reserve judgement but it doesn't really seem worth breaking thus out into a separate function.

You do need to ensure N is treating Index as unsigned before you can shrink the extend. MaskedGatherScatterSDNode has a function to query this.

[MattDevereau]

Update commit message
Added unsigned index check
Inlined logic instead of using a function

[paulwalker-arm]

@MattDevereau: I've recently landed D130533 which I think can benefit you here. Updating isVectorShrinkable so it first handles your explicit extension cases before it drops into the BUILD_VECTOR handling might do what you need.

[MattDevereau]

@paulwalker-arm I've extended isVectorShrinkable to shrink sign extended and zero extended gathers/scatters, however i'm seeing the following change to a pair of gather/scatter tests in sve-fixed-length-masked-gather.ll:

; CHECK-LABEL: masked_gather_32b_scaled_sext_f64:
; CHECK:       // %bb.0:
; CHECK-NEXT:    ptrue p0.d, vl32
; CHECK-NEXT:    ld1d { z0.d }, p0/z, [x0]
; CHECK-NEXT:    ld1sw { z1.d }, p0/z, [x1]
; CHECK-NEXT:    fcmeq p1.d, p0/z, z0.d, #0.0
; CHECK-NEXT:    ld1d { z0.d }, p1/z, [x2, z1.d, lsl #3]
; CHECK-NEXT:    st1d { z0.d }, p0, [x0]
; CHECK-NEXT:    ret
  %cvals = load <32 x

; CHECK-LABEL: masked_gather_32b_scaled_sext_f64:
; CHECK:       // %bb.0:
; CHECK-NEXT:    ptrue p0.d, vl32
; CHECK-NEXT:    ptrue p1.s, vl32
; CHECK-NEXT:    ld1d { z0.d }, p0/z, [x0]
; CHECK-NEXT:    ld1w { z1.s }, p1/z, [x1]
; CHECK-NEXT:    fcmeq p1.d, p0/z, z0.d, #0.0
; CHECK-NEXT:    sunpklo z0.d, z1.s
; CHECK-NEXT:    ld1d { z0.d }, p1/z, [x2, z0.d, lsl #3]
; CHECK-NEXT:    st1d { z0.d }, p0, [x0]
; CHECK-NEXT:    ret

define void @masked_gather_32b_scaled_sext_f64(<32 x double>* %a, <32 x i32>* %b, double* %base) vscale_range(16,0) #0 {
  %cvals = load <32 x double>, <32 x double>* %a
  %idxs = load <32 x i32>, <32 x i32>* %b
  %ext = sext <32 x i32> %idxs to <32 x i64>
  %ptrs = getelementptr double, double* %base, <32 x i64> %ext
  %mask = fcmp oeq <32 x double> %cvals, zeroinitializer
  %vals = call <32 x double> @llvm.masked.gather.v32f64(<32 x double*> %ptrs, i32 8, <32 x i1> %mask, <32 x double> undef)
  store <32 x double> %vals, <32 x double>* %a
  ret void

The test specifically wants the sign-extend which makes sense, but with my changes in we ignore the sign extend and then unpack it pointlessly. I'm assuming I need to check that the gather predicate doesn't come from a comparison with a wider vector before shrinking the vector?

[MattDevereau]

These two tests currently have regressions:
b/llvm/test/CodeGen/AArch64/sve-fixed-length-masked-gather.ll:@masked_gather_32b_scaled_sext_f64
b/llvm/test/CodeGen/AArch64/sve-fixed-length-masked-scatter.ll:@masked_scatter_32b_scaled_sext_f64

[paulwalker-arm]

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

296

I'm probably just being paranoid but with this code now sitting after the call to getScalarSizeInBits() can you add something like assert(N->getValueType(0).isVector() && "Expected a vector!"); here? just in case.

303

It's not sufficient to only consider the result type of the extend. For example, take

isVectorShrinkable(zext_to_i64(i48_node, 32, false)

With the current implementation you'll return true but:

zext_to_i64(trunc_to_i32(i48_node)) != zext_to_i64(i48_node)

For the sext & zext cases I think you also need N's operand to be <= NewEltSize

llvm/test/CodeGen/AArch64/sve-fixed-length-masked-gather.ll

1036–1041 ↗

(On Diff #454483)

I've had a look and believe the issue is that for fixed length vectors we don't want to shrink Index when the main datatype of the gather/scatter is a vector of 64bit values because the data elements must line up with the offset elements and when that is not the case operation legalisation will "fix" it by explicitly extending which ever is the smaller type.

This is fixable within findMoreOptimalIndexType by just bailing about of such types just before the // Can indices be trivially shrunk? block.

The reason this is a fixed-length only problem is because for scalable vectors we use an "unpacked" format where each element of a nxv2i32 vector sits within the bottom half of each element of a nxv2i64 vector so element of differing sizes remain aligned to each other.

[paulwalker-arm]

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

17873

Please pull this out into a separate check before this block, along with a suitable comment. The "fix" is not really related to isVectorShrinkable, it is just that today that is the only logic applicable for fixed length vectors. However, this might change in the future hence why I prefer an isolated check.

[MattDevereau]

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

17873

I'm not sure if I'm missing something obvious here but I don't understand what you're expecting this to look like when this check is seperated. The way this function's logic is structured means it's difficult to bail out cleanly from individual checks here, as we want to fall through this if block instead of returning false. I could use a nested if block, e.g.

//comment
if (!(DataVT.getScalarSizeInBits() == 64 && DataVT.isFixedLengthVector())){
  if (!ISD::isVectorShrinkable... ){
    ...
  }
}

To sort of separate the "temporary" condition, but I believe it still needs to be and logic for the correct behaviour here
Could you please elaborate on what you're expecting?

[paulwalker-arm]

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

17873

I was thinking of:

// Fixed length vectors are always "packed" so there's no value in the index having a smaller element type than the data.
if (DataVT.isFixedLengthVector() && DataVT.getScalarSizeInBits() == 64)
  return Changed;

It is my assertion that we don't want to fall through because all code after this point is trying to rewrite Index to be a vector of i32s, which is never going to be good for fixed length vector types because Index will just get re-extended during operation legalisation.

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

for (const SDValue &Op : N->op_values()) {
     if (!isa<ConstantFPSDNode>(Op))
       return false;
   }
   return true;
 }
 
 bool ISD::isVectorShrinkable(const SDNode *N, unsigned NewEltSize,
                              bool Signed) {
-  if (N->getOpcode() != ISD::BUILD_VECTOR)
-    return false;
+  assert(N->getValueType(0).isVector() && "Expected a vector!");
 
   unsigned EltSize = N->getValueType(0).getScalarSizeInBits();
   if (EltSize <= NewEltSize)
     return false;
 
+  if (N->getOpcode() == ISD::ZERO_EXTEND) {
+    return (N->getOperand(0).getValueType().getScalarSizeInBits() <=
+            NewEltSize) &&
+           !Signed;
+  }
+  if (N->getOpcode() == ISD::SIGN_EXTEND) {
+    return (N->getOperand(0).getValueType().getScalarSizeInBits() <=
+            NewEltSize) &&
+           Signed;
+  }
+  if (N->getOpcode() != ISD::BUILD_VECTOR)
+    return false;
+
   for (const SDValue &Op : N->op_values()) {
     if (Op.isUndef())
       continue;
     if (!isa<ConstantSDNode>(Op))
       return false;
 
     APInt C = cast<ConstantSDNode>(Op)->getAPIntValue().trunc(EltSize);
     if (Signed && C.trunc(NewEltSize).sext(EltSize) != C)

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

if (auto Shift = DAG.getSplatValue(ShiftOp))
                             Add.getOperand(0), ShiftOp);
         return true;
       }
   }
 
   return false;
 }
 
 // Analyse the specified address returning true if a more optimal addressing
 // mode is available. When returning true all parameters are updated to reflect
 // their recommended values.
 static bool findMoreOptimalIndexType(const MaskedGatherScatterSDNode *N,
                                      SDValue &BasePtr, SDValue &Index,
                                      SelectionDAG &DAG) {
   // Try to iteratively fold parts of the index into the base pointer to
   // simplify the index as much as possible.
   bool Changed = false;
   while (foldIndexIntoBase(BasePtr, Index, N->getScale(), SDLoc(N), DAG))
     Changed = true;
 
   // Only consider element types that are pointer sized as smaller types can
   // be easily promoted.
   EVT IndexVT = Index.getValueType();
   if (IndexVT.getVectorElementType() != MVT::i64 || IndexVT == MVT::nxv2i64)
     return Changed;
 
   // Can indices be trivially shrunk?
-  if (ISD::isVectorShrinkable(Index.getNode(), 32, N->isIndexSigned())) {
+  EVT DataVT = N->getOperand(1).getValueType();
+  if (ISD::isVectorShrinkable(Index.getNode(), 32, N->isIndexSigned()) &&
+      !(DataVT.getScalarSizeInBits() == 64 && DataVT.isFixedLengthVector())) {
     EVT NewIndexVT = IndexVT.changeVectorElementType(MVT::i32);
     Index = DAG.getNode(ISD::TRUNCATE, SDLoc(N), NewIndexVT, Index);
     return true;
   }
 
   // Match:
   //   Index = step(const)
   int64_t Stride = 0;

llvm/test/CodeGen/AArch64/sve-gather-scatter-dag-combine.ll

; CHECK-NEXT: ret
   call void @llvm.masked.store.nxv2i8(<vscale x 2 x i8> %load,
                                       <vscale x 2 x i8> *%res_out,
                                       i32 8,
                                       <vscale x 2 x i1> %pred)
 
   ret <vscale x 2 x i64> %res
 }
 
-define <vscale x 16 x i8> @narrow_i64_gather_index_i8(i8* %out, i8* %in, <vscale x 16 x i8> %d, i64 %ptr){
-; CHECK-LABEL: narrow_i64_gather_index_i8:
+define <vscale x 16 x i8> @narrow_i64_gather_index_i8_zext(i8* %out, i8* %in, <vscale x 16 x i8> %d, i64 %ptr){
+; CHECK-LABEL: narrow_i64_gather_index_i8_zext:
 ; CHECK:       // %bb.0:
 ; CHECK-NEXT:    add x8, x1, x2
-; CHECK-NEXT:    ptrue p0.d
-; CHECK-NEXT:    ld1b { z0.d }, p0/z, [x1, x2]
-; CHECK-NEXT:    ld1b { z1.d }, p0/z, [x8, #1, mul vl]
-; CHECK-NEXT:    ld1b { z2.d }, p0/z, [x8, #2, mul vl]
-; CHECK-NEXT:    ld1b { z3.d }, p0/z, [x8, #3, mul vl]
-; CHECK-NEXT:    ld1b { z4.d }, p0/z, [x8, #4, mul vl]
-; CHECK-NEXT:    ld1b { z5.d }, p0/z, [x8, #5, mul vl]
-; CHECK-NEXT:    ld1b { z6.d }, p0/z, [x8, #6, mul vl]
-; CHECK-NEXT:    ld1b { z7.d }, p0/z, [x8, #7, mul vl]
-; CHECK-NEXT:    ld1b { z7.d }, p0/z, [x1, z7.d]
-; CHECK-NEXT:    ld1b { z6.d }, p0/z, [x1, z6.d]
-; CHECK-NEXT:    ld1b { z5.d }, p0/z, [x1, z5.d]
-; CHECK-NEXT:    ld1b { z4.d }, p0/z, [x1, z4.d]
-; CHECK-NEXT:    ld1b { z3.d }, p0/z, [x1, z3.d]
-; CHECK-NEXT:    ld1b { z2.d }, p0/z, [x1, z2.d]
-; CHECK-NEXT:    ld1b { z0.d }, p0/z, [x1, z0.d]
-; CHECK-NEXT:    ld1b { z1.d }, p0/z, [x1, z1.d]
-; CHECK-NEXT:    uzp1 z6.s, z6.s, z7.s
-; CHECK-NEXT:    uzp1 z4.s, z4.s, z5.s
-; CHECK-NEXT:    uzp1 z2.s, z2.s, z3.s
-; CHECK-NEXT:    uzp1 z0.s, z0.s, z1.s
-; CHECK-NEXT:    uzp1 z1.h, z4.h, z6.h
-; CHECK-NEXT:    uzp1 z0.h, z0.h, z2.h
-; CHECK-NEXT:    uzp1 z0.b, z0.b, z1.b
+; CHECK-NEXT:    ptrue p0.s
+; CHECK-NEXT:    ld1b { z0.s }, p0/z, [x1, x2]
+; CHECK-NEXT:    ld1b { z1.s }, p0/z, [x8, #1, mul vl]
+; CHECK-NEXT:    ld1b { z2.s }, p0/z, [x8, #2, mul vl]
+; CHECK-NEXT:    ld1b { z3.s }, p0/z, [x8, #3, mul vl]
+; CHECK-NEXT:    ld1b { z3.s }, p0/z, [x1, z3.s, uxtw]
+; CHECK-NEXT:    ld1b { z2.s }, p0/z, [x1, z2.s, uxtw]
+; CHECK-NEXT:    ld1b { z0.s }, p0/z, [x1, z0.s, uxtw]
+; CHECK-NEXT:    ld1b { z1.s }, p0/z, [x1, z1.s, uxtw]
+; CHECK-NEXT:    uzp1 z2.h, z2.h, z3.h
+; CHECK-NEXT:    uzp1 z0.h, z0.h, z1.h
+; CHECK-NEXT:    uzp1 z0.b, z0.b, z2.b
 ; CHECK-NEXT:    ret
   %1 = getelementptr inbounds i8, i8* %in, i64 %ptr
   %2 = bitcast i8* %1 to <vscale x 16 x i8>*
   %wide.load = load <vscale x 16 x i8>, <vscale x 16 x i8>* %2, align 1
   %3 = zext <vscale x 16 x i8> %wide.load to <vscale x 16 x i64>
   %4 = getelementptr inbounds i8, i8* %in, <vscale x 16 x i64> %3
   %wide.masked.gather = call <vscale x 16 x i8> @llvm.masked.gather.nxv16i8.nxv16p0(<vscale x 16 x i8*> %4, i32 1, <vscale x 16 x i1> shufflevector (<vscale x 16 x i1> insertelement (<vscale x 16 x i1> poison, i1 true, i32 0), <vscale x 16 x i1> poison, <vscale x 16 x i32> zeroinitializer), <vscale x 16 x i8> undef)
   ret <vscale x 16 x i8> %wide.masked.gather
 }
 
-define <vscale x 8 x i16> @narrow_i64_gather_index_i16(i16* %out, i16* %in, <vscale x 8 x i16> %d, i64 %ptr){
-; CHECK-LABEL: narrow_i64_gather_index_i16:
+define <vscale x 16 x i8> @narrow_i64_gather_index_i8_sext(i8* %out, i8* %in, <vscale x 16 x i8> %d, i64 %ptr){
+; CHECK-LABEL: narrow_i64_gather_index_i8_sext:
+; CHECK:       // %bb.0:
+; CHECK-NEXT:    add x8, x1, x2
+; CHECK-NEXT:    ptrue p0.s
+; CHECK-NEXT:    ld1sb { z0.s }, p0/z, [x1, x2]
+; CHECK-NEXT:    ld1sb { z1.s }, p0/z, [x8, #1, mul vl]
+; CHECK-NEXT:    ld1sb { z2.s }, p0/z, [x8, #2, mul vl]
+; CHECK-NEXT:    ld1sb { z3.s }, p0/z, [x8, #3, mul vl]
+; CHECK-NEXT:    ld1b { z3.s }, p0/z, [x1, z3.s, sxtw]
+; CHECK-NEXT:    ld1b { z2.s }, p0/z, [x1, z2.s, sxtw]
+; CHECK-NEXT:    ld1b { z0.s }, p0/z, [x1, z0.s, sxtw]
+; CHECK-NEXT:    ld1b { z1.s }, p0/z, [x1, z1.s, sxtw]
+; CHECK-NEXT:    uzp1 z2.h, z2.h, z3.h
+; CHECK-NEXT:    uzp1 z0.h, z0.h, z1.h
+; CHECK-NEXT:    uzp1 z0.b, z0.b, z2.b
+; CHECK-NEXT:    ret
+  %1 = getelementptr inbounds i8, i8* %in, i64 %ptr
+  %2 = bitcast i8* %1 to <vscale x 16 x i8>*
+  %wide.load = load <vscale x 16 x i8>, <vscale x 16 x i8>* %2, align 1
+  %3 = sext <vscale x 16 x i8> %wide.load to <vscale x 16 x i64>
+  %4 = getelementptr inbounds i8, i8* %in, <vscale x 16 x i64> %3
+  %wide.masked.gather = call <vscale x 16 x i8> @llvm.masked.gather.nxv16i8.nxv16p0(<vscale x 16 x i8*> %4, i32 1, <vscale x 16 x i1> shufflevector (<vscale x 16 x i1> insertelement (<vscale x 16 x i1> poison, i1 true, i32 0), <vscale x 16 x i1> poison, <vscale x 16 x i32> zeroinitializer), <vscale x 16 x i8> undef)
+  ret <vscale x 16 x i8> %wide.masked.gather
+}
+
+define <vscale x 8 x i16> @narrow_i64_gather_index_i16_zext(i16* %out, i16* %in, <vscale x 8 x i16> %d, i64 %ptr){
+; CHECK-LABEL: narrow_i64_gather_index_i16_zext:
 ; CHECK:       // %bb.0:
 ; CHECK-NEXT:    add x8, x1, x2, lsl #1
-; CHECK-NEXT:    ptrue p0.d
-; CHECK-NEXT:    ld1h { z0.d }, p0/z, [x1, x2, lsl #1]
-; CHECK-NEXT:    ld1h { z1.d }, p0/z, [x8, #1, mul vl]
-; CHECK-NEXT:    ld1h { z2.d }, p0/z, [x8, #2, mul vl]
-; CHECK-NEXT:    ld1h { z3.d }, p0/z, [x8, #3, mul vl]
-; CHECK-NEXT:    ld1h { z3.d }, p0/z, [x1, z3.d, lsl #1]
-; CHECK-NEXT:    ld1h { z2.d }, p0/z, [x1, z2.d, lsl #1]
-; CHECK-NEXT:    ld1h { z0.d }, p0/z, [x1, z0.d, lsl #1]
-; CHECK-NEXT:    ld1h { z1.d }, p0/z, [x1, z1.d, lsl #1]
-; CHECK-NEXT:    uzp1 z2.s, z2.s, z3.s
-; CHECK-NEXT:    uzp1 z0.s, z0.s, z1.s
-; CHECK-NEXT:    uzp1 z0.h, z0.h, z2.h
+; CHECK-NEXT:    ptrue p0.s
+; CHECK-NEXT:    ld1h { z0.s }, p0/z, [x1, x2, lsl #1]
+; CHECK-NEXT:    ld1h { z1.s }, p0/z, [x8, #1, mul vl]
+; CHECK-NEXT:    ld1h { z0.s }, p0/z, [x1, z0.s, uxtw #1]
+; CHECK-NEXT:    ld1h { z1.s }, p0/z, [x1, z1.s, uxtw #1]
+; CHECK-NEXT:    uzp1 z0.h, z0.h, z1.h
 ; CHECK-NEXT:    ret
   %1 = getelementptr inbounds i16, i16* %in, i64 %ptr
   %2 = bitcast i16* %1 to <vscale x 8 x i16>*
   %wide.load = load <vscale x 8 x i16>, <vscale x 8 x i16>* %2, align 1
   %3 = zext <vscale x 8 x i16> %wide.load to <vscale x 8 x i64>
   %4 = getelementptr inbounds i16, i16* %in, <vscale x 8 x i64> %3
   %wide.masked.gather = call <vscale x 8 x i16> @llvm.masked.gather.nxv8i16.nxv8p0(<vscale x 8 x i16*> %4, i32 1, <vscale x 8 x i1> shufflevector (<vscale x 8 x i1> insertelement (<vscale x 8 x i1> poison, i1 true, i32 0), <vscale x 8 x i1> poison, <vscale x 8 x i32> zeroinitializer), <vscale x 8 x i16> undef)
   ret <vscale x 8 x i16> %wide.masked.gather
 }
 
+define <vscale x 8 x i16> @narrow_i64_gather_index_i16_sext(i16* %out, i16* %in, <vscale x 8 x i16> %d, i64 %ptr){
+; CHECK-LABEL: narrow_i64_gather_index_i16_sext:
+; CHECK:       // %bb.0:
+; CHECK-NEXT:    add x8, x1, x2, lsl #1
+; CHECK-NEXT:    ptrue p0.s
+; CHECK-NEXT:    ld1sh { z0.s }, p0/z, [x1, x2, lsl #1]
+; CHECK-NEXT:    ld1sh { z1.s }, p0/z, [x8, #1, mul vl]
+; CHECK-NEXT:    ld1h { z0.s }, p0/z, [x1, z0.s, sxtw #1]
+; CHECK-NEXT:    ld1h { z1.s }, p0/z, [x1, z1.s, sxtw #1]
+; CHECK-NEXT:    uzp1 z0.h, z0.h, z1.h
+; CHECK-NEXT:    ret
+  %1 = getelementptr inbounds i16, i16* %in, i64 %ptr
+  %2 = bitcast i16* %1 to <vscale x 8 x i16>*
+  %wide.load = load <vscale x 8 x i16>, <vscale x 8 x i16>* %2, align 1
+  %3 = sext <vscale x 8 x i16> %wide.load to <vscale x 8 x i64>
+  %4 = getelementptr inbounds i16, i16* %in, <vscale x 8 x i64> %3
+  %wide.masked.gather = call <vscale x 8 x i16> @llvm.masked.gather.nxv8i16.nxv8p0(<vscale x 8 x i16*> %4, i32 1, <vscale x 8 x i1> shufflevector (<vscale x 8 x i1> insertelement (<vscale x 8 x i1> poison, i1 true, i32 0), <vscale x 8 x i1> poison, <vscale x 8 x i32> zeroinitializer), <vscale x 8 x i16> undef)
+  ret <vscale x 8 x i16> %wide.masked.gather
+}
+
 define <vscale x 4 x i32> @no_narrow_i64_gather_index_i32(i32* %out, i32* %in, <vscale x 4 x i32> %d, i64 %ptr){
 ; CHECK-LABEL: no_narrow_i64_gather_index_i32:
 ; CHECK:       // %bb.0:
 ; CHECK-NEXT:    ptrue p0.s
 ; CHECK-NEXT:    ld1w { z0.s }, p0/z, [x1, x2, lsl #2]
 ; CHECK-NEXT:    ld1w { z0.s }, p0/z, [x1, z0.s, uxtw #2]
 ; CHECK-NEXT:    ret
   %1 = getelementptr inbounds i32, i32* %in, i64 %ptr