[SVE] Add flag to specify SVE register size, using this to calculate legal vector types.

[paulwalker-arm]

Summary

Adds aarch64-sve-vector-bits-{min,max} to allow the size of SVE
data registers (in bits) to be specified. This allows the code
generator to make assumptions it normally couldn't. As a starting
point this information is used to mark fixed length vector types
that can fit within the specified size as legal.

[paulwalker-arm]

Rebase and ping.

[efriedma]

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

3500

Restricting i1 types like this seems a little weird to me. We don't need to pretend to be NEON. We want these types to be legal for similar reasons we want the other fixed vector types to be legal. I guess we could leave the check for now if it makes it easier to get simple testcases working, but I don't think this is what we want long-term.

For non-i1 types, should we check the element type is legal? integer_fixedlen_vector_valuetypes() and fp_fixedlen_vector_valuetypes() can return some exotic types.

3512

Non-power-of-two types are an extra complication, yes; better to leave it out for now.

llvm/lib/Target/AArch64/AArch64Subtarget.cpp

345

We probably want function attributes?

It looks like this patch doesn't use the max size; what do you anticipate using it for?

[paulwalker-arm]

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

3500

Part of this is me not looking for trouble at this stage and wanting to wait to see if there's a performance reason to make them legal. I think the majority of the cases can be handled after the conversion to scalable vector types, which just leaves the cross-block scenarios.

However, from a functional point of view I'm trying not to make any ABI related changes. Currently the handling of i1 based vectors (well all fixed length vectors) has already been decided for NEON and so I cannot just change it in isolation without breaking compatibility with existing libraries (which don't know about SVE).

In the future we'll need a function attribute that corresponds to an as yet undefined ABI. When that is in place we can relax the i1 restriction. For now though my focus is purely on block level code generation.

llvm/lib/Target/AArch64/AArch64Subtarget.cpp

345

When creating the patch I did spot "min-legal-vector-width" exists as a function attribute but as mentioned above I'm trying to avoid establishing anything ABI related at this stage.

As part of D80385 where I've added the createPredicateForFixedVector function there's a comment that mentions the use case for the max size. Essentially when min==max we can avoid the need to create explicitly sized predicates and instead use "PTRUE ALL" as a route to select unpredicated instructions when available.

[paulwalker-arm]

Updated useSVEForFixedLengthVectors to reject vectors of illegal element types.

[efriedma]

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

3500

We can mess with the ABI separately if we need to say the values are sign-extended or whatever.

I'm concerned that if we don't make this legal now, we're going to end up writing a bunch of code to work around the fact that we can't produce i1 vectors after legalization.

3503

isTypeLegal(VT.getVectorElementType()) probably doesn't do what you want; i8 is not a legal type.

llvm/lib/Target/AArch64/AArch64Subtarget.cpp

345

I guess we can leave out function attributes for now, but it's a temporary solution at best; LLVM flags are not properly usable from clang.

[paulwalker-arm]

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

3500

Do you have any specific examples. I ask because my experience is the opposite. By keeping fixed length i1 vectors as illegal I am able to remove a lot of work that just isn't necessary to achieve my objective of supporting larger fixed length vectors.

I'll admit that here I'm potentially trading a bit on performance but there's nothing binding here, it's just the order of implementation I prefer.

None of this affects scalable vectors so it doesn't affect the post operation legalisation side of things.

3503

That'll teach me for taking things too literally without thinking about what I'm writing :)

I've changed this to an explicit list and deliberately kept it minimal as bf and 128bit element types are things I don't really care about at this stage.

llvm/lib/Target/AArch64/AArch64Subtarget.cpp

345

Yep, I understand. The flags allow me to write more concise tests so even when we nail down the way front-ends enables this feature I suspect the flags will still have value.

[paulwalker-arm]

Rebased so now all dependent filecheck work is available.
Corrected the test for legal vector element types.
Updated test to include some vector element type variation.

[efriedma]

Okay, it sounds like we're on the same page. LGTM

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

3500

Say we have something like (a==b|c==d)? e : f. In NEON, we do something with blending in vector registers. In SVE, we do not want the condition to ever be moved from predicate registers to vector registers. So now you're pattern-matching some combination of logic operators and blends to try to recover the obvious SVE code.

If you aren't really doing that sort of optimization in the first iteration, transitioning later might not be a big deal.

llvm/lib/Target/AArch64/AArch64ISelLowering.h

void ReplaceExtractSubVectorResults(SDNode *N,
                                       SelectionDAG &DAG) const;
 
   bool shouldNormalizeToSelectSequence(LLVMContext &, EVT) const override;
 
   void finalizeLowering(MachineFunction &MF) const override;
 
   bool shouldLocalize(const MachineInstr &MI,
                       const TargetTransformInfo *TTI) const override;
+
+  bool useSVEForFixedLengthVectors() const;
+  bool useSVEForFixedLengthVectorVT(MVT VT) const;
 };
 
 namespace AArch64 {
 FastISel *createFastISel(FunctionLoweringInfo &funcInfo,
                          const TargetLibraryInfo *libInfo);
 } // end namespace AArch64
 
 } // end namespace llvm
 
 #endif

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

if (Subtarget->hasSVE()) {
     addRegisterClass(MVT::nxv8f16, &AArch64::ZPRRegClass);
     addRegisterClass(MVT::nxv2bf16, &AArch64::ZPRRegClass);
     addRegisterClass(MVT::nxv4bf16, &AArch64::ZPRRegClass);
     addRegisterClass(MVT::nxv8bf16, &AArch64::ZPRRegClass);
     addRegisterClass(MVT::nxv2f32, &AArch64::ZPRRegClass);
     addRegisterClass(MVT::nxv4f32, &AArch64::ZPRRegClass);
     addRegisterClass(MVT::nxv2f64, &AArch64::ZPRRegClass);
 
+    if (useSVEForFixedLengthVectors()) {
+      for (MVT VT : MVT::integer_fixedlen_vector_valuetypes())
+        if (useSVEForFixedLengthVectorVT(VT))
+          addRegisterClass(VT, &AArch64::ZPRRegClass);
+
+      for (MVT VT : MVT::fp_fixedlen_vector_valuetypes())
+        if (useSVEForFixedLengthVectorVT(VT))
+          addRegisterClass(VT, &AArch64::ZPRRegClass);
+    }
+
     for (auto VT : { MVT::nxv16i8, MVT::nxv8i16, MVT::nxv4i32, MVT::nxv2i64 }) {
       setOperationAction(ISD::SADDSAT, VT, Legal);
       setOperationAction(ISD::UADDSAT, VT, Legal);
       setOperationAction(ISD::SSUBSAT, VT, Legal);
       setOperationAction(ISD::USUBSAT, VT, Legal);
     }
 
     for (auto VT :
case ISD::ATOMIC_LOAD_AND:
     return LowerATOMIC_LOAD_AND(Op, DAG);
   case ISD::DYNAMIC_STACKALLOC:
     return LowerDYNAMIC_STACKALLOC(Op, DAG);
   case ISD::VSCALE:
     return LowerVSCALE(Op, DAG);
   }
 }
 
+bool AArch64TargetLowering::useSVEForFixedLengthVectors() const {
+  // Prefer NEON unless larger SVE registers are available.
+  return Subtarget->hasSVE() && Subtarget->getMinSVEVectorSizeInBits() >= 256;
+}
+
+bool AArch64TargetLowering::useSVEForFixedLengthVectorVT(MVT VT) const {
+  assert(VT.isFixedLengthVector());
+  if (!useSVEForFixedLengthVectors())
+    return false;
+
+  // Fixed length predicates should be promoted to i8.
+  // NOTE: This is consistent with how NEON (and thus 64/128bit vectors) work.
+  if (VT.getVectorElementType() == MVT::i1)
+    return false;
+
+  // Don't use SVE for vectors we cannot scalarize if required.
+  if (!isTypeLegal(VT.getVectorElementType()))
+    return false;
+
+  // Ensure NEON MVTs only belong to a single register class.
+  if (VT.getSizeInBits() <= 128)
+    return false;
+
+  // Don't use SVE for types that don't fit.
+  if (VT.getSizeInBits() > Subtarget->getMinSVEVectorSizeInBits())
+    return false;
+
+  // TODO: Perhaps an artificial restriction, but worth having whilst getting
+  // the base fixed length SVE support in place.
+  if (!VT.isPow2VectorType())
+    return false;
+
+  return true;
+}
+
 //===----------------------------------------------------------------------===//
 //                      Calling Convention Implementation
 //===----------------------------------------------------------------------===//
 
 /// Selects the correct CCAssignFn for a given CallingConvention value.
 CCAssignFn *AArch64TargetLowering::CCAssignFnForCall(CallingConv::ID CC,
                                                      bool IsVarArg) const {
   switch (CC) {

llvm/lib/Target/AArch64/AArch64Subtarget.h

bool isCallingConvWin64(CallingConv::ID CC) const {
     case CallingConv::Win64:
       return true;
     default:
       return false;
     }
   }
 
   void mirFileLoaded(MachineFunction &MF) const override;
+
+  // Return the known range for the bit length of SVE data registers. A value
+  // of 0 means nothing is known about that particular limit beyong what's
+  // implied by the architecture.
+  unsigned getMaxSVEVectorSizeInBits() const;
+  unsigned getMinSVEVectorSizeInBits() const;
 };
 } // End llvm namespace
 
 #endif

llvm/lib/Target/AArch64/AArch64Subtarget.cpp

 UseAddressTopByteIgnored("aarch64-use-tbi", cl::desc("Assume that top byte of "
                          "an address is ignored"), cl::init(false), cl::Hidden);
 
 static cl::opt<bool>
     UseNonLazyBind("aarch64-enable-nonlazybind",
                    cl::desc("Call nonlazybind functions via direct GOT load"),
                    cl::init(false), cl::Hidden);
 
+static cl::opt<unsigned> SVEVectorBitsMax(
+    "aarch64-sve-vector-bits-max",
+    cl::desc("Assume SVE vector registers are at most this big, "
+             "with zero meaning no maximum size is assumed."),
+    cl::init(0), cl::Hidden);
+
+static cl::opt<unsigned> SVEVectorBitsMin(
+    "aarch64-sve-vector-bits-min",
+    cl::desc("Assume SVE vector registers are at least this big, "
+             "with zero meaning no minimum size is assumed."),
+    cl::init(0), cl::Hidden);
+
 AArch64Subtarget &
 AArch64Subtarget::initializeSubtargetDependencies(StringRef FS,
                                                   StringRef CPUString) {
   // Determine default and user-specified characteristics
 
   if (CPUString.empty())
     CPUString = "generic";
 
void AArch64Subtarget::mirFileLoaded(MachineFunction &MF) const {
   // We usually compute max call frame size after ISel. Do the computation now
   // if the .mir file didn't specify it. Note that this will probably give you
   // bogus values after PEI has eliminated the callframe setup/destroy pseudo
   // instructions, specify explicitly if you need it to be correct.
   MachineFrameInfo &MFI = MF.getFrameInfo();
   if (!MFI.isMaxCallFrameSizeComputed())
     MFI.computeMaxCallFrameSize(MF);
 }
+
+unsigned AArch64Subtarget::getMaxSVEVectorSizeInBits() const {
+  assert(HasSVE && "Tried to get SVE vector length without SVE support!");
+  assert(SVEVectorBitsMax % 128 == 0 &&
+         "SVE requires vector length in multiples of 128!");
+  assert((SVEVectorBitsMax >= SVEVectorBitsMin || SVEVectorBitsMax == 0) &&
+         "Minimum SVE vector size should not be larger than its maximum!");
+  if (SVEVectorBitsMax == 0)
+    return 0;
+  return (std::max(SVEVectorBitsMin, SVEVectorBitsMax) / 128) * 128;
+}
+
+unsigned AArch64Subtarget::getMinSVEVectorSizeInBits() const {
+  assert(HasSVE && "Tried to get SVE vector length without SVE support!");
+  assert(SVEVectorBitsMin % 128 == 0 &&
+         "SVE requires vector length in multiples of 128!");
+  assert((SVEVectorBitsMax >= SVEVectorBitsMin || SVEVectorBitsMax == 0) &&
+         "Minimum SVE vector size should not be larger than its maximum!");
+  if (SVEVectorBitsMax == 0)
+    return (SVEVectorBitsMin / 128) * 128;
+  return (std::min(SVEVectorBitsMin, SVEVectorBitsMax) / 128) * 128;
+}

llvm/lib/Target/AArch64/AArch64TargetTransformInfo.h

if (Vector) {
         return 32;
       return 0;
     }
     return 31;
   }
 
   unsigned getRegisterBitWidth(bool Vector) const {
     if (Vector) {
+      if (ST->hasSVE())
+        return std::max(ST->getMinSVEVectorSizeInBits(), 128u);
       if (ST->hasNEON())
         return 128;
       return 0;
     }
     return 64;
   }
 
   unsigned getMinVectorRegisterBitWidth() {

llvm/test/Analysis/CostModel/AArch64/sve-fixed-length.ll

+; RUN: opt < %s -cost-model -analyze | FileCheck %s -D#VBITS=128
+; RUN: opt < %s -cost-model -analyze -aarch64-sve-vector-bits-min=128 | FileCheck %s -D#VBITS=128
+; RUN: opt < %s -cost-model -analyze -aarch64-sve-vector-bits-min=256 | FileCheck %s -D#VBITS=256
+; RUN: opt < %s -cost-model -analyze -aarch64-sve-vector-bits-min=384 | FileCheck %s -D#VBITS=256
+; RUN: opt < %s -cost-model -analyze -aarch64-sve-vector-bits-min=512 | FileCheck %s -D#VBITS=512
+; RUN: opt < %s -cost-model -analyze -aarch64-sve-vector-bits-min=640 | FileCheck %s -D#VBITS=512
+; RUN: opt < %s -cost-model -analyze -aarch64-sve-vector-bits-min=768 | FileCheck %s -D#VBITS=512
+; RUN: opt < %s -cost-model -analyze -aarch64-sve-vector-bits-min=896 | FileCheck %s -D#VBITS=512
+; RUN: opt < %s -cost-model -analyze -aarch64-sve-vector-bits-min=1024 | FileCheck %s -D#VBITS=1024
+; RUN: opt < %s -cost-model -analyze -aarch64-sve-vector-bits-min=1152 | FileCheck %s -D#VBITS=1024
+; RUN: opt < %s -cost-model -analyze -aarch64-sve-vector-bits-min=1280 | FileCheck %s -D#VBITS=1024
+; RUN: opt < %s -cost-model -analyze -aarch64-sve-vector-bits-min=1408 | FileCheck %s -D#VBITS=1024
+; RUN: opt < %s -cost-model -analyze -aarch64-sve-vector-bits-min=1536 | FileCheck %s -D#VBITS=1024
+; RUN: opt < %s -cost-model -analyze -aarch64-sve-vector-bits-min=1664 | FileCheck %s -D#VBITS=1024
+; RUN: opt < %s -cost-model -analyze -aarch64-sve-vector-bits-min=1792 | FileCheck %s -D#VBITS=1024
+; RUN: opt < %s -cost-model -analyze -aarch64-sve-vector-bits-min=1920 | FileCheck %s -D#VBITS=1024
+; RUN: opt < %s -cost-model -analyze -aarch64-sve-vector-bits-min=2048 | FileCheck %s -D#VBITS=2048
+
+; VBITS represents the useful bit size of a vector register from the code
+; generator's point of view. It is clamped to power-of-2 values because
+; only power-of-2 vector lengths are considered legal, regardless of the
+; user specified vector length.
+
+target triple = "aarch64-unknown-linux-gnu"
+
+; Ensure the cost of legalisation is removed as the vector length grows.
+define void @add() #0 {
+; CHECK-LABEL: Printing analysis 'Cost Model Analysis' for function 'add':
+; CHECK: cost of [[#div(127,VBITS)+1]] for instruction:   %add128 = add <4 x i32> undef, undef
+; CHECK: cost of [[#div(255,VBITS)+1]] for instruction:   %add256 = add <8 x i32> undef, undef
+; CHECK: cost of [[#div(511,VBITS)+1]] for instruction:   %add512 = add <16 x i32> undef, undef
+; CHECK: cost of [[#div(1023,VBITS)+1]] for instruction:   %add1024 = add <32 x i32> undef, undef
+; CHECK: cost of [[#div(2047,VBITS)+1]] for instruction:   %add2048 = add <64 x i32> undef, undef
+  %add128 = add <4 x i32> undef, undef
+  %add256 = add <8 x i32> undef, undef
+  %add512 = add <16 x i32> undef, undef
+  %add1024 = add <32 x i32> undef, undef
+  %add2048 = add <64 x i32> undef, undef
+  ret void
+}
+
+attributes #0 = { "target-features"="+sve" }