[RISCV] Implement getVScaleForTuning and thus prefer scalable vectorization when enabled

[reames]

Summary

LoopVectorizer uses getVScaleForTuning for deciding how to discount the cost of a potential vector factor by the amount of work performed. Without the callback implemented, the vectorizer was defaulting to an estimated vscale of 1. This results in fixed vectorization looking falsely profitable (since it used the command line VLEN).

The test change is pretty limited since a) we don't have much coverage of the vectorizer with scalable vectors at all, and b) what little coverage we have mostly uses i64 element types. There's a separate issue with <vscale x 1 x i64> which prevents us from getting to this stage of costing, and thus only the one test explicitly written to avoid that is visible in the diff. However, this is actually a very wide impact change as it changes the practical vectorization result when both fixed and scalable is enabled to scalable.

As an aside, I think the vectorizer is at little too strongly biased towards scalable when both are legal, but we can explore that separately. For now, let's just get the cost model working the way it was intended.

[craig.topper]

LGTM

llvm/lib/Target/RISCV/RISCVTargetTransformInfo.h

InstructionCost getIntImmCostIntrin(Intrinsic::ID IID, unsigned Idx,
                                       const APInt &Imm, Type *Ty,
                                       TTI::TargetCostKind CostKind);
 
   TargetTransformInfo::PopcntSupportKind getPopcntSupport(unsigned TyWidth);
 
   bool shouldExpandReduction(const IntrinsicInst *II) const;
   bool supportsScalableVectors() const { return ST->hasVInstructions(); }
   Optional<unsigned> getMaxVScale() const;
+  Optional<unsigned> getVScaleForTuning() const;
 
   TypeSize getRegisterBitWidth(TargetTransformInfo::RegisterKind K) const;
 
   unsigned getRegUsageForType(Type *Ty);
 
   InstructionCost getMaskedMemoryOpCost(unsigned Opcode, Type *Src,
                                         Align Alignment, unsigned AddressSpace,
                                         TTI::TargetCostKind CostKind);

llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp

 }
 
 Optional<unsigned> RISCVTTIImpl::getMaxVScale() const {
   if (ST->hasVInstructions())
     return ST->getRealMaxVLen() / RISCV::RVVBitsPerBlock;
   return BaseT::getMaxVScale();
 }
 
+Optional<unsigned> RISCVTTIImpl::getVScaleForTuning() const {
+  if (ST->hasVInstructions())
+    return ST->getRealMinVLen() / RISCV::RVVBitsPerBlock;
+  return BaseT::getVScaleForTuning();
+}
+
 TypeSize
 RISCVTTIImpl::getRegisterBitWidth(TargetTransformInfo::RegisterKind K) const {
   unsigned LMUL = PowerOf2Floor(
       std::max<unsigned>(std::min<unsigned>(RVVRegisterWidthLMUL, 8), 1));
   switch (K) {
   case TargetTransformInfo::RGK_Scalar:
     return TypeSize::getFixed(ST->getXLen());
   case TargetTransformInfo::RGK_FixedWidthVector:

llvm/test/Transforms/LoopVectorize/RISCV/scalable-basics.ll

 ; VLENUNK-NEXT:    [[IV_NEXT]] = add nuw nsw i64 [[IV]], 1
 ; VLENUNK-NEXT:    [[EXITCOND_NOT:%.*]] = icmp eq i64 [[IV_NEXT]], 1024
 ; VLENUNK-NEXT:    br i1 [[EXITCOND_NOT]], label [[FOR_END]], label [[FOR_BODY]], !llvm.loop [[LOOP2:![0-9]+]]
 ; VLENUNK:       for.end:
 ; VLENUNK-NEXT:    ret void
 ;
 ; VLEN128-LABEL: @vector_add_i32(
 ; VLEN128-NEXT:  entry:
-; VLEN128-NEXT:    br i1 false, label [[SCALAR_PH:%.*]], label [[VECTOR_PH:%.*]]
+; VLEN128-NEXT:    [[TMP0:%.*]] = call i64 @llvm.vscale.i64()
+; VLEN128-NEXT:    [[TMP1:%.*]] = mul i64 [[TMP0]], 4
+; VLEN128-NEXT:    [[MIN_ITERS_CHECK:%.*]] = icmp ult i64 1024, [[TMP1]]
+; VLEN128-NEXT:    br i1 [[MIN_ITERS_CHECK]], label [[SCALAR_PH:%.*]], label [[VECTOR_PH:%.*]]
 ; VLEN128:       vector.ph:
-; VLEN128-NEXT:    [[BROADCAST_SPLATINSERT:%.*]] = insertelement <4 x i32> poison, i32 [[V:%.*]], i32 0
-; VLEN128-NEXT:    [[BROADCAST_SPLAT:%.*]] = shufflevector <4 x i32> [[BROADCAST_SPLATINSERT]], <4 x i32> poison, <4 x i32> zeroinitializer
-; VLEN128-NEXT:    [[BROADCAST_SPLATINSERT2:%.*]] = insertelement <4 x i32> poison, i32 [[V]], i32 0
-; VLEN128-NEXT:    [[BROADCAST_SPLAT3:%.*]] = shufflevector <4 x i32> [[BROADCAST_SPLATINSERT2]], <4 x i32> poison, <4 x i32> zeroinitializer
+; VLEN128-NEXT:    [[TMP2:%.*]] = call i64 @llvm.vscale.i64()
+; VLEN128-NEXT:    [[TMP3:%.*]] = mul i64 [[TMP2]], 4
+; VLEN128-NEXT:    [[N_MOD_VF:%.*]] = urem i64 1024, [[TMP3]]
+; VLEN128-NEXT:    [[N_VEC:%.*]] = sub i64 1024, [[N_MOD_VF]]
+; VLEN128-NEXT:    [[BROADCAST_SPLATINSERT:%.*]] = insertelement <vscale x 2 x i32> poison, i32 [[V:%.*]], i32 0
+; VLEN128-NEXT:    [[BROADCAST_SPLAT:%.*]] = shufflevector <vscale x 2 x i32> [[BROADCAST_SPLATINSERT]], <vscale x 2 x i32> poison, <vscale x 2 x i32> zeroinitializer
+; VLEN128-NEXT:    [[BROADCAST_SPLATINSERT2:%.*]] = insertelement <vscale x 2 x i32> poison, i32 [[V]], i32 0
+; VLEN128-NEXT:    [[BROADCAST_SPLAT3:%.*]] = shufflevector <vscale x 2 x i32> [[BROADCAST_SPLATINSERT2]], <vscale x 2 x i32> poison, <vscale x 2 x i32> zeroinitializer
 ; VLEN128-NEXT:    br label [[VECTOR_BODY:%.*]]
 ; VLEN128:       vector.body:
 ; VLEN128-NEXT:    [[INDEX:%.*]] = phi i64 [ 0, [[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], [[VECTOR_BODY]] ]
-; VLEN128-NEXT:    [[TMP0:%.*]] = add i64 [[INDEX]], 0
-; VLEN128-NEXT:    [[TMP1:%.*]] = add i64 [[INDEX]], 4
-; VLEN128-NEXT:    [[TMP2:%.*]] = getelementptr inbounds i32, ptr [[A:%.*]], i64 [[TMP0]]
-; VLEN128-NEXT:    [[TMP3:%.*]] = getelementptr inbounds i32, ptr [[A]], i64 [[TMP1]]
-; VLEN128-NEXT:    [[TMP4:%.*]] = getelementptr inbounds i32, ptr [[TMP2]], i32 0
-; VLEN128-NEXT:    [[WIDE_LOAD:%.*]] = load <4 x i32>, ptr [[TMP4]], align 4
-; VLEN128-NEXT:    [[TMP5:%.*]] = getelementptr inbounds i32, ptr [[TMP2]], i32 4
-; VLEN128-NEXT:    [[WIDE_LOAD1:%.*]] = load <4 x i32>, ptr [[TMP5]], align 4
-; VLEN128-NEXT:    [[TMP6:%.*]] = add <4 x i32> [[WIDE_LOAD]], [[BROADCAST_SPLAT]]
-; VLEN128-NEXT:    [[TMP7:%.*]] = add <4 x i32> [[WIDE_LOAD1]], [[BROADCAST_SPLAT3]]
-; VLEN128-NEXT:    store <4 x i32> [[TMP6]], ptr [[TMP4]], align 4
-; VLEN128-NEXT:    store <4 x i32> [[TMP7]], ptr [[TMP5]], align 4
-; VLEN128-NEXT:    [[INDEX_NEXT]] = add nuw i64 [[INDEX]], 8
-; VLEN128-NEXT:    [[TMP8:%.*]] = icmp eq i64 [[INDEX_NEXT]], 1024
-; VLEN128-NEXT:    br i1 [[TMP8]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop [[LOOP4:![0-9]+]]
+; VLEN128-NEXT:    [[TMP4:%.*]] = add i64 [[INDEX]], 0
+; VLEN128-NEXT:    [[TMP5:%.*]] = call i64 @llvm.vscale.i64()
+; VLEN128-NEXT:    [[TMP6:%.*]] = mul i64 [[TMP5]], 2
+; VLEN128-NEXT:    [[TMP7:%.*]] = add i64 [[TMP6]], 0
+; VLEN128-NEXT:    [[TMP8:%.*]] = mul i64 [[TMP7]], 1
+; VLEN128-NEXT:    [[TMP9:%.*]] = add i64 [[INDEX]], [[TMP8]]
+; VLEN128-NEXT:    [[TMP10:%.*]] = getelementptr inbounds i32, ptr [[A:%.*]], i64 [[TMP4]]
+; VLEN128-NEXT:    [[TMP11:%.*]] = getelementptr inbounds i32, ptr [[A]], i64 [[TMP9]]
+; VLEN128-NEXT:    [[TMP12:%.*]] = getelementptr inbounds i32, ptr [[TMP10]], i32 0
+; VLEN128-NEXT:    [[WIDE_LOAD:%.*]] = load <vscale x 2 x i32>, ptr [[TMP12]], align 4
+; VLEN128-NEXT:    [[TMP13:%.*]] = call i32 @llvm.vscale.i32()
+; VLEN128-NEXT:    [[TMP14:%.*]] = mul i32 [[TMP13]], 2
+; VLEN128-NEXT:    [[TMP15:%.*]] = getelementptr inbounds i32, ptr [[TMP10]], i32 [[TMP14]]
+; VLEN128-NEXT:    [[WIDE_LOAD1:%.*]] = load <vscale x 2 x i32>, ptr [[TMP15]], align 4
+; VLEN128-NEXT:    [[TMP16:%.*]] = add <vscale x 2 x i32> [[WIDE_LOAD]], [[BROADCAST_SPLAT]]
+; VLEN128-NEXT:    [[TMP17:%.*]] = add <vscale x 2 x i32> [[WIDE_LOAD1]], [[BROADCAST_SPLAT3]]
+; VLEN128-NEXT:    store <vscale x 2 x i32> [[TMP16]], ptr [[TMP12]], align 4
+; VLEN128-NEXT:    [[TMP18:%.*]] = call i32 @llvm.vscale.i32()
+; VLEN128-NEXT:    [[TMP19:%.*]] = mul i32 [[TMP18]], 2
+; VLEN128-NEXT:    [[TMP20:%.*]] = getelementptr inbounds i32, ptr [[TMP10]], i32 [[TMP19]]
+; VLEN128-NEXT:    store <vscale x 2 x i32> [[TMP17]], ptr [[TMP20]], align 4
+; VLEN128-NEXT:    [[TMP21:%.*]] = call i64 @llvm.vscale.i64()
+; VLEN128-NEXT:    [[TMP22:%.*]] = mul i64 [[TMP21]], 4
+; VLEN128-NEXT:    [[INDEX_NEXT]] = add nuw i64 [[INDEX]], [[TMP22]]
+; VLEN128-NEXT:    [[TMP23:%.*]] = icmp eq i64 [[INDEX_NEXT]], [[N_VEC]]
+; VLEN128-NEXT:    br i1 [[TMP23]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop [[LOOP4:![0-9]+]]
 ; VLEN128:       middle.block:
-; VLEN128-NEXT:    [[CMP_N:%.*]] = icmp eq i64 1024, 1024
+; VLEN128-NEXT:    [[CMP_N:%.*]] = icmp eq i64 1024, [[N_VEC]]
 ; VLEN128-NEXT:    br i1 [[CMP_N]], label [[FOR_END:%.*]], label [[SCALAR_PH]]
 ; VLEN128:       scalar.ph:
-; VLEN128-NEXT:    [[BC_RESUME_VAL:%.*]] = phi i64 [ 1024, [[MIDDLE_BLOCK]] ], [ 0, [[ENTRY:%.*]] ]
+; VLEN128-NEXT:    [[BC_RESUME_VAL:%.*]] = phi i64 [ [[N_VEC]], [[MIDDLE_BLOCK]] ], [ 0, [[ENTRY:%.*]] ]
 ; VLEN128-NEXT:    br label [[FOR_BODY:%.*]]
 ; VLEN128:       for.body:
 ; VLEN128-NEXT:    [[IV:%.*]] = phi i64 [ [[BC_RESUME_VAL]], [[SCALAR_PH]] ], [ [[IV_NEXT:%.*]], [[FOR_BODY]] ]
 ; VLEN128-NEXT:    [[ARRAYIDX:%.*]] = getelementptr inbounds i32, ptr [[A]], i64 [[IV]]
 ; VLEN128-NEXT:    [[ELEM:%.*]] = load i32, ptr [[ARRAYIDX]], align 4
 ; VLEN128-NEXT:    [[ADD:%.*]] = add i32 [[ELEM]], [[V]]
 ; VLEN128-NEXT:    store i32 [[ADD]], ptr [[ARRAYIDX]], align 4
 ; VLEN128-NEXT:    [[IV_NEXT]] = add nuw nsw i64 [[IV]], 1