Skip to content

Commit

Permalink
[InstCombine] foldSelectRotate - generalize to foldSelectFunnelShift
Browse files Browse the repository at this point in the history 
This is the last of the rotate->funnel shift InstCombine generalizations for PR46896

We still have foldGuardedRotateToFunnelShift to deal with in AggressiveInstCombine

Differential Revision: https://reviews.llvm.org/D90382
  • Loading branch information
@RKSimon
RKSimon committed Oct 31, 2020
1 parent 9e406ee commit 538fdb0
Show file tree
Hide file tree
Showing 2 changed files with 34 additions and 40 deletions.
53 changes: 31 additions & 22 deletions llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
View file
Expand Up @@ -2312,42 +2312,43 @@ static Instruction *factorizeMinMaxTree(SelectPatternFlavor SPF, Value *LHS,
return SelectInst::Create(CmpABC, MinMaxOp, ThirdOp);
}

/// Try to reduce a rotate pattern that includes a compare and select into a
/// funnel shift intrinsic. Example:
/// Try to reduce a funnel/rotate pattern that includes a compare and select
/// into a funnel shift intrinsic. Example:
/// rotl32(a, b) --> (b == 0 ? a : ((a >> (32 - b)) | (a << b)))
/// --> call llvm.fshl.i32(a, a, b)
static Instruction *foldSelectRotate(SelectInst &Sel,
InstCombiner::BuilderTy &Builder) {
// The false value of the select must be a rotate of the true value.
/// fshl32(a, b, c) --> (c == 0 ? a : ((b >> (32 - c)) | (a << c)))
/// --> call llvm.fshl.i32(a, b, c)
/// fshr32(a, b, c) --> (c == 0 ? b : ((a >> (32 - c)) | (b << c)))
/// --> call llvm.fshr.i32(a, b, c)
static Instruction *foldSelectFunnelShift(SelectInst &Sel,
InstCombiner::BuilderTy &Builder) {
// This must be a power-of-2 type for a bitmasking transform to be valid.
unsigned Width = Sel.getType()->getScalarSizeInBits();
if (!isPowerOf2_32(Width))
return nullptr;

BinaryOperator *Or0, *Or1;
if (!match(Sel.getFalseValue(), m_OneUse(m_Or(m_BinOp(Or0), m_BinOp(Or1)))))
return nullptr;

Value *TVal = Sel.getTrueValue();
Value *SA0, *SA1;
if (!match(Or0, m_OneUse(m_LogicalShift(m_Specific(TVal),
Value *SV0, *SV1, *SA0, *SA1;
if (!match(Or0, m_OneUse(m_LogicalShift(m_Value(SV0),
m_ZExtOrSelf(m_Value(SA0))))) ||
!match(Or1, m_OneUse(m_LogicalShift(m_Specific(TVal),
!match(Or1, m_OneUse(m_LogicalShift(m_Value(SV1),
m_ZExtOrSelf(m_Value(SA1))))) ||
Or0->getOpcode() == Or1->getOpcode())
return nullptr;

// Canonicalize to or(shl(TVal, SA0), lshr(TVal, SA1)).
// Canonicalize to or(shl(SV0, SA0), lshr(SV1, SA1)).
if (Or0->getOpcode() == BinaryOperator::LShr) {
std::swap(Or0, Or1);
std::swap(SV0, SV1);
std::swap(SA0, SA1);
}
assert(Or0->getOpcode() == BinaryOperator::Shl &&
Or1->getOpcode() == BinaryOperator::LShr &&
"Illegal or(shift,shift) pair");

// We should now have this pattern:
// select ?, TVal, (or (shl TVal, SA0), (lshr TVal, SA1))
// This must be a power-of-2 rotate for a bitmasking transform to be valid.
unsigned Width = Sel.getType()->getScalarSizeInBits();
if (!isPowerOf2_32(Width))
return nullptr;

// Check the shift amounts to see if they are an opposite pair.
Value *ShAmt;
if (match(SA1, m_OneUse(m_Sub(m_SpecificInt(Width), m_Specific(SA0)))))
Expand All @@ -2357,20 +2358,28 @@ static Instruction *foldSelectRotate(SelectInst &Sel,
else
return nullptr;

// We should now have this pattern:
// select ?, TVal, (or (shl SV0, SA0), (lshr SV1, SA1))
// The false value of the select must be a funnel-shift of the true value:
// IsFShl -> TVal must be SV0 else TVal must be SV1.
bool IsFshl = (ShAmt == SA0);
Value *TVal = Sel.getTrueValue();
if ((IsFshl && TVal != SV0) || (!IsFshl && TVal != SV1))
return nullptr;

// Finally, see if the select is filtering out a shift-by-zero.
Value *Cond = Sel.getCondition();
ICmpInst::Predicate Pred;
if (!match(Cond, m_OneUse(m_ICmp(Pred, m_Specific(ShAmt), m_ZeroInt()))) ||
Pred != ICmpInst::ICMP_EQ)
return nullptr;

// This is a rotate that avoids shift-by-bitwidth UB in a suboptimal way.
// This is a funnel/rotate that avoids shift-by-bitwidth UB in a suboptimal way.
// Convert to funnel shift intrinsic.
bool IsFshl = (ShAmt == SA0);
Intrinsic::ID IID = IsFshl ? Intrinsic::fshl : Intrinsic::fshr;
Function *F = Intrinsic::getDeclaration(Sel.getModule(), IID, Sel.getType());
ShAmt = Builder.CreateZExt(ShAmt, Sel.getType());
return IntrinsicInst::Create(F, { TVal, TVal, ShAmt });
return IntrinsicInst::Create(F, { SV0, SV1, ShAmt });
}

static Instruction *foldSelectToCopysign(SelectInst &Sel,
Expand Down Expand Up @@ -3014,8 +3023,8 @@ Instruction *InstCombinerImpl::visitSelectInst(SelectInst &SI) {
if (Instruction *Select = foldSelectBinOpIdentity(SI, TLI, *this))
return Select;

if (Instruction *Rot = foldSelectRotate(SI, Builder))
return Rot;
if (Instruction *Funnel = foldSelectFunnelShift(SI, Builder))
return Funnel;

if (Instruction *Copysign = foldSelectToCopysign(SI, Builder))
return Copysign;
Expand Down
21 changes: 3 additions & 18 deletions llvm/test/Transforms/InstCombine/funnel.ll
View file
Expand Up @@ -285,12 +285,7 @@ define i8 @fshr_commute_8bit(i32 %x, i32 %y, i32 %shift) {

define i8 @fshr_select(i8 %x, i8 %y, i8 %shamt) {
; CHECK-LABEL: @fshr_select(
; CHECK-NEXT: [[CMP:%.*]] = icmp eq i8 [[SHAMT:%.*]], 0
; CHECK-NEXT: [[SUB:%.*]] = sub i8 8, [[SHAMT]]
; CHECK-NEXT: [[SHR:%.*]] = lshr i8 [[Y:%.*]], [[SHAMT]]
; CHECK-NEXT: [[SHL:%.*]] = shl i8 [[X:%.*]], [[SUB]]
; CHECK-NEXT: [[OR:%.*]] = or i8 [[SHL]], [[SHR]]
; CHECK-NEXT: [[R:%.*]] = select i1 [[CMP]], i8 [[Y]], i8 [[OR]]
; CHECK-NEXT: [[R:%.*]] = call i8 @llvm.fshr.i8(i8 [[X:%.*]], i8 [[Y:%.*]], i8 [[SHAMT:%.*]])
; CHECK-NEXT: ret i8 [[R]]
;
%cmp = icmp eq i8 %shamt, 0
Expand All @@ -306,12 +301,7 @@ define i8 @fshr_select(i8 %x, i8 %y, i8 %shamt) {

define i16 @fshl_select(i16 %x, i16 %y, i16 %shamt) {
; CHECK-LABEL: @fshl_select(
; CHECK-NEXT: [[CMP:%.*]] = icmp eq i16 [[SHAMT:%.*]], 0
; CHECK-NEXT: [[SUB:%.*]] = sub i16 16, [[SHAMT]]
; CHECK-NEXT: [[SHR:%.*]] = lshr i16 [[Y:%.*]], [[SUB]]
; CHECK-NEXT: [[SHL:%.*]] = shl i16 [[X:%.*]], [[SHAMT]]
; CHECK-NEXT: [[OR:%.*]] = or i16 [[SHR]], [[SHL]]
; CHECK-NEXT: [[R:%.*]] = select i1 [[CMP]], i16 [[X]], i16 [[OR]]
; CHECK-NEXT: [[R:%.*]] = call i16 @llvm.fshl.i16(i16 [[X:%.*]], i16 [[Y:%.*]], i16 [[SHAMT:%.*]])
; CHECK-NEXT: ret i16 [[R]]
;
%cmp = icmp eq i16 %shamt, 0
Expand All @@ -327,12 +317,7 @@ define i16 @fshl_select(i16 %x, i16 %y, i16 %shamt) {

define <2 x i64> @fshl_select_vector(<2 x i64> %x, <2 x i64> %y, <2 x i64> %shamt) {
; CHECK-LABEL: @fshl_select_vector(
; CHECK-NEXT: [[CMP:%.*]] = icmp eq <2 x i64> [[SHAMT:%.*]], zeroinitializer
; CHECK-NEXT: [[SUB:%.*]] = sub <2 x i64> <i64 64, i64 64>, [[SHAMT]]
; CHECK-NEXT: [[SHR:%.*]] = lshr <2 x i64> [[X:%.*]], [[SUB]]
; CHECK-NEXT: [[SHL:%.*]] = shl <2 x i64> [[Y:%.*]], [[SHAMT]]
; CHECK-NEXT: [[OR:%.*]] = or <2 x i64> [[SHL]], [[SHR]]
; CHECK-NEXT: [[R:%.*]] = select <2 x i1> [[CMP]], <2 x i64> [[Y]], <2 x i64> [[OR]]
; CHECK-NEXT: [[R:%.*]] = call <2 x i64> @llvm.fshl.v2i64(<2 x i64> [[Y:%.*]], <2 x i64> [[X:%.*]], <2 x i64> [[SHAMT:%.*]])
; CHECK-NEXT: ret <2 x i64> [[R]]
;
%cmp = icmp eq <2 x i64> %shamt, zeroinitializer
Expand Down

0 comments on commit 538fdb0

Please sign in to comment.