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[KnownBits] Improve KnownBits::rem(X, Y) in cases where we can dedu…
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…ce low-bits of output

The first `cttz(Y)` bits in `X` are translated 1-1 in the output.

Alive2 Links:
    https://alive2.llvm.org/ce/z/Qc47p7
    https://alive2.llvm.org/ce/z/19ut5H

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D149421
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@goldsteinn
goldsteinn committed May 8, 2023
1 parent 7b1f123 commit 7770b0a
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Showing 4 changed files with 34 additions and 45 deletions.
5 changes: 5 additions & 0 deletions llvm/include/llvm/Support/KnownBits.h
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Original file line number Diff line number Diff line change
Expand Up @@ -420,6 +420,11 @@ struct KnownBits {

void print(raw_ostream &OS) const;
void dump() const;

private:
// Internal helper for getting the initial KnownBits for an `srem` or `urem`
// operation with the low-bits set.
static KnownBits remGetLowBits(const KnownBits &LHS, const KnownBits &RHS);
};

inline KnownBits operator&(KnownBits LHS, const KnownBits &RHS) {
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31 changes: 19 additions & 12 deletions llvm/lib/Support/KnownBits.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -546,16 +546,27 @@ KnownBits KnownBits::udiv(const KnownBits &LHS, const KnownBits &RHS) {
return Known;
}

KnownBits KnownBits::urem(const KnownBits &LHS, const KnownBits &RHS) {
KnownBits KnownBits::remGetLowBits(const KnownBits &LHS, const KnownBits &RHS) {
unsigned BitWidth = LHS.getBitWidth();
if (!RHS.isZero() && RHS.Zero[0]) {
// rem X, Y where Y[0:N] is zero will preserve X[0:N] in the result.
unsigned RHSZeros = RHS.countMinTrailingZeros();
APInt Mask = APInt::getLowBitsSet(BitWidth, RHSZeros);
APInt OnesMask = LHS.One & Mask;
APInt ZerosMask = LHS.Zero & Mask;
return KnownBits(ZerosMask, OnesMask);
}
return KnownBits(BitWidth);
}

KnownBits KnownBits::urem(const KnownBits &LHS, const KnownBits &RHS) {
assert(!LHS.hasConflict() && !RHS.hasConflict());
KnownBits Known(BitWidth);

KnownBits Known = remGetLowBits(LHS, RHS);
if (RHS.isConstant() && RHS.getConstant().isPowerOf2()) {
// The upper bits are all zero, the lower ones are unchanged.
APInt LowBits = RHS.getConstant() - 1;
Known.Zero = LHS.Zero | ~LowBits;
Known.One = LHS.One & LowBits;
// NB: Low bits set in `remGetLowBits`.
APInt HighBits = ~(RHS.getConstant() - 1);
Known.Zero |= HighBits;
return Known;
}

Expand All @@ -568,16 +579,12 @@ KnownBits KnownBits::urem(const KnownBits &LHS, const KnownBits &RHS) {
}

KnownBits KnownBits::srem(const KnownBits &LHS, const KnownBits &RHS) {
unsigned BitWidth = LHS.getBitWidth();
assert(!LHS.hasConflict() && !RHS.hasConflict());
KnownBits Known(BitWidth);

KnownBits Known = remGetLowBits(LHS, RHS);
if (RHS.isConstant() && RHS.getConstant().isPowerOf2()) {
// The low bits of the first operand are unchanged by the srem.
// NB: Low bits are set in `remGetLowBits`.
APInt LowBits = RHS.getConstant() - 1;
Known.Zero = LHS.Zero & LowBits;
Known.One = LHS.One & LowBits;

// If the first operand is non-negative or has all low bits zero, then
// the upper bits are all zero.
if (LHS.isNonNegative() || LowBits.isSubsetOf(LHS.Zero))
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25 changes: 4 additions & 21 deletions llvm/test/Analysis/ValueTracking/knownbits-rem-lowbits.ll
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Original file line number Diff line number Diff line change
Expand Up @@ -3,11 +3,7 @@

define i8 @urem_low_bits_know(i8 %xx, i8 %yy) {
; CHECK-LABEL: @urem_low_bits_know(
; CHECK-NEXT: [[X:%.*]] = or i8 [[XX:%.*]], 2
; CHECK-NEXT: [[Y:%.*]] = and i8 [[YY:%.*]], -4
; CHECK-NEXT: [[REM:%.*]] = urem i8 [[X]], [[Y]]
; CHECK-NEXT: [[R:%.*]] = and i8 [[REM]], 2
; CHECK-NEXT: ret i8 [[R]]
; CHECK-NEXT: ret i8 2
;
%x = or i8 %xx, 2
%y = and i8 %yy, -4
Expand All @@ -18,12 +14,7 @@ define i8 @urem_low_bits_know(i8 %xx, i8 %yy) {

define i8 @urem_low_bits_know2(i8 %xx, i8 %yy) {
; CHECK-LABEL: @urem_low_bits_know2(
; CHECK-NEXT: [[XO:%.*]] = and i8 [[XX:%.*]], -4
; CHECK-NEXT: [[X:%.*]] = or i8 [[XO]], 2
; CHECK-NEXT: [[Y:%.*]] = and i8 [[YY:%.*]], -4
; CHECK-NEXT: [[REM:%.*]] = urem i8 [[X]], [[Y]]
; CHECK-NEXT: [[R:%.*]] = and i8 [[REM]], 3
; CHECK-NEXT: ret i8 [[R]]
; CHECK-NEXT: ret i8 2
;
%xo = or i8 %xx, 2
%x = and i8 %xo, 254
Expand Down Expand Up @@ -80,11 +71,7 @@ define i8 @urem_fail_low_bits_unknown2(i8 %xx, i8 %yy) {

define i8 @srem_low_bits_know(i8 %xx, i8 %yy) {
; CHECK-LABEL: @srem_low_bits_know(
; CHECK-NEXT: [[X:%.*]] = or i8 [[XX:%.*]], 10
; CHECK-NEXT: [[Y:%.*]] = and i8 [[YY:%.*]], -4
; CHECK-NEXT: [[REM:%.*]] = srem i8 [[X]], [[Y]]
; CHECK-NEXT: [[R:%.*]] = and i8 [[REM]], 2
; CHECK-NEXT: ret i8 [[R]]
; CHECK-NEXT: ret i8 2
;
%x = or i8 %xx, 10
%y = and i8 %yy, -4
Expand All @@ -95,11 +82,7 @@ define i8 @srem_low_bits_know(i8 %xx, i8 %yy) {

define i8 @srem_low_bits_know2(i8 %xx, i8 %yy) {
; CHECK-LABEL: @srem_low_bits_know2(
; CHECK-NEXT: [[X:%.*]] = or i8 [[XX:%.*]], 1
; CHECK-NEXT: [[Y:%.*]] = and i8 [[YY:%.*]], -2
; CHECK-NEXT: [[REM:%.*]] = srem i8 [[X]], [[Y]]
; CHECK-NEXT: [[R:%.*]] = and i8 [[REM]], 1
; CHECK-NEXT: ret i8 [[R]]
; CHECK-NEXT: ret i8 1
;
%x = or i8 %xx, 1
%y = and i8 %yy, -2
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18 changes: 6 additions & 12 deletions llvm/test/Transforms/LoopVectorize/AArch64/masked-call.ll
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Original file line number Diff line number Diff line change
Expand Up @@ -33,8 +33,7 @@ define void @test_widen(ptr noalias %a, ptr readnone %b) #4 {
; TFNONE-NEXT: [[TMP9:%.*]] = icmp eq i64 [[INDEX_NEXT]], [[N_VEC]]
; TFNONE-NEXT: br i1 [[TMP9]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop [[LOOP0:![0-9]+]]
; TFNONE: middle.block:
; TFNONE-NEXT: [[CMP_N:%.*]] = icmp eq i64 1025, [[N_VEC]]
; TFNONE-NEXT: br i1 [[CMP_N]], label [[FOR_COND_CLEANUP:%.*]], label [[SCALAR_PH]]
; TFNONE-NEXT: br i1 false, label [[FOR_COND_CLEANUP:%.*]], label [[SCALAR_PH]]
; TFNONE: scalar.ph:
; TFNONE-NEXT: [[BC_RESUME_VAL:%.*]] = phi i64 [ [[N_VEC]], [[MIDDLE_BLOCK]] ], [ 0, [[ENTRY:%.*]] ]
; TFNONE-NEXT: br label [[FOR_BODY:%.*]]
Expand Down Expand Up @@ -193,8 +192,7 @@ define void @test_if_then(ptr noalias %a, ptr readnone %b) #4 {
; TFNONE-NEXT: [[TMP11:%.*]] = icmp eq i64 [[INDEX_NEXT]], [[N_VEC]]
; TFNONE-NEXT: br i1 [[TMP11]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop [[LOOP4:![0-9]+]]
; TFNONE: middle.block:
; TFNONE-NEXT: [[CMP_N:%.*]] = icmp eq i64 1025, [[N_VEC]]
; TFNONE-NEXT: br i1 [[CMP_N]], label [[FOR_COND_CLEANUP:%.*]], label [[SCALAR_PH]]
; TFNONE-NEXT: br i1 false, label [[FOR_COND_CLEANUP:%.*]], label [[SCALAR_PH]]
; TFNONE: scalar.ph:
; TFNONE-NEXT: [[BC_RESUME_VAL:%.*]] = phi i64 [ [[N_VEC]], [[MIDDLE_BLOCK]] ], [ 0, [[ENTRY:%.*]] ]
; TFNONE-NEXT: br label [[FOR_BODY:%.*]]
Expand Down Expand Up @@ -395,8 +393,7 @@ define void @test_widen_if_then_else(ptr noalias %a, ptr readnone %b) #4 {
; TFNONE-NEXT: [[TMP12:%.*]] = icmp eq i64 [[INDEX_NEXT]], [[N_VEC]]
; TFNONE-NEXT: br i1 [[TMP12]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop [[LOOP6:![0-9]+]]
; TFNONE: middle.block:
; TFNONE-NEXT: [[CMP_N:%.*]] = icmp eq i64 1025, [[N_VEC]]
; TFNONE-NEXT: br i1 [[CMP_N]], label [[FOR_COND_CLEANUP:%.*]], label [[SCALAR_PH]]
; TFNONE-NEXT: br i1 false, label [[FOR_COND_CLEANUP:%.*]], label [[SCALAR_PH]]
; TFNONE: scalar.ph:
; TFNONE-NEXT: [[BC_RESUME_VAL:%.*]] = phi i64 [ [[N_VEC]], [[MIDDLE_BLOCK]] ], [ 0, [[ENTRY:%.*]] ]
; TFNONE-NEXT: br label [[FOR_BODY:%.*]]
Expand Down Expand Up @@ -607,8 +604,7 @@ define void @test_widen_nomask(ptr noalias %a, ptr readnone %b) #4 {
; TFNONE-NEXT: [[TMP9:%.*]] = icmp eq i64 [[INDEX_NEXT]], [[N_VEC]]
; TFNONE-NEXT: br i1 [[TMP9]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop [[LOOP8:![0-9]+]]
; TFNONE: middle.block:
; TFNONE-NEXT: [[CMP_N:%.*]] = icmp eq i64 1025, [[N_VEC]]
; TFNONE-NEXT: br i1 [[CMP_N]], label [[FOR_COND_CLEANUP:%.*]], label [[SCALAR_PH]]
; TFNONE-NEXT: br i1 false, label [[FOR_COND_CLEANUP:%.*]], label [[SCALAR_PH]]
; TFNONE: scalar.ph:
; TFNONE-NEXT: [[BC_RESUME_VAL:%.*]] = phi i64 [ [[N_VEC]], [[MIDDLE_BLOCK]] ], [ 0, [[ENTRY:%.*]] ]
; TFNONE-NEXT: br label [[FOR_BODY:%.*]]
Expand Down Expand Up @@ -666,8 +662,7 @@ define void @test_widen_nomask(ptr noalias %a, ptr readnone %b) #4 {
; TFFALLBACK-NEXT: [[TMP9:%.*]] = icmp eq i64 [[INDEX_NEXT]], [[N_VEC]]
; TFFALLBACK-NEXT: br i1 [[TMP9]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop [[LOOP8:![0-9]+]]
; TFFALLBACK: middle.block:
; TFFALLBACK-NEXT: [[CMP_N:%.*]] = icmp eq i64 1025, [[N_VEC]]
; TFFALLBACK-NEXT: br i1 [[CMP_N]], label [[FOR_COND_CLEANUP:%.*]], label [[SCALAR_PH]]
; TFFALLBACK-NEXT: br i1 false, label [[FOR_COND_CLEANUP:%.*]], label [[SCALAR_PH]]
; TFFALLBACK: scalar.ph:
; TFFALLBACK-NEXT: [[BC_RESUME_VAL:%.*]] = phi i64 [ [[N_VEC]], [[MIDDLE_BLOCK]] ], [ 0, [[ENTRY:%.*]] ]
; TFFALLBACK-NEXT: br label [[FOR_BODY:%.*]]
Expand Down Expand Up @@ -731,8 +726,7 @@ define void @test_widen_optmask(ptr noalias %a, ptr readnone %b) #4 {
; TFNONE-NEXT: [[TMP9:%.*]] = icmp eq i64 [[INDEX_NEXT]], [[N_VEC]]
; TFNONE-NEXT: br i1 [[TMP9]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop [[LOOP10:![0-9]+]]
; TFNONE: middle.block:
; TFNONE-NEXT: [[CMP_N:%.*]] = icmp eq i64 1025, [[N_VEC]]
; TFNONE-NEXT: br i1 [[CMP_N]], label [[FOR_COND_CLEANUP:%.*]], label [[SCALAR_PH]]
; TFNONE-NEXT: br i1 false, label [[FOR_COND_CLEANUP:%.*]], label [[SCALAR_PH]]
; TFNONE: scalar.ph:
; TFNONE-NEXT: [[BC_RESUME_VAL:%.*]] = phi i64 [ [[N_VEC]], [[MIDDLE_BLOCK]] ], [ 0, [[ENTRY:%.*]] ]
; TFNONE-NEXT: br label [[FOR_BODY:%.*]]
Expand Down

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