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ValueTracking: simplify udiv/urem recurrences #108973

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merged 3 commits into from
Nov 7, 2024
Merged

ValueTracking: simplify udiv/urem recurrences #108973

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9498f46
ValueTracking: simplify udiv/urem recurrences
artagnon Oct 7, 2024
5de6461
ValueTracking: integrate udiv/urem with shift code
artagnon Nov 5, 2024
23acb5e
ValueTracking: fix nit in comment
artagnon Nov 6, 2024
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26 changes: 19 additions & 7 deletions llvm/lib/Analysis/ValueTracking.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1426,16 +1426,23 @@ static void computeKnownBitsFromOperator(const Operator *I,
// this is sufficient to catch some interesting cases.
unsigned Opcode = BO->getOpcode();

// If this is a shift recurrence, we know the bits being shifted in.
// We can combine that with information about the start value of the
// recurrence to conclude facts about the result.
switch (Opcode) {
// If this is a shift recurrence, we know the bits being shifted in. We
// can combine that with information about the start value of the
// recurrence to conclude facts about the result. If this is a udiv
// recurrence, we know that the result can never exceed either the
// numerator or the start value, whichever is greater.
case Instruction::LShr:
case Instruction::AShr:
case Instruction::Shl: {
case Instruction::Shl:
case Instruction::UDiv:
if (BO->getOperand(0) != I)
break;
[[fallthrough]];

// For a urem recurrence, the result can never exceed the start value. The
// phi could either be the numerator or the denominator.
case Instruction::URem: {
// We have matched a recurrence of the form:
// %iv = [R, %entry], [%iv.next, %backedge]
// %iv.next = shift_op %iv, L
Expand All @@ -1453,8 +1460,10 @@ static void computeKnownBitsFromOperator(const Operator *I,
Known.Zero.setLowBits(Known2.countMinTrailingZeros());
break;
case Instruction::LShr:
// A lshr recurrence will preserve the leading zeros of the
// start value
case Instruction::UDiv:
case Instruction::URem:
// lshr, udiv, and urem recurrences will preserve the leading zeros of
// the start value.
Known.Zero.setHighBits(Known2.countMinLeadingZeros());
break;
case Instruction::AShr:
Expand Down Expand Up @@ -1542,6 +1551,7 @@ static void computeKnownBitsFromOperator(const Operator *I,
}
break;
}

default:
break;
}
Expand Down Expand Up @@ -9039,12 +9049,14 @@ bool llvm::matchSimpleRecurrence(const PHINode *P, BinaryOperator *&BO,
switch (Opcode) {
default:
continue;
// TODO: Expand list -- xor, div, gep, uaddo, etc..
// TODO: Expand list -- xor, gep, uadd.sat etc.
case Instruction::LShr:
case Instruction::AShr:
case Instruction::Shl:
case Instruction::Add:
case Instruction::Sub:
case Instruction::UDiv:
case Instruction::URem:
case Instruction::And:
case Instruction::Or:
case Instruction::Mul:
Expand Down
10 changes: 2 additions & 8 deletions llvm/test/Analysis/ValueTracking/recurrence-knownbits.ll
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Original file line number Diff line number Diff line change
Expand Up @@ -86,12 +86,9 @@ define i64 @test_udiv(i1 %c) {
; CHECK-NEXT: entry:
; CHECK-NEXT: br label [[LOOP:%.*]]
; CHECK: loop:
; CHECK-NEXT: [[IV:%.*]] = phi i64 [ 9, [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ]
; CHECK-NEXT: [[IV_NEXT]] = udiv i64 [[IV]], 3
; CHECK-NEXT: br i1 [[C:%.*]], label [[EXIT:%.*]], label [[LOOP]]
; CHECK: exit:
; CHECK-NEXT: [[RES:%.*]] = and i64 [[IV]], 16
; CHECK-NEXT: ret i64 [[RES]]
; CHECK-NEXT: ret i64 0
;
entry:
br label %loop
Expand Down Expand Up @@ -132,12 +129,9 @@ define i64 @test_urem(i1 %c) {
; CHECK-NEXT: entry:
; CHECK-NEXT: br label [[LOOP:%.*]]
; CHECK: loop:
; CHECK-NEXT: [[IV:%.*]] = phi i64 [ 3, [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ]
; CHECK-NEXT: [[IV_NEXT]] = urem i64 9, [[IV]]
; CHECK-NEXT: br i1 [[C:%.*]], label [[EXIT:%.*]], label [[LOOP]]
; CHECK: exit:
; CHECK-NEXT: [[RES:%.*]] = and i64 [[IV]], 4
; CHECK-NEXT: ret i64 [[RES]]
; CHECK-NEXT: ret i64 0
;
entry:
br label %loop
Expand Down
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