[llvm] fef6613 - ValueTracking: simplify udiv/urem recurrences (#108973)
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Thu Nov 7 03:41:40 PST 2024
Author: Ramkumar Ramachandra
Date: 2024-11-07T11:41:35Z
New Revision: fef6613e9fc05bca8e315c65e8f8da796860a3cf
URL: https://github.com/llvm/llvm-project/commit/fef6613e9fc05bca8e315c65e8f8da796860a3cf
DIFF: https://github.com/llvm/llvm-project/commit/fef6613e9fc05bca8e315c65e8f8da796860a3cf.diff
LOG: ValueTracking: simplify udiv/urem recurrences (#108973)
A urem recurrence has the property that the result can never exceed the
start value. A udiv recurrence has the property that the result can
never exceed either the start value or the numerator, whichever is
greater. Implement a simplification based on these properties.
Added:
Modified:
llvm/lib/Analysis/ValueTracking.cpp
llvm/test/Analysis/ValueTracking/recurrence-knownbits.ll
Removed:
################################################################################
diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index ed3fa35c5b8610..52d3468fbe9cae 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -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
@@ -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:
@@ -1542,6 +1551,7 @@ static void computeKnownBitsFromOperator(const Operator *I,
}
break;
}
+
default:
break;
}
@@ -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:
diff --git a/llvm/test/Analysis/ValueTracking/recurrence-knownbits.ll b/llvm/test/Analysis/ValueTracking/recurrence-knownbits.ll
index 726b52271c6afb..2907266fe26333 100644
--- a/llvm/test/Analysis/ValueTracking/recurrence-knownbits.ll
+++ b/llvm/test/Analysis/ValueTracking/recurrence-knownbits.ll
@@ -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
@@ -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
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