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[ValueTracking] Teach isKnownNonZero about monotonically increasing PHIs

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If a PHI starts at a non-negative constant, monotonically increases
(only adds of a constant are supported at the moment) and that add
does not wrap, then the PHI is known never to be zero.

llvm-svn: 248796
This commit is contained in:
James Molloy 2015-09-29 14:08:45 +00:00
parent 808a3386e2
commit 7af5f0f32e
2 changed files with 69 additions and 0 deletions
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20
lib/Analysis/ValueTracking.cpp
View File

@ -1904,6 +1904,26 @@ bool isKnownNonZero(Value *V, const DataLayout &DL, unsigned Depth,
isKnownNonZero(SI->getFalseValue(), DL, Depth, Q))
return true;
}
// PHI
else if (PHINode *PN = dyn_cast<PHINode>(V)) {
// Try and detect a recurrence that monotonically increases from a
// starting value, as these are common as induction variables.
if (PN->getNumIncomingValues() == 2) {
Value *Start = PN->getIncomingValue(0);
Value *Induction = PN->getIncomingValue(1);
if (isa<ConstantInt>(Induction) && !isa<ConstantInt>(Start))
std::swap(Start, Induction);
if (ConstantInt *C = dyn_cast<ConstantInt>(Start)) {
if (!C->isZero() && !C->isNegative()) {
ConstantInt *X;
if ((match(Induction, m_NSWAdd(m_Specific(PN), m_ConstantInt(X))) ||
match(Induction, m_NUWAdd(m_Specific(PN), m_ConstantInt(X)))) &&
!X->isNegative())
return true;
}
}
}
}
if (!BitWidth) return false;
APInt KnownZero(BitWidth, 0);

49
test/Analysis/ValueTracking/monotonic-phi.ll Normal file
View File

@ -0,0 +1,49 @@
; RUN: opt -instsimplify -S < %s | FileCheck %s
; CHECK-LABEL: @test1
define i1 @test1(i8 %p, i8* %pq, i8 %n, i8 %r) {
entry:
br label %loop
loop:
%A = phi i8 [ 1, %entry ], [ %next, %loop ]
%next = add nsw i8 %A, 1
%cmp1 = icmp eq i8 %A, %n
br i1 %cmp1, label %exit, label %loop
exit:
%add = or i8 %A, %r
%cmp = icmp eq i8 %add, 0
; CHECK: ret i1 false
ret i1 %cmp
}
; CHECK-LABEL: @test2
define i1 @test2(i8 %p, i8* %pq, i8 %n, i8 %r) {
entry:
br label %loop
loop:
%A = phi i8 [ 1, %entry ], [ %next, %loop ]
%next = add i8 %A, 1
%cmp1 = icmp eq i8 %A, %n
br i1 %cmp1, label %exit, label %loop
exit:
%add = or i8 %A, %r
%cmp = icmp eq i8 %add, 0
; CHECK-NOT: ret i1 false
ret i1 %cmp
}
; CHECK-LABEL: @test3
define i1 @test3(i8 %p, i8* %pq, i8 %n, i8 %r) {
entry:
br label %loop
loop:
%A = phi i8 [ 1, %entry ], [ %next, %loop ]
%next = add nuw i8 %A, 1
%cmp1 = icmp eq i8 %A, %n
br i1 %cmp1, label %exit, label %loop
exit:
%add = or i8 %A, %r
%cmp = icmp eq i8 %add, 0
; CHECK: ret i1 false
ret i1 %cmp
}