Eliminate switch cases that can never match, for example removes all

negative switch cases if the branch condition is known to be positive.
Inspired by a recent improvement to GCC's VRP.

llvm-svn: 152405
This commit is contained in:
Duncan Sands 2012-03-09 13:45:18 +00:00
parent 1f5568c55f
commit cca89124a2
2 changed files with 186 additions and 1 deletions

View File

@ -37,6 +37,7 @@ namespace {
bool processPHI(PHINode *P);
bool processMemAccess(Instruction *I);
bool processCmp(CmpInst *C);
bool processSwitch(SwitchInst *SI);
public:
static char ID;
@ -173,6 +174,84 @@ bool CorrelatedValuePropagation::processCmp(CmpInst *C) {
return true;
}
/// processSwitch - Simplify a switch instruction by removing cases which can
/// never fire. If the uselessness of a case could be determined locally then
/// constant propagation would already have figured it out. Instead, walk the
/// predecessors and statically evaluate cases based on information available
/// on that edge. Cases that cannot fire no matter what the incoming edge can
/// safely be removed. If a case fires on every incoming edge then the entire
/// switch can be removed and replaced with a branch to the case destination.
bool CorrelatedValuePropagation::processSwitch(SwitchInst *SI) {
Value *Cond = SI->getCondition();
BasicBlock *BB = SI->getParent();
// If the condition was defined in same block as the switch then LazyValueInfo
// currently won't say anything useful about it, though in theory it could.
if (isa<Instruction>(Cond) && cast<Instruction>(Cond)->getParent() == BB)
return false;
// If the switch is unreachable then trying to improve it is a waste of time.
pred_iterator PB = pred_begin(BB), PE = pred_end(BB);
if (PB == PE) return false;
// Analyse each switch case in turn. This is done in reverse order so that
// removing a case doesn't cause trouble for the iteration.
bool Changed = false;
for (SwitchInst::CaseIt CI = SI->caseEnd(), CE = SI->caseBegin(); CI-- != CE;
) {
ConstantInt *Case = CI.getCaseValue();
// Check to see if the switch condition is equal to/not equal to the case
// value on every incoming edge, equal/not equal being the same each time.
LazyValueInfo::Tristate State = LazyValueInfo::Unknown;
for (pred_iterator PI = PB; PI != PE; ++PI) {
// Is the switch condition equal to the case value?
LazyValueInfo::Tristate Value = LVI->getPredicateOnEdge(CmpInst::ICMP_EQ,
Cond, Case, *PI, BB);
// Give up on this case if nothing is known.
if (Value == LazyValueInfo::Unknown) {
State = LazyValueInfo::Unknown;
break;
}
// If this was the first edge to be visited, record that all other edges
// need to give the same result.
if (PI == PB) {
State = Value;
continue;
}
// If this case is known to fire for some edges and known not to fire for
// others then there is nothing we can do - give up.
if (Value != State) {
State = LazyValueInfo::Unknown;
break;
}
}
if (State == LazyValueInfo::False) {
// This case never fires - remove it.
CI.getCaseSuccessor()->removePredecessor(BB);
SI->removeCase(CI); // Does not invalidate the iterator.
Changed = true;
} else if (State == LazyValueInfo::True) {
// This case always fires. Arrange for the switch to be turned into an
// unconditional branch by replacing the switch condition with the case
// value.
SI->setCondition(Case);
Changed = true;
break;
}
}
if (Changed)
// If the switch has been simplified to the point where it can be replaced
// by a branch then do so now.
ConstantFoldTerminator(BB);
return Changed;
}
bool CorrelatedValuePropagation::runOnFunction(Function &F) {
LVI = &getAnalysis<LazyValueInfo>();
@ -200,6 +279,13 @@ bool CorrelatedValuePropagation::runOnFunction(Function &F) {
}
}
Instruction *Term = FI->getTerminator();
switch (Term->getOpcode()) {
case Instruction::Switch:
BBChanged |= processSwitch(cast<SwitchInst>(Term));
break;
}
FnChanged |= BBChanged;
}

View File

@ -79,4 +79,103 @@ Impossible:
LessThanOrEqualToTwo:
ret i32 0
}
}
define i32 @switch1(i32 %s) {
; CHECK: @switch1
entry:
%cmp = icmp slt i32 %s, 0
br i1 %cmp, label %negative, label %out
negative:
switch i32 %s, label %out [
; CHECK: switch i32 %s, label %out
i32 0, label %out
; CHECK-NOT: i32 0
i32 1, label %out
; CHECK-NOT: i32 1
i32 -1, label %next
; CHECK: i32 -1, label %next
i32 -2, label %next
; CHECK: i32 -2, label %next
i32 2, label %out
; CHECK-NOT: i32 2
i32 3, label %out
; CHECK-NOT: i32 3
]
out:
%p = phi i32 [ 1, %entry ], [ -1, %negative ], [ -1, %negative ], [ -1, %negative ], [ -1, %negative ]
ret i32 %p
next:
%q = phi i32 [ 0, %negative ], [ 0, %negative ]
ret i32 %q
}
define i32 @switch2(i32 %s) {
; CHECK: @switch2
entry:
%cmp = icmp sgt i32 %s, 0
br i1 %cmp, label %positive, label %out
positive:
switch i32 %s, label %out [
i32 0, label %out
i32 -1, label %next
i32 -2, label %next
]
; CHECK: br label %out
out:
%p = phi i32 [ -1, %entry ], [ 1, %positive ], [ 1, %positive ]
ret i32 %p
next:
%q = phi i32 [ 0, %positive ], [ 0, %positive ]
ret i32 %q
}
define i32 @switch3(i32 %s) {
; CHECK: @switch3
entry:
%cmp = icmp sgt i32 %s, 0
br i1 %cmp, label %positive, label %out
positive:
switch i32 %s, label %out [
i32 -1, label %out
i32 -2, label %next
i32 -3, label %next
]
; CHECK: br label %out
out:
%p = phi i32 [ -1, %entry ], [ 1, %positive ], [ 1, %positive ]
ret i32 %p
next:
%q = phi i32 [ 0, %positive ], [ 0, %positive ]
ret i32 %q
}
define void @switch4(i32 %s) {
; CHECK: @switch4
entry:
%cmp = icmp eq i32 %s, 0
br i1 %cmp, label %zero, label %out
zero:
switch i32 %s, label %out [
i32 0, label %next
i32 1, label %out
i32 -1, label %out
]
; CHECK: br label %next
out:
ret void
next:
ret void
}