LLVM LoopPass values used outside the loop - llvm

Question

I'm writing an LLVM LoopPass in which I need to know which values
are used outside the loop. For that I have this code:
virtual bool runOnLoop(Loop *loop, LPPassManager &LPM)
{
for (auto it = loop->block_begin(); it != loop->block_end(); it++)
{
for (auto inst = (*it)->begin(); inst != (*it)->end(); inst++)
{
if (Is_Used_Outside_This_loop(loop,(Instruction *) inst))
{
errs() << inst->getName().str();
errs() << " is used outside the loop\n";
}
}
}
// ...
}
The inner function seemed right at first, but with the *.ll file below,
it gives incorrect classification for %tmp5 since it is used twice
inside a basic block of the loop.
bool Is_Used_Outside_This_loop(Loop *loop, Value *v)
{
int n=0;
int numUses = v->getNumUses();
for (auto it = loop->block_begin(); it != loop->block_end(); it++)
{
if (v->isUsedInBasicBlock(*it))
{
n++;
}
}
if (n == numUses) return false;
else return true;
}
The following *.ll code shows that %tmp5 is used twice
inside a basic block of the loop. When I carefully searched the API,
I couldn't find anything like Value::numUsesInBasicBlock( ... )
; Function Attrs: nounwind uwtable
define internal void #foo(i8* %s) #0 {
entry:
%s.addr = alloca i8*, align 8
%c = alloca i8, align 1
store i8* %s, i8** %s.addr, align 8
store i8 0, i8* %c, align 1
br label %while.cond
while.cond: ; preds = %while.body, %entry
%tmp = load i8*, i8** %s.addr, align 8
%tmp1 = load i8, i8* %tmp, align 1
%conv = sext i8 %tmp1 to i32
%cmp = icmp eq i32 %conv, 97
br i1 %cmp, label %lor.end, label %lor.rhs
lor.rhs: ; preds = %while.cond
%tmp2 = load i8*, i8** %s.addr, align 8
%tmp3 = load i8, i8* %tmp2, align 1
%conv2 = sext i8 %tmp3 to i32
%cmp3 = icmp eq i32 %conv2, 98
br label %lor.end
lor.end:; preds = %lor.rhs, %while.cond
%tmp4 = phi i1 [ true, %while.cond ], [ %cmp3, %lor.rhs ]
br i1 %tmp4, label %while.body, label %while.end
while.body: ; preds = %lor.end
%tmp5 = load i8*, i8** %s.addr, align 8
%incdec.ptr = getelementptr inbounds i8, i8* %tmp5, i32 1
store i8* %incdec.ptr, i8** %s.addr, align 8
%tmp6 = load i8, i8* %tmp5, align 1
store i8 %tmp6, i8* %c, align 1
br label %while.cond
while.end: ; preds = %lor.end
%tmp7 = load i8*, i8** %s.addr, align 8
%tmp8 = load i8, i8* %tmp7, align 1
%conv5 = sext i8 %tmp8 to i32
%cmp6 = icmp eq i32 %conv5, 99
br i1 %cmp6, label %if.then, label %if.end
if.then: ; preds = %while.end
%tmp9 = load i8*, i8** %s.addr, align 8
%incdec.ptr8 = getelementptr inbounds i8, i8* %tmp9, i32 1
store i8* %incdec.ptr8, i8** %s.addr, align 8
br label %if.end
if.end: ; preds = %if.then, %while.end
ret void
}
Clearly, there's got to be a way of doing this, right? Thanks!

Answer 1

The problem is your exit condition. You ask for uses, but then compare with the number of blocks that the value is used in.
So, if the value is used twice in the same basic block, the number of uses is 2 and the n counter of basic blocks that used the value is only incremented once, hence the mismatch of n and numUses.
Maybe a more concise way to do what you want is:
void FindUsesNotIn(
llvm::SmallPtrSetImpl<llvm::BasicBlock *> &Blocks,
llvm::SmallPtrSetImpl<llvm::Value *> &OutUses) {
for(const auto &b : Blocks)
for(auto &i : *b)
for(const auto &u : i.users()) {
auto *userInst = llvm::dyn_cast<llvm::Instruction>(u);
if(userInst && !Blocks.count(userInst->getParent())) {
OutUses.insert(&i);
break;
}
}
}
and then in the runOnLoop method have something like this:
virtual bool runOnLoop(llvm::Loop *loop, llvm::LPPassManager &LPM) {
llvm::SmallPtrSet<llvm::BasicBlock*, 10> loopBlocks(loop->block_begin(), loop->block_end());
llvm::SmallPtrSet<llvm::Value *, 10> outs;
FindUsesNotIn(loopBlocks, outs);
for(const auto *e : outs)
llvm::dbgs() << *e << '\n';
return false;
}

Answer 2

Here's how I solved it, though it looks overly complicated:
virtual bool runOnLoop(Loop *loop, LPPassManager &LPM)
{
for (auto it = loop->block_begin(); it != loop->block_end(); it++)
{
for (auto inst = (*it)->begin(); inst != (*it)->end(); inst++)
{
int n=0;
for (auto use = inst->use_begin(); use != inst->use_end(); use++)
{
Instruction *i = (Instruction *) use->getUser();
if (BasicBlockBelongsToLoop(i->getParent(),loop))
{
n++;
}
}
assert(n <= inst->getNumUses());
if (n < inst->getNumUses())
{
errs() << inst->getName().str();
errs() << " is used outside the loop\n";
}
}
}
// ...
I also couldn't found in the API, how to check if a basic block belongs to a loop,
so I had to write my own BasicBlockBelongsToLoop, here it is:
bool BasicBlockBelongsToLoop(BasicBlock *BB, Loop *loop)
{
for (auto it = loop->block_begin(); it != loop->block_end(); it++)
{
if (BB == (*it))
{
return true;
}
}
return false;
}