# RFC: Recursive inlining

Hi,

The other day on IRC myself, Chandler, Hal and Pablo discussed recursive inlining. I’ve pondered a bit since then, and thought I’d get some background on the list with the aim of stimulating discussion.

Our goal is to inline recursive functions. We have several workloads with recursive calls, and inlining them to a certain degree can improve performance by up to 30%.

Inlining recursive functions is a slightly different problem to inlining non-recursive functions in that the inlining can never terminate, so you need to choose a factor (number of times) to inline. In this respect it is similar to loop unrolling, and Chandler suggested we should model it as such.

We originally thought he meant by this that the mechanics of inlining would stay the same, but the heuristics would have to be changed. This is the approach taken by Rugina & Rinard 1, which is almost the sum of the literature on this topic I could find, which surprised me. However, the actual suggestion was to remove the recursion entirely, and replace it with a loop.

Before I get into the mechanics of this, I’d like to explain what the possible optimization opportunities are for recursive functions turned into a loop.

1. Unrolling
• This leads to bigger basic blocks (if they can be merged) and exposes more pipeline-level parallelism.
• CSE between iterations.
• Reduces backbranch overhead (generally negligible impact).
1. Commoning / LICM
• Recursion doesn’t have a vehicle for moving common code out into a dominating block (compare with LICM for loops).

OK, first example:

void ex1(i) {
if (i == 0) return;
a[i] = i+1;
ex1(i-1);
}

gets converted to:

while (1) {
if (i == 0) break;
a[i] = i+1;
i = i-1;
}

Now, the above is the simplest case and is pure tail recursion. We can eliminate this currently with the TailRecursionElimination pass. The more complex case is where we have non-tail recusion and live variables.

void ex2(i) {
if (i == 0) return;
j = a[i];
ex2(i-1)
b[j] = i;
}

This requires a stack being modelled and could be converted to:

cnt = 0;
while (1) {
if (i == 0) break;
j = a[i];
stack[cnt++] = j;
stack[cnt++] = i;
++cnt;
–i;
}
while (cnt) {
i = stack[–cnt];
j = stack[–cnt];
b[j] = i;
}

The above is as far as the discussion got on IRC. The following are my half-baked thoughts.

The above transform works because the access pattern to the stack is strictly linear. Pushes are followed by pops. But I don’t think this is the most interesting case for recursion in real programs. I see (qualitatively, poorly) 3 reasons for recursion in code that should go fast:

1. A fibonnacci function in a microbenchmark or compiler shootout. I think we can safely ignore this usecase…
2. Divide and conquer algorithms. Quicksort, FFT butterflies, etc.
3. Traversing a data structure, where the recursion is simply to get to the leaves which is where all the fun stuff happens.

Points 2 and 3 both share a trait that they’re dividing/fanning out at each step:

void ex3(i) {
if (is_base(i)) {
f();
return;
}
g(i);
ex3(i / 2);
ex3(i / 2 + 1);
}

The access pattern of such a function is not linear. It is a pre-order walk of a (binary, in this case) tree. As such we can’t use the two-loop transform above, but would have to form one loop and just manually implement the stack operations:

cnt = 0
stack[cnt++] = i;
while (1) {
i = stack[–cnt];
if (is_base(i)) {
f(); continue;
}
g(i);
stack[cnt++] = i / 2 + 1;
stack[cnt++] = i / 2;
}

OK, it’s still a well-formed loop, we can still do useful operations on it like LICM or, if it’s profitable, unrolling.

Now what about a more complex case, like in the FFT butterfly algorithm:

void ex4(i) {
if (is_base(i)) {
f(i); return;
}
g(i);
ex4(i / 2);
ex4(i / 2 + 1);
h(i);
}

Here, we have to do work after the last recursive call. This presents a problem, because we don’t have any of the caller’s context when dealing with a node. Is a node N the first recursive call? or the second? Call-stack recursion can save two things: the state of variables and the return address which serves as another kind of state. The obvious answer is to turn this into a sort of state machine:

cnt = 0
stack[cnt++] = i;
stack[cnt++] = STATE_1;
while (1) {
state = stack[–cnt];
i = stack[–cnt];

if (state == STATE_FINISH) {
h(i);
continue;
}

if (is_base(i)) {
f(); continue;
}

g(i);
stack[cnt++] = i; // Push ourself again, in the final state this time
stack[cnt++] = STATE_FINISH;
stack[cnt++] = i / 2 + 1;
stack[cnt++] = STATE_1;
stack[cnt++] = i / 2;
stack[cnt++] = STATE_1;
}

This solution can be generalised for if there is code in between the recursive calls (just add more states). It’s getting more complex in terms of control flow though - the question is, is it worth it?

Unrolling this loop is probably not going to usefully create larger basic blocks due to how complex the control flow is. I suppose we have the opportunity, if we unroll, of CSE and jump threading values through equivalent blocks that may gain us time. However, we can still do LICM which is probably a major improvement, and there are no calls/returns to speculate.

This approach would have to be extended if the recursive calls didn’t return void (for example it was a recursive reduction), but it’s not impossible.

Sorry for the long post. What do people think?

Cheers,

James

First off, thanks for the examples. They are very nice.

Breaking it down into an explicit state machine is I think exactly the
right way to model this.

This solution can be generalised for if there is code in between the
recursive calls (just add more states). It's getting more complex in terms
of control flow though - the question is, is it worth it?

Unrolling this loop is probably not going to usefully create larger basic
blocks due to how complex the control flow is. I suppose we have the
opportunity, if we unroll, of CSE and jump threading values through
equivalent blocks that may gain us time. However, we can still do LICM
which is probably a major improvement, and there are no calls/returns to
speculate.

This approach would have to be extended if the recursive calls didn't
return void (for example it was a recursive reduction), but it's not
impossible.

Sorry for the long post. What do people think?

Personally, I think this would be an amazing transformation for LLVM to do.
But I agree that it gets very complex in the limit. I suspect there will
need to be some complexity bound on doing this transformation, but as of
yet I'm not sure what it would be. Here is what is in my head.

Let's imagine we do this transform. Let's imagine that *no other
optimizations fire*. How close can we make the generated code resemble what
would be executed by actually calling the function recursively? Can we use
the actual stack and lower the adjustments much like we would were we to
imagine the dynamic trace of instructions in the recursive variant?

My suspicion is that for a decent chunk of real world recursion, we can
transform it into a loop that is not significantly lower to execute. We can
use the same stack adjustment code that would be executed in a recursive
call, etc. My further suspicion is that the complexity of control flow is
not the critical concern here, but the amount of stack usage, and how much
work we have to do to use a minimal amount of the stack. I could be
first impression is that the code is actually not bad... except for the
stack manipulation. I think we'll actually generate very reasonable code
for the CFGs here.

So, if we can do this transform in a way that produces generated code that
essentially executes the same dynamic trace of instructions that the
recursive code would execute, I think we're in *very* safe territory. The
primary code size will be switching from the call-stack approach to this
manual stack approach. It'll be like the cost of peeling the outer
iteration. Unless the code size is of extreme concern, I don't think there
is much downside.

As for the upside, I think it is huge. LICM alone is a game-changer. GVN
starts to work. PRE can be applied. Will SCEV be able to reason about the
trip count? probably not. But the entire rest of the optimizer essentially
gets turned *on* by this change.

goto state_0;
if (is_base(i)){
}
goto state_0;

Thinking about this some more I realise I could explain this more clearly. I think we can manipulate the bitcode fairly easily, and in the same way for all recursive functions.

before;

void func(%arg1, …) {

entry:
// …

terminate:

// …

ret;

recursive_call:

// …

call func(%arg1_tmp, …)

// …

end:

// …

ret;
}

after;

From: "James Molloy" <james@jamesmolloy.co.uk>
To: llvmdev@cs.uiuc.edu, "Chandler Carruth" <chandlerc@google.com>, "Hal Finkel" <hfinkel@anl.gov>, "pablo barrio"
<pablo.barrio@arm.com>
Sent: Thursday, February 5, 2015 4:36:16 AM
Subject: RFC: Recursive inlining

Hi,

The other day on IRC myself, Chandler, Hal and Pablo discussed
recursive inlining. I've pondered a bit since then, and thought I'd
get some background on the list with the aim of stimulating
discussion.

Our goal is to inline recursive functions. We have several workloads
with recursive calls, and inlining them to a certain degree can
improve performance by up to 30%.

Inlining recursive functions is a slightly different problem to
inlining non-recursive functions in that the inlining can never
terminate, so you need to choose a factor (number of times) to
inline. In this respect it is similar to loop unrolling, and
Chandler suggested we should model it as such.

We originally thought he meant by this that the mechanics of inlining
would stay the same, but the heuristics would have to be changed.
This is the approach taken by Rugina & Rinard , which is almost
the sum of the literature on this topic I could find, which
surprised me. However, the actual suggestion was to remove the
recursion entirely, and replace it with a loop.

Before I get into the mechanics of this, I'd like to explain what the
possible optimization opportunities are for recursive functions
turned into a loop.

1. Unrolling
- This leads to bigger basic blocks (if they can be merged) and
exposes more pipeline-level parallelism.
- CSE between iterations.
- Reduces backbranch overhead (generally negligible impact).
2. Commoning / LICM
- Recursion doesn't have a vehicle for moving common code out into a
dominating block (compare with LICM for loops).

OK, first example:

void ex1(i) {
if (i == 0) return;
a[i] = i+1;
ex1(i-1);
}

gets converted to:

while (1) {
if (i == 0) break;
a[i] = i+1;
i = i-1;
}

Now, the above is the simplest case and is pure tail recursion. We
can eliminate this currently with the TailRecursionElimination pass.
The more complex case is where we have non-tail recusion and live
variables.

void ex2(i) {
if (i == 0) return;
j = a[i];
ex2(i-1)
b[j] = i;
}

This requires a stack being modelled and could be converted to:

cnt = 0;
while (1) {
if (i == 0) break;
j = a[i];
stack[cnt++] = j;
stack[cnt++] = i;
++cnt;
--i;
}
while (cnt) {
i = stack[--cnt];
j = stack[--cnt];
b[j] = i;
}

The above is as far as the discussion got on IRC. The following are
my half-baked thoughts.

The above transform works because the access pattern to the stack is
strictly linear. Pushes are followed by pops. But I don't think this
is the most interesting case for recursion in real programs. I see
(qualitatively, poorly) 3 reasons for recursion in code that should
go fast:

1. A fibonnacci function in a microbenchmark or compiler shootout. I
think we can safely ignore this usecase...
2. Divide and conquer algorithms. Quicksort, FFT butterflies, etc.
3. Traversing a data structure, where the recursion is simply to get
to the leaves which is where all the fun stuff happens.

Points 2 and 3 both share a trait that they're dividing/fanning out
at each step:

void ex3(i) {
if (is_base(i)) {
f();
return;
}
g(i);
ex3(i / 2);
ex3(i / 2 + 1);
}

The access pattern of such a function is not linear. It is a
pre-order walk of a (binary, in this case) tree. As such we can't
use the two-loop transform above, but would have to form one loop
and just manually implement the stack operations:

cnt = 0
stack[cnt++] = i;
while (1) {
i = stack[--cnt];
if (is_base(i)) {
f(); continue;
}
g(i);
stack[cnt++] = i / 2 + 1;
stack[cnt++] = i / 2;
}

OK, it's still a well-formed loop, we can still do useful operations
on it like LICM or, if it's profitable, unrolling.

Now what about a more complex case, like in the FFT butterfly
algorithm:

void ex4(i) {
if (is_base(i)) {
f(i); return;
}
g(i);
ex4(i / 2);
ex4(i / 2 + 1);
h(i);
}

Here, we have to do work after the last recursive call. This presents
a problem, because we don't have any of the caller's context when
dealing with a node. Is a node N the first recursive call? or the
second? Call-stack recursion can save two things: the state of
variables and the return address which serves as another kind of
state. The obvious answer is to turn this into a sort of state
machine:

cnt = 0
stack[cnt++] = i;
stack[cnt++] = STATE_1;
while (1) {
state = stack[--cnt];
i = stack[--cnt];

if (state == STATE_FINISH) {
h(i);
continue;
}

if (is_base(i)) {
f(); continue;
}

g(i);
stack[cnt++] = i; // Push ourself again, in the final state this time
stack[cnt++] = STATE_FINISH;
stack[cnt++] = i / 2 + 1;
stack[cnt++] = STATE_1;
stack[cnt++] = i / 2;
stack[cnt++] = STATE_1;
}

As we had briefly mentioned on IRC, one way of forming this 'stack', and its associated 'cnt' variable, is to use dynamic stack allocation. You can do this in general at the IR level (using allocas that just happen not to be in the entry block). If each stack block holds a pointer to the previous one (as a linked list), then you just need to maintain a pointer to the current block, and you don't need a separate 'cnt' variable (you might also keep a pointer to the next block for allocation reuse). Is that what you had in mind?

-Hal

Hi Hal,

As we had briefly mentioned on IRC, one way of forming this ‘stack’, and its associated ‘cnt’ variable, is to use dynamic stack allocation.

I hadn’t really reached a decision on the mechanics yet. However, your suggestion while it can work within the current abilities of the IR, has the disadvantage that it is using an extra stack slot for the link pointer. I think if we are going to apply this optimization, we must make sure we aren’t going to blow the stack.

As users may already be just close enough to the max-stack boundary that their applications don’t crash, I think we should aim as much as possible to keep the stack usage no higher than the recursive equivalent. Holding the “state” is the equivalent of a return address, so adding a stack link pointer would be the equivalent of a frame address I suppose… That could be acceptable?

As I say, I haven’t thought about the deep mechanics just yet.

James

Hi all,

This thread is really interesting. I’m just trying to understand here, so please correct me if I’m wrong.

Wouldn’t we potentially save stack usage just by turning the recursion into loops? For example, at the assembly level, the ABI often requires to preserve some registers between calls which translate into stack pushes, and we will get rid of that. My understanding is that this transform does the same thing as the lowering to assembly does for function calls, with the exception that all the architecture-independent passes will have a broader context for analysis. So I would expect to even improve the stack usage.

Otherwise, we could analyze the expected stack usage for a single recursive call. Then, compare it to the expected usage after the transformation for each loop iteration. This could be used as a heuristic to decide whether to do the conversion or not? E.g. we only allow to increase the stack growth by 1.2x.

Thanks,

Pablo

Pablo, Most of the time the ABI only requires saving of registers that
are actually used in the function. So there may be cases where those
will not need saving, but at the same time, chances are that the loop
version will have to save/restore approximately the same amount of
context - and the caller-save registers [including return address] are
part of the callee context in recursion.

But yes, there is a small chance that the loop version saves on
"save/restore register on stack".

If the stack grows more than in actual recursion, then it's probably
questionable if the inlining/unwinding of the recursion is worth doing
- since most of the purpose here is to avoid the call/return and
argument passing - but if the code ends up saving/restoring MORE data,
it's unlikely to be of benefit [in my limited understanding of the
problem].

From: "James Molloy" <james@jamesmolloy.co.uk>
To: "Hal Finkel" <hfinkel@anl.gov>
Cc: llvmdev@cs.uiuc.edu, "Chandler Carruth" <chandlerc@google.com>, "pablo barrio" <pablo.barrio@arm.com>
Sent: Thursday, February 5, 2015 9:18:35 AM
Subject: Re: RFC: Recursive inlining

Hi Hal,

> As we had briefly mentioned on IRC, one way of forming this
> 'stack', and its associated 'cnt' variable, is to use dynamic
> stack allocation.

I hadn't really reached a decision on the mechanics yet. However,
your suggestion while it can work within the current abilities of
the IR, has the disadvantage that it is using an extra stack slot
for the link pointer. I think if we are going to apply this
optimization, we must make sure we aren't going to blow the stack.

As users may already be just close enough to the max-stack boundary
that their applications don't crash, I think we should aim as much
as possible to keep the stack usage no higher than the recursive
equivalent. Holding the "state" is the equivalent of a return
frame address I suppose... That could be acceptable?

I agree. We should aim not to increase the amount of stack space used. This will require some target knowledge (because dynamic stack allocation has some target-dependent overhead because of alignment and other ABI requirements).

-Hal

Sorry for jumping late into this thread, I’m quite late in my mail reading after CGO. Here’s my experience with this topic (I presented this on a binary translation workshop of CGO):

I sort of stumbled upon this transformation by accident in the last year. I do research with binary translation and LLVM – I have a prototype that reads MIPS binary code and mimics the behavior of a MIPS guest machine with LLVM IR code, and then use an LLVM backend to emulate the guest code on other host machines (x86 and ARM).

I guess you already see where this is going after all this state machine discussion. One day I decided to experiment with MIPS function calls being modelled in the IR as jumps to basic blocks instead of calls to LLVM functions. In this way, the whole MIPS program would be translated into a single LLVM function, removing any recursion, if present.

After this, two of my very simple benchmarks (Fibonacci and Ackermann, two recursive ones) began to show better performance after being compiled to MIPS and then retranslated to x86, which didn’t make sense, because binary translation always imposes some overhead that comes from emulating the behavior of another machine. Upon further investigation, the culprit was that the recursive functions were transformed into a loop in a way that is similar to what you described: the stack you refer to is my MIPS guest stack that I need to emulate on top of x86. Nothing special, I’m just emulating a processor in software, so, upon hitting a return instruction, I check the return address to decide to which basic block to jump to (whether it’s time to exit the loop or not), similar to your control.

The performance gain was so expressive that it was mitigating the binary translator overhead and yet beating native x86 performance by 2x in the first implementation (LLVM 3.3 running in an old Intel Core2quad). Now in trunk and running on a recent processor, it is a much more modest speedup of 20%. But I only observed this for simple benchmarks, such as Fibonacci and Ackermann. If you didn’t implement this transformation yet and you are curious about its impact, I can test this framework with other simple workloads, as long as it doesn’t break my prototype It’s not ideal though, since it has a considerable overhead from emulating a guest MIPS platform.

However, when translating to ARM, I could not observe the same gains seen on x86 because the way I performed the binary translation imposed a higher overhead on ARM (Cortex A9) platforms than on x86 (Haswell) ones.

Here are the results for LLVM trunk of Jan 19 (my last rebase). I’m using the full -O3 opt pipeline after I translate the code:

Fibonacci:
Native x86: 1.56s
Translated MIPS to x86: 1.26s (-20.75%)

Native ARM: 6.42s
Translated MIPS to ARM: 6.49s (overhead of 9%)

Ackermann:
Native x86: 0.94s
Translated MIPS to x86: 0.57s (-38.30%)

Regards,
Rafael Auler

Hi,

Apologies for the very late response.

We have manually tried the idea with a very simple Fibonacci sequence code. While being very very simple, the recursion cannot be handled by TRE. Because there are two recursive callsites, it also needs to keep some sort of state across iterations of the "while(stack not empty)" loop.

We get between 2.5 and 8x slowdowns depending on which processor, and the branch mispredictions are huge (specially in x86: around 100 times higher). We are replacing the function call mechanism by a stack + switch statement to mimic the control flow for callsites. My understanding is that the branches in the switch are hardly predictable: they depend on a value taken from the stack at each iteration. Things could get harder as we increase the number of recursive callsites in a function (we increase the size of the switch).

Also, virtually all processors beyond a certain level of sophistication (let's say, meant to run an OS) feature a return-stack prediction mechanism. This results in highly-efficient branch prediction for calls+returns. Since we are transforming the "call jumps" into "switch jumps", we are probably misusing that piece of hardware.

Of course, this is not to say that this is a dead end. I paste the code here (original and "unrolled" version) so that you see the problem, and maybe you can spot something that I missed. Put your favorite queue implementation in there.

Cheers,
Pablo

unsigned fibonacci(unsigned n){

if (n == 0)
return 0;
else if (n == 1)
return 1;
else
return fibonacci(n - 1) + fibonacci(n - 2);
}

unsigned fibonacci_opt(unsigned n){

// S0 = before 1st call, S1 = after 1st call, S2 = after 2nd call
enum {S0, S1, S2};

Stack s;
int ret;
init(&s, 256);
push(&s, n);
push(&s, S0);

while (!empty(s)){

/* print(&s); */

int state = pop(&s);
n = pop(&s);

int a, b;

switch(state){
case S0:

if (n <= 0)
ret = 0;

else if (n == 1)
ret = 1;

else{
// On "fake return" we need to move to the end of the first "recursive call".
push(&s, n);
push(&s, S1);

// Go for the first "recursive call"
push(&s, n - 1);
push(&s, S0);
}
break;

case S1:
// On "fake return", we want to recover the return of the previous call.
push(&s, ret);
push(&s, S2);

// Go for the second "recursive call"
push(&s, n - 2);
push(&s, S0);
break;

case S2:
ret += n;
}
}

delete(&s);
return ret;
}

Yes, it will be critical to use a branching pattern that similarly fits
modern predictors. I don't think this will be hard though. Again, I'm
continuing to work on a prototype actual pass.

I’m not completely sure, but you are probably getting a slowdown here because LLVM transforms the native Fibonacci with a tail call optimization (not elimination, but it just rewrites the recursion) that greatly improves Fibonacci, and it is unable to apply this transformation to your explicit- stack implementation.

To see this, compile your regular fibonacci and checkout the x86 assembly to see how LLVM transforms it.

The manual transformation of this form seems to work well enough for me;

unsigned fibonacci(unsigned n){
if (n <= 1)
return n;
return fibonacci(n - 1) + fibonacci(n - 2);
}

time fib(40)
real 0m1.100s
user 0m1.096s
sys 0m0.004s

after;

unsigned fibonacci(unsigned argn){
unsigned stack;
unsigned pos=0;
unsigned n=argn;
unsigned ret;
unsigned tmp1;
stack[pos++]=0;

start:
if (n <= 1){
ret=n;
goto pop;
}

stack[pos++]=n; // push n as it’s live
stack[pos++]=1; // push state
n= n - 1; // set call argument
goto start;

pop:
// pop state and jump
switch(stack[–pos]){
case 0:
return ret;
case 1:
n=stack[–pos]; // pop n
stack[pos++]=ret; // push ret as it’s live
stack[pos++]=2; // push state
n= n - 2; // set call argument
goto start;
case 2:
tmp1 = stack[–pos]; // pop temporary return value
ret = tmp1 + ret;
goto pop;
}
}

time fib(40)
real 0m0.772s
user 0m0.772s
sys 0m0.003s

But with your 3 return example the stack based code is ending up slower after optimisations;

if (n == 0){
ret=0;
goto pop;
}
if (n == 1){
ret=1;
goto pop;
}

time fib(40)
real 0m1.233s
user 0m1.233s
sys 0m0.000s

Chandler, thank you very much for working on this.

My manual implementation was a) using an extra unnecessary push+pop, and b) resulting in a very twisted and unoptimizable control flow. The one by Jeremy Lakeman is better.

Let me know if I can be of help with benchmarking/tuning for ARM architectures or anything else.

Thank you Jeremy, I compared the CFG after optimizations for my version against yours, and mine just looks atrocious, although I’m not sure why.