llvm dev - Feb 2016 - Jit: use @llvm.lifetime.end to optimize away stores to globals used as temporaries

Hi all,

Is it allowed to use the @llvm.lifetime.end intrinsic to optimize away stores to
global variables that are used as temporaries?

eg I want use the Jit engine to generate code for the following function
'f':

struct State {
  int a;
  int tmp;
  int b;
};

void f0(State* s) {  s->tmp = s->a; }
void f1(State* s) { s->b = s->tmp; }

void f(State* s)
{
  f0(s);
  f1(s);
}

The Jit engine generates the following code:

define void @_Z1fP5State(%struct.State* %s) #1 {
  %1 = getelementptr inbounds %struct.State, %struct.State* %s, i64 0, i32 0
  %2 = load i32, i32* %1, align 4, !tbaa !1
  %3 = getelementptr inbounds %struct.State, %struct.State* %s, i64 0, i32 1
  store i32 %2, i32* %3, align 4, !tbaa !6
  %4 = getelementptr inbounds %struct.State, %struct.State* %s, i64 0, i32 2
  store i32 %2, i32* %4, align 4, !tbaa !7
  ret void
}

Which gives (on x86_64):

_Z1fP5State:
        movl              (%rdi), %eax
        movl              %eax, 4(%rdi)
        movl              %eax, 8(%rdi)
        retq

The 'tmp' state variable is only needed during execution of the function
'f'.
After the function has finished to value can be discarded.

How can I tell the optimizer to optimize away to stores to the 'tmp'
variable?
Can I use the @llvm.lifetime.end intrinsic to do this?

Eg If I change the function 'f' as follows:

extern "C" void llvm_lifetime_end(unsigned long long, void*);
void cleanup(State* s)
{
  // llvm_lifetime_end will be replaced with @llvm.lifetime.end
  llvm_lifetime_end(sizeof(s->tmp), (void*)&s->tmp);
}

void f(State* s)
{
  f0(s);
  f1(s);
  cleanup(s);
}

And then use a pass to replace the function llvm_lifetime_end with the intrinsic
@llvm.lifetime.end.

I get the following code:

define void @_Z1fP5State(%struct.State* nocapture %s) #0 {
  %1 = getelementptr inbounds %struct.State, %struct.State* %s, i64 0, i32 0
  %2 = load i32, i32* %1, align 4, !tbaa !1
  %3 = getelementptr inbounds %struct.State, %struct.State* %s, i64 0, i32 1
  %4 = getelementptr inbounds %struct.State, %struct.State* %s, i64 0, i32 2
  store i32 %2, i32* %4, align 4, !tbaa !7
  %5 = bitcast i32* %3 to i8*
  tail call void @llvm.lifetime.end(i64 4, i8* %5)
  ret void
}

_Z1fP5State:
                movl      (%rdi), %eax
                movl      %eax, 8(%rdi)
                retq


Is this a supported usecase of the @llvm.lifetime.end intrinsic? ie is it
allowed to use the intrinsic on a global variable instead of a local variable?
Or is there a better way to get the same result?
Is it ok to only have an @llvm.lifetime.end intrinsic, without a matching
@llvm.lifetime.start intrinisic?


If not is there another way to tell the optimizer to optimize away the stores to
the 'tmp' state variable?

Thanks,
Tom
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Tom Aernoudt via llvm-dev schrieb:
> Hi all,
>
> The 'tmp' state variable is only needed during execution of the
function
> 'f'.
>
That's what you say, because you called it 'tmp' :), but the
compiler
sees no difference to 'a' or 'b'. As soon as s->tmp would
"escape" f()
the compiler will have to write it.

I think you may want to keep external state and temporal context separate

e.g.

struct State {
   int a;
   int b;
}


struct Temp {
   int tmp;
}

static void f0( State *s, Temp *t)
{
   t->tmp = s->a;
}

...

void f(State* s)

{
    Temp  tmp;

    f0( s, &tmp);
    f1( s, &tmp);
}

Using @llvm.lifetime.end is IMO a deadend, since this should just 
signify that the alloca is gone.


Ciao
    Nat!

> From: Nat! [mailto:nat at mulle-kybernetik.com]
> Tom Aernoudt via llvm-dev schrieb:
> > Hi all,
> >
> > The 'tmp' state variable is only needed during execution of
the function
> > 'f'.
> >
> That's what you say, because you called it 'tmp' :), but the
compiler
> sees no difference to 'a' or 'b'. As soon as s->tmp
would "escape" f()
> the compiler will have to write it.
> 
> I think you may want to keep external state and temporal context separate
> 
> e.g.
> 
> struct State {
>    int a;
>    int b;
> }
> 
> 
> struct Temp {
>    int tmp;
> }
> 
> static void f0( State *s, Temp *t)
> {
>    t->tmp = s->a;
> }
> 
> ...
> 
> void f(State* s)
> 
> {
>     Temp  tmp;
> 
>     f0( s, &tmp);
>     f1( s, &tmp);
> }
> 
> Using @llvm.lifetime.end is IMO a deadend, since this should just
> signify that the alloca is gone.
>
In the real code, the state variables ('tmp',...) do not have trivial
constructors. Constructing them as local variables in the function 'f'
adds a lot of extra code that may not always be optimized away.
It will also require a lot of change to existing code.

Using the @llvm.lifetime.end intrinsic seems to do exactly what we want to
achieve, ie optimize away Store instructions to the 'tmp' variables with
no/minimal changes to existing code (only the runtime generated functions
'f' have to be changed to call the @llvm.lifetime.end intrinsics).
Is using the intrinsic on something else as an Alloca allowed? Or is the
behavior I see just a side effect of the current implementation and may it break
in the future?

Regards,
Tom