[LLVMdev] RFC: PerfGuide for frontend authors

[Björn Steinbrink]

Hi,

On 2015.02.28 10:53:35 -0600, Hal Finkel wrote:
> ----- Original Message -----
> > From: "Philip Reames" <listmail at philipreames.com>

> > > 6. Use the lifetime.start/lifetime.end and
> > > invariant.start/invariant.end intrinsics where possible
> > Do you find these help in practice?  The few experiments I ran were
> > neutral at best and harmful in one or two cases.  Do you have
> > suggestions on how and when to use them?
> 
> Good point, we should be more specific here. My, admittedly limited,
> experience with these is that they're most useful when their
> properties are not dynamic -- which perhaps means that they
> post-dominate the entry, and are applied to allocas in the entry block
> -- and the larger the objects in question, the more the potential
> stack-space savings, etc.

my experience adding support for the lifetime intrinsics to the rust
compiler is largely positive (because our code is very stack heavy at
the moment), but we still suffer from missed memcpy optimizations.
That happens because I made the lifetime regions as small as possible,
and sometimes an alloca starts its lifetime too late for the optimization
to happen.  My new (but not yet implemented) approach to to "align" the
calls to lifetime.start for allocas with overlapping lifetimes unless
there's actually a possibility for stack slot sharing.

For example we currently translate:

    let a = [0; 1000000]; // Array of 1000000 zeros
    {
      let b = a;
    }
    let c = something;

to roughly this:

    lifetime.start(a)
    memset(a, 0, 1000000)
    
    lifetime.start(b)
    memcpy(b, a)
    lifetime.end(b)
    
    lifetime.start(c)
    lifetime.end(c)
    
    lifetime.end(a)

The lifetime.start call for "b" stops the call-slot (I think)
optimization from being applied. So instead this should be translated to
something like:

    lifetime.start(a)
    lifetime.start(b)
    memset(a, 0, 1000000)
    
    memcpy(b, a)
    lifetime.end(b)
    
    lifetime.start(c)
    lifetime.end(c)
    
    lifetime.end(a)

extending the lifetime of "b" because it overlaps with that of "a"
anyway. The lifetime of "c" still starts after the end of "b"'s lifetime
because there's actually a possibility for stack slot sharing.

Björn