[llvm-dev] Global removal pass - potential for improvement?

Doerfert, Johannes via llvm-dev llvm-dev at lists.llvm.org
Tue Jan 28 09:44:47 PST 2020

Hi Karl, Roman,

On 01/28, Roman Lebedev via llvm-dev wrote:
> On Tue, Jan 28, 2020 at 8:09 PM Karl Rehm via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> > I was looking into how the global optimization pass fares against
> > things like what's reported in
> > https://bugs.llvm.org/show_bug.cgi?id=44676

I need to take a closer look but I would have expected BasicAA to be
able to determine that `do_log` and `R` cannot alias. In the -Os version
(lower right here https://gcc.godbolt.org/z/KLaeiH), the write to `R`
clobbers the read from `do_log` which prevents us from removing the
load/store pair. My reasoning would have been that we know the size of
`do_log` to be less than the size accessed via `R`. What exactly goes
wrong or if my logic is flawed needs to be examined. I would start
looking at the debug generated by the code parts touched here:

> > Looking at this, I think it would be pretty trivial to optimize that
> > down given that there are already threading assumptions made:
> > https://godbolt.org/z/u6ZqoB

Optimizing more aggressively based on forward process guarantees will
get us in more trouble than we are already in. I don't have the link
handy but as far as I remember the proposed solution was to have a
forward process guarantee function attribute. I would recommend we look
into that first before we start more aggressive optimizations which will
cause problems for a lot of (non C/C++) folks.

> > Is this something I can look into?

Sure :)

> > Another thing is that currently *all* external calls break this
> > optimization, including calls to intrinsics that probably shouldn't:
> > https://godbolt.org/z/pK7Cew
> I think during load propagation, there is a legality check "here's a
> load, and here's a store.
> Is there anything in between that may have clobbered that memory location?".

Right now we only have `__attribute__((pure/const))` but we want to
expose all LLVM-IR attributes to the user soon [0] which will allow way
more fine-grained control. Intrinsics are a different story again.

> For calls, there are some attributes that are helpful here:
> https://llvm.org/docs/LangRef.html#function-attributes
> So in this case, i guess `@llvm.x86.flags.write` intrinsic maybe can
> be annotated with readonly attribute,
> thus signalling that it won't clobber that memory location?

While target specific intrinsics are a bit more complicated we see the
problem often with generic intrinsic already. We proposed the other day
[1] to change the default semantics of non-target specific intrinsics
such that you have to opt-in for certain effects.

For the above example you want `llvm.x86.flags.write` to be `writeonly` and
`inaccesiblememonly`. Also `nosync`, `willreturn`, ...


[0] https://www.youtube.com/watch?v=elmio6AoyK0
[1] http://lists.llvm.org/pipermail/llvm-dev/2019-August/134404.html
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