[llvm-dev] [RFC] How to manifest information in LLVM-IR, or, revisiting llvm.assume

[Doerfert, Johannes via llvm-dev]

[Update]

Of the 4 problems listed below, A) and B) have been (mostly) addressed.
C) and D) remain open.

Over the last few months Gauthier (@tyker) made *a lot* of progress on
this. There is infrastructure to deal with operand bundles on
`llvm.assume`, e.g.,to simplify those operand bundles or to identify
assumes you don't need. Instcombine (among other things) can "cheaply"
reason about uses that can be removed without traversing def-use chains,
etc. There is some work to do to unify the ephemeral value handling and
the "droppable" uses stuff but we are moving in the right direction.

Yesterday, we switched the most commonly generated assuming (alignment)
to the bundle representation. Hopefully that will not only reduce IR
size (with all the usual consequences) but also make it easier to
identify and deal with (attribute) assumptions.

See https://reviews.llvm.org/D71739 for the switch. It simplifies the
handling quite a bit, i.a., compare
   https://reviews.llvm.org/D81854?vs=on&id=273782
with
   https://reviews.llvm.org/D81854?vs=on&id=270795

I would like to ask people to try the `--enable-knowledge-retention`
flag. It will preserve a lot of the (attribute) information we currently
loose during transformations like inlining or dead code elimination.
There is probably some tuning necessary but any input on the effects
would be very much appreciated.


~ Johannes


P.S.
   Feedback and help is always welcome ;)


On 12/16/19 5:16 PM, Doerfert, Johannes wrote:
 > Abstract:
 >
 > It is often hard or impossible to encode complex, e.g., non-boolean,
 > information in an `llvm.assume(i1)`. This RFC describes various problems
 > we have right now and provides alternative design ideas.
 >
 >
 >
 > Some Existing Problems:
 >
 > A) The boolean requirement.
 >   The current `llvm.assume(i1)` expects a boolean that is known to hold
 >   true at runtime (once the `llvm.assume` call is reached). However,
 >   forming this boolean for "arbitrary" information is hard or even
 >   impossible. Alignment, which is an easy case, already requires 3 extra
 >   instructions, one of which is a `ptrtoint` and therefore not really
 >   optimizer friendly. Dereferenceability, is even scarier. Thus, we are
 >   currently limited to (almost) boolean information when we want to
 >   manifest information in the IR (which happens due to inlining or code
 >   motion, see https://reviews.llvm.org/D69477 for an example).
 >
 > B) The one-use checks.
 >   Various pattern matching passes check the number of uses of a value.
 >   However, while `llvm.assume` is eventually eliminated by the backend
 >   it will still increase the use count of the operand. I doubt we are
 >   able to not increase the use count at all (and keep everything else
 >   working), but what we can do is make sure the uses in "assumptions"
 >   are easy to spot, thus filter. This is not the case right now because
 >   of the additional instructions we need to make the values boolean.
 >   Even if you have `__builtin_assume(ptr);` the `ptr` use will not be
 >   the `llvm.assume` call but a `icmp`.
 >
 > C) The side-effect avoidance.
 >   `__assume`, `__builtin_assume`, `__builtin_assume_aligned`, and OpenMP
 >   `omp assume` are all defined to not evaluate their argument, thus to
 >   not cause the side effects that the evaluation of the argument would
 >   otherwise imply. The way we implement this restriction is by not
 >   emitting the argument expression in IR if it might cause a side
 >   effect. We warn the user if that happens. While this is generally
 >   speaking fine, it would be interesting to lift the *implementation*
 >   restriction. One benefit would be that we could implement `assert`
 >   through `__builtin_assume` properly.
 >
 > D) The singleton ranges.
 >   An `llvm.assume` will only provide information for a single program
 >   point not a range. Even if the beginning and the end of a range have
 >   an `llvm.assume`, there are cases where the information will not be
 >   as good as a proper range assumption. OpenMP 5.1 introduces such
 >   range assumptions but other situations would benefit from them as
 >   well. Take for example function attributes and inlining. Since we know
 >   they hold for the entire function and not only when it is entered we
 >   could encode the information over the entire range of the inlined
 >   code.
 >
 >
 > Some Site Notes:
 >
 > - It seems of little use that we interleave the code for the assumed
 >   expression with the user code. Having the `llvm.assume` allows us to
 >   tie information to a program point, beyond that we just clutter the
 >   function with instructions that we later remove anyway.
 >
 > - Reconstructing information from the pattern of instructions that feed
 >   into the `llvm.assume` is also not optimal, especially since we do
 >   not need to "optimize" these instructions anyway.
 >
 > - The current (=arbitrary) side-effects of `llvm.assume` are too strong.
 >   We have `inaccessiblemem_or_argmemonly` and we might be able to come
 >   up with something even more specialized for this, e.g.,
 >   `control_dependences_only` to indicate that there are only control
 >   dependences.
 > -
 >
 >
 > Some Design Ideas:
 >
 > 1) Use named operand bundles to encode information.
 >    If we want to encode property XYZ for a value %V holds at a certain
 >    program point and the property is dependent on %N we could encode
 >    that as:
 >      `llvm.assume() ["XYZ"(%V, %N)]`
 >    There are various benefits, including:
 >      - It is freely extensible.
 >      - The property is directly tied to the value. Thus, no need for
 >        encoding patterns that introduce extra instructions and uses and
 >        which we need to preserve and decode later.
 >      - Allows dynamic properties even when corresponding attributes do
 >        not, e.g., `llvm.assume() ["align"(%arg_ptr, %N)]` is fine and
 >        once `%N` becomes a constant, or we determine a lower bound, we
 >        can introduce the `align(C)` attribute for `%arg_ptr`.
 >
 > 2) Outline assumption expression code (with side-effects).
 >   If we potentially have side-effects, or we simply have a non-trivial
 >   expression that requires to be lowered into instructions, we can
 >   outline the assumption expression code and tie it to the
 >   `llvm.assume` via another operand bundle property. It could look
 >   something like this:
 >     `__builtin_assume(foo(i) == bar(j));`
 >   will cause us to generate
 >     ```
 >     /// Must return true!
 >     static bool llvm.assume.expression_#27(int i, int j) {
 >       return foo(i) == bar(j);
 >     }
 >     ```
 >   and a `llvm.assume` call like this:
 >     `llvm.assume() ["assume_fn"(@llvm.assume.expression_#27, %i, %j))]
 >   So we generate the expression in a new function which we (only) tie to
 >   the `llvm.assume()` through the "assume_fn" operand bundle. This will
 >   make sure we do not accidentally evaluate the code, or assume it is
 >   evaluated and produced side-effects. We can still optimize the code
 >   and use the information that we learn from it at the `llvm.assume`
 >   site though.
 >
 >  3) Use tokens to mark ranges.
 >    We have tokens which can be used to tie two instructions together,
 >    basically forming a range (with some conditions on the initial CFG).
 >    If we tie two `llvm.assume` calls together we can say that the
 >    information provided by the first holds for any point dominated by it
 >    and post-dominated by the second.
 >
 >
 >
 > I tried to be brief so I hope I could still get some ideas across
 > without too much confusion. In any case, please let me know what you
 > think!
 >
 >
 > Thanks,
 >   Johannes