[LLVMdev] RFC: Missing canonicalization in LLVM

Pete Cooper
Wed, 21 Jan 2015 22:43:47 -0000

> On Jan 21, 2015, at 2:18 PM, Chandler Carruth <chandlerc at
gmail.com> wrote:
>
> On Wed, Jan 21, 2015 at 2:16 PM, Chandler Carruth <chandlerc at
gmail.com <mailto:chandlerc at gmail.com>> wrote:
> So, we've run into some test cases which are pretty alarming.
>
> When inlining code in various different paths we can end up with this IR:
>
> define void @f(float* %value, i8* %b) {
> entry:
>   %0 = load float* %value, align 4
>   %1 = bitcast i8* %b to float*
>   store float %0, float* %1, align 1
>   ret void
> }
>
> define void @g(float* %value, i8* %b) {
> entry:
>   %0 = bitcast float* %value to i32*
>   %1 = load i32* %0, align 4
>   %2 = bitcast i8* %b to i32*
>   store i32 %1, i32* %2, align 1
>   ret void
> }
>
> Now, I don't really care one way or the other about these two IR
inputs, but it's pretty concerning that we get these two equivalent bits of
code and nothing canonicalizes to one or the other.
>
> So, the naive first blush approach here would be to canonicalize on the
first -- it has fewer instructions after all -- but I don't think that's
the right approach for two reasons:
>
> 1) It will be a *very* narrow canonicalization that only works with overly
specific sets of casted pointers.
> 2) It doesn't effectively move us toward the optimizer treating IR with
different pointee types for pointer types indistinguishably. Some day, I
continue to think we should get rid of the pointee types entirely.
>
> To see why #1 and #2 are problematic, assume another round of inlining took
place and we suddenly had the following IR:
>
>
> And the missing IR example:
>
> define void @f(i8* %value, i8* %b) {
> entry:
>   %0 = bitcast i8* %value to float*
>   %1 = load float* %0, align 4
>   %2 = bitcast i8* %b to float*
>   store float %0, float* %1, align 1
>   ret void
> }
>
> define void @g(i8* %value, i8* %b) {
> entry:
>   %0 = bitcast i8* %value to i32*
>   %1 = load i32* %0, align 4
>   %2 = bitcast i8* %b to i32*
>   store i32 %1, i32* %2, align 1
>   ret void
> }
>
>
> AFAICT, this is the same and we still don't have a good
canonicalization story.
>
> What seems like the obvious important and missing canonicalization is that
when we have a loaded value that is *only* used by storing it back into memory,
we don't canonicalize the type of that *value* (ignoring the pointer types)
to a single value type.
>
> So, the only really suitable type for this kind of stuff is 'iN'
where N matches the number of bits loaded or stored.
>
> I have this change implemented. It is trivial and unsurprising. However,
the effects of this are impossible to predict so I wanted to make sure it made
sense to others. Essentially, I expect random and hard to track down performance
fluctuations across the board. Some things may get better, others may get worse,
and they will probably all be bugs elsewhere in the stack.
>
> So, thoughts?
The first thing that springs to mind is that I don’t trust the backend to get
this right.  I don’t think it will understand when an i32 load/store would have
been preferable to a float one or vice versa.  I have no evidence of this, but
given how strongly typed tablegen is, I don’t think it can make a good choice
here.

So I think we probably need to teach the backend how to undo whatever canonical
form we choose if it has a reason to.  And the best long term solution is for
tablegen to have sized load/stores, not typed ones.

One (potentially expensive) way to choose the canonical form here is to look at
the users of the load and see what type works best.  If we load an i32, but bit
cast and do an fp operation on it, then a float load was best.  If we just load
it then store, then in theory either type works.

Pete
> -Chandler
>
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