[llvm-dev] Alignment analysis

[Thomas Raoux via llvm-dev]

Hi Johannes,

It is not really the `sext` that is confusing the analysis, the
problem is that the computeKnownBits will only go one level up after
the phi, so if there is any instruction in between the `shl` and the
phi the analysis will fail.

Here is the same example with long int and an extra `add` in between:
https://godbolt.org/z/r8asbaWWc

This is done on purpose due to this limitation:
```
// Recurse, but cap the recursion to one level, because we don't
// want to waste time spinning around in loops.
computeKnownBits(IncValue, Known2, MaxAnalysisRecursionDepth - 1, RecQ);
```
https://github.com/llvm/llvm-project/blob/d2e8fb331835fcc565929720781a5fd64e66fc17/llvm/lib/Analysis/ValueTracking.cpp#L1548

On Tue, Jan 25, 2022 at 7:00 AM Johannes Doerfert
<johannesdoerfert at gmail.com> wrote:
>
> Hi Thomas,
>
> the `sext` of offset confuses the alignment computation.
> If you make offset and offset1 `long int` it seems to work
> fine: https://godbolt.org/z/zYdro6YPz
>
> I haven't dug in deeper but this might help. Probably worth
> to file a bug/issue.
>
> ~ Johannes
>
> On 1/25/22 00:26, Thomas Raoux via llvm-dev wrote:
> > Hi,
> >
> > I have been looking at a way to get alignment information propagation
> > on load/store instructions. I see that instcombine does some alignment
> > propagation based on value tracking helper ComputeKnownBits, however
> > because of compile time heuristic it won't recurse much when the value
> > comes from a phi
> > (https://github.com/llvm/llvm-project/blob/d2e8fb331835fcc565929720781a5fd64e66fc17/llvm/lib/Analysis/ValueTracking.cpp#L1550)
> >
> > Is there a way to do more aggressive alignment propagation? I'm
> > generating code going through the NVPTX target and because of the
> > missing alignment information the backend will have to break up vector
> > loads into scalar ones causing performance problems. (I also tried
> > AlignmentFromAssumptionsPass but it doesn't help).
> >
> > I assume this may be a problem for other backends as well. How is it handled?
> >
> > Here is a simple showing how limited the propagation is:
> > https://godbolt.org/z/ej1vWb7ax
> >
> > In the code below `p[offset]` should be aligned as the offset comes
> > from a phi(0, offset << 8) however the alignment analysis doesn't
> > detect it:
> >
> > ```
> > float *getAlignedPtr() __attribute__((assume_aligned (32)));
> > void anchor();
> >
> > float loadaligned(int offset, bool c) {
> > float* p = getAlignedPtr();
> > int offset1 = offset << 8;
> > if(c) {
> > anchor();
> > offset = offset1;
> > } else {
> > offset = 0;
> > }
> > // p[offset1] and p[0] are marked as aligned but not p[offset]
> > return p[offset] + p[offset1] + p[0];
> > }
> > ```
> >



-- 
Thomas