[llvm-dev] Alignment analysis

[Johannes Doerfert via llvm-dev]

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];
> }
> ```
>