[PATCH] D102334: [AMDGPU] Sort LDS globals based on thier size and alignment.

[Jay Foad via Phabricator via llvm-commits]

foad added a comment.

> [AMDGPU] Sort LDS globals based on thier size and alignment.

Typo "their" :-)

> In order to increase the probability of aligned accessing of LDS memory
> operations,

Are you expecting this to affect instruction selection? Or just to increase the probability that the selected instructions will go fast because they will see an aligned address at run time?

> sort LDS globals based on thier size and alignment, and then
> allocate memory in that sorted order.

Sorting seems like an odd way  to do this, because it *might* happen to over-align a global, but only by chance depending on what other globals get sorted before it.

Instead how about explicitly over-aligning all globals with something like:

  for each lds global:
    if size > 8:
      // we might want to use a b96 or b128 load
      alignment = max(alignment, 16)
    else if size > 4:
      // we might want to use a b64 load
      alignment = max(alignment, 8)
    else if size > 2:
      // we might want to use a b32 load
      alignment = max(alignment, 4)
    else if size > 1
      // we might want to use a b16 load
      alignment = max(alignment, 2)

(You could still sort them after doing that, but the only reason would be to try to minimise wasted space due to alignment gaps.)


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D102334/new/

https://reviews.llvm.org/D102334