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used guard_or_false instead of guard_size_oblivious inside maybe_reduce (#154172)

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This was added in https://github.com/pytorch/pytorch/pull/119562
the idea in this loop seems to be the following.
```
    if (TORCH_GUARD_SIZE_OBLIVIOUS(size.sym_eq(1))) {
      // NB: we could short circuit this once needs_reduce is true but there's
      // no point since the reduction function will guard on this anyway
      if (!c10::guard_or_false(size.sym_eq(target), __FILE__, __LINE__)) {
        needs_reduce = true;
      }
    } else {
      if (!size.sym_eq(target).expect_true(__FILE__, __LINE__)) {
        fail();
      }
    }
  ```
  1. if we know size ==1
       1.1 : if we know for sure size == target --> no reduce needed.
       1.2 : we know for sure that size != target  --> we do reduction.
       1.3: we could not tell if size == target or not --> we do reduction.
  2. if we do now know if size ==1 or not
     we add a runtime assertions that size ==target and we fail at runtime if size is not equal to target.

We could have simplified 1.1 and always do reduction under 1.1, since doing 1.3 without runtime checks implies
that it is safe, but i feel the reason could be perf here? idk.

anyway using TORCH_GUARD_OR_FALSE instead of TORCH_GUARD_SIZE_OBLIVIOUS here is appropriate.
there is really no clear reason for size oblivious reasoning. or for this logic not to apply when size is not size like
size is always >=0 anyway. but bad reasoning can make us not able to infer that although we know its true here.

 python test/dynamo/test_misc.py -k test_validate_outputs_unbacked

Pull Request resolved: https://github.com/pytorch/pytorch/pull/154172
Approved by: https://github.com/bobrenjc93
ghstack dependencies: #154154, #154164, #154167
This commit is contained in:
Laith Sakka
2025-05-22 17:49:58 -07:00
committed by PyTorch MergeBot
parent ab5137b048
commit e33feddb72
1 changed files with 1 additions and 1 deletions
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2
torch/csrc/autograd/input_metadata.cpp
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@ -91,7 +91,7 @@ at::Tensor InputMetadata::maybe_reduce(
const auto& target = desired[target_dim - i - 1];
// The conditions here are written carefully so that we are able to
// infer deferred runtime asserts
if (TORCH_GUARD_SIZE_OBLIVIOUS(size.sym_eq(1))) {
if (TORCH_GUARD_OR_FALSE(size.sym_eq(1))) {
// NB: we could short circuit this once needs_reduce is true but there's
// no point since the reduction function will guard on this anyway
if (!c10::guard_or_false(size.sym_eq(target), __FILE__, __LINE__)) {