[PATCH] D30695: [Polly][ScopDetection] Only allow SCoP-wide available base pointers

[Michael Kruse via Phabricator via llvm-commits]

Meinersbur created this revision.
Meinersbur added a project: Polly.

Simplify ` ScopDetection::isInvariant`. Essentially deny everything that is defined within the SCoP and is not load-hoisted.

The previous understanding of "invariant" has a few holes:

- Expressions without side-effects with only invariant arguments, but are defined withing the SCoP's region with the exception of selects [sic!] and PHIs. These should be part of the index expression and not of the base pointer.

- Function calls with `!mayHaveSideEffects()` (`readnone nounwind`). These must always return the same value and have no side-effect. However, we do not know which value it returns. For instance, a call

  @C = external global i32
  ...
  %ptr = call float* @getNextBasePtr(float* %A, float %B)

Might return

- `%A`, so it aliases with it in the SCoP
- `%B`, so it aliases with it in the SCoP
- `@C`, so it aliases with it in the SCoP
- a new pointer everytime it is called, such as `malloc()`
- a pointer into the allocated block of one of the aforementioned
- any of the above, at random

Hence we cannot know whether it aliases with something. It might still be useful with `-polly-ignore-aliasing`, if the user really wants unsafe code generation. Functions such as `getNextBasePtr` are usually inlined anyway (or not marked with `readnone nounwind` because they are defined in another translation unit).

On the other side it is a problem for the code generator if the base pointer is not available anywhere in the SCoP, when it needs to regenerate the access function (when `setNewAccessRelation` is called, such as in https://reviews.llvm.org/D28518, DeLICM and JSONImporter). The code generator currently assumes that base pointers are availably globally and generates invalid code (referring to the base pointer of the original code).


https://reviews.llvm.org/D30695

Files:
  lib/Analysis/ScopDetection.cpp
  test/Isl/CodeGen/invariant_verify_function_failed.ll
  test/ScopDetect/base_pointer.ll
  test/ScopDetect/base_pointer_is_inst_inside_invariant_1___%for.i---%exit.jscop
  test/ScopDetect/base_pointer_setNewAccessRelation.ll
  test/ScopDetectionDiagnostics/ReportVariantBasePtr-01.ll

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