[flang-commits] [PATCH] D105500: [flang] Prevent rewrite of shape inquiries to non constant explicit expr

[Jean Perier via Phabricator via flang-commits]

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Shape analysis was building expressions using non constant explicit bounds.
This caused issues because shape analysis is actively used in front-end
expression rewrites, regardless of the context. When rewriting expressions
inside execution parts it is not guaranteed that non constant specification
expressions will evaluate to the same value they would inside the specification
parts.

Example:

  fortran
  subroutine foo(n)
    real :: x(n)
    n = n + 100
    print *, size(x, 1)
  end subroutine

Was unparsed with `f18 -funparse` to the following program that is not
semantically equivalent:

  fortran
  SUBROUTINE foo (n)
   REAL x(n)
    n=n+100_4
   PRINT *, int(int(n,kind=8)-1_8+1_8,kind=4)
  END SUBROUTINE

Currently, this impacted at least all rewrites of LBOUND, UBOUND, SIZE, SHAPE,
and SIZEOF.

This patch prevents usage of non constant explicit bounds in shape analysis
results, except when requested by the shape analysis user (in contexts where it
is known to be safe to use these). Otherwise, evaluate::DescriptorInquiry are
generated. The only place where keeping non constant explicit bounds in shape
analysis appeared to be needed so far was runtime type info generation (for
derived types).

I considered fixing it at the folding level (by discarding non constant results from
shape analysis), but it seemed less solid (more places to secure, hard to enforce all
usages of shape analysis do not end-up used somewhere else now or in the future). Also,
Being able to rewrite `size(x+y)` where y is an array to `size(y)` is safe and still an
improvement, and naively discarding non constant shape analysis results in folding was
preventing that from happening.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D105500

Files:
  flang/include/flang/Evaluate/shape.h
  flang/lib/Evaluate/shape.cpp
  flang/lib/Semantics/runtime-type-info.cpp
  flang/test/Semantics/modfile33.f90

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