Hello, this is a fix for PR102043, which is a wrong code bug caused by the middle-end concluding from array indexing that the array index is non-negative. This is a wrong assumption for "reversed arrays", that is arrays whose descriptor makes accesses to the array from last element to first element. More generally the wrong cases are arrays with a descriptor having a negative stride for at least one dimension.
I have been trying to fix this by stopping the front-end from generating bogus code, by either stripping array-ness from descriptor data pointers, or by changing the initialization of data pointers to point to the first element in memory order instead of the first element in access order (which is the last in memory order for reversed arrays). Both ways are very impacting changes to the frontend and I haven’t been able to eliminate all the regressions in time using either way. However, Richi showed with a patch attached to the PR that array references are crucial for the problem to appear, and everything works if array indexing is replaced with pointer arithmetic. This is much simpler and doesn’t imply invasive changes to the frontend. I have built on top of his patch to keep the array indexing in cases where the change to pointer arithmetic is not necessary, either because the array is not a fortran array with a descriptor, or because it’s known to be contiguous. This has the benefit of reducing the churn in the dumped code patterns used in the testsuite. It also avoids ICE regression such as interface_12.f90 or result_in_spec.f90, but I can’t exclude that those could be a real problem made latent. Patches 1 to 3 are preliminary changes to avoid regressions. The main change is number 4, the last in the series. Regression tested on x86_64-pc-linux-gnu. OK for master? Mikael Morin (4): fortran: Pre-evaluate string pointers. [PR102043] fortran: Update index extraction code. [PR102043] fortran: Generate an array temporary reference [PR102043] fortran: Use pointer arithmetic to index arrays [PR102043] gcc/fortran/trans-array.cc | 60 +++++- gcc/fortran/trans-expr.cc | 9 +- gcc/fortran/trans-io.cc | 48 ++++- gcc/fortran/trans.cc | 42 +++- gcc/fortran/trans.h | 4 +- .../gfortran.dg/array_reference_3.f90 | 195 ++++++++++++++++++ gcc/testsuite/gfortran.dg/c_loc_test_22.f90 | 4 +- gcc/testsuite/gfortran.dg/dependency_49.f90 | 3 +- gcc/testsuite/gfortran.dg/finalize_10.f90 | 2 +- .../gfortran.dg/negative_stride_1.f90 | 25 +++ .../gfortran.dg/vector_subscript_8.f90 | 16 ++ .../gfortran.dg/vector_subscript_9.f90 | 21 ++ 12 files changed, 401 insertions(+), 28 deletions(-) create mode 100644 gcc/testsuite/gfortran.dg/array_reference_3.f90 create mode 100644 gcc/testsuite/gfortran.dg/negative_stride_1.f90 create mode 100644 gcc/testsuite/gfortran.dg/vector_subscript_8.f90 create mode 100644 gcc/testsuite/gfortran.dg/vector_subscript_9.f90 -- 2.35.1
