3dbb055f54
Flang was generating invalid IR when there was a GOTO to the body of a DO loop. This happened because the value of step, computed at the beginning of the loop, was being reused at the end of the loop, that, for unstructured loops, is in another basic block. Because of this, a GOTO could skip the beginning of the loop, that defined step, and yet try to use it at the end of the loop, which is invalid. Instead of reusing the step value, it can be recomputed if it is a constant, or stored and loaded to/from a temporary variable, for non-constant step expressions. Note that, while this change prevents the generation of invalid IR on the presence of jumps to DO loop bodies, what happens if the program reaches the end of a DO loop without ever passing through its beginning is undefined behavior, as some control variables, such as trip, will be uninitialized. It doesn't seem worth the effort and overhead to ensure this legacy extension will behave correctly in this case. This is consistent with at least gfortran, that doesn't behave correctly if step is not equal to one. Fixes: https://github.com/llvm/llvm-project/issues/65036
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