[flang][cuda] Transfer device data used in host operations (#228292) An assignment to host data from an expression with device data is lowered as an implicit data transfer only when the expression also contains a constant or a host symbol. Constants in subscripts are ignored, so expressions such as a(3)*a(4), -a(3), -d1, or a conversion a(3) from real(8) to real(4) were lowered as plain transfers. The scalar result was then computed on the host directly from device memory and stored to the left-hand side, and array expressions failed the cuf.data_transfer verifier. Treat any right-hand side with device data that is not a variable as an implicit transfer. The device data is copied to a host temporary and the expression is evaluated on the host. An assignment of such an expression to device data is now diagnosed as an unsupported transfer, as a = a + 10 already is. GitOrigin-RevId: 65a82906cff9c7a7a33844812d35ce159d88a354
Flang is a ground-up implementation of a Fortran front end written in modern C++. It started off as the f18 project (https://github.com/flang-compiler/f18) with an aim to replace the previous flang project (https://github.com/flang-compiler/flang) and address its various deficiencies. F18 was subsequently accepted into the LLVM project and rechristened as Flang.
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To better understand Fortran as a language and the specific grammar accepted by flang, read Fortran For C Programmers and flang's specifications of the Fortran grammar and the OpenMP grammar.
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