llvm-project

7f8451c8 - [RISCV] Use vsetvli instead of vlenb in Prologue/Epilogue (#113756)

[RISCV] Use vsetvli instead of vlenb in Prologue/Epilogue (#113756) Currently, we use `csrr` with `vlenb` to obtain the `VLEN`, but this is not the only option. We can also use `vsetvli` with `e8`/`m1` to get `VLENMAX`, which is equal to the VLEN. This method is preferable on some microarchitectures and makes it easier to obtain values like `VLEN * 2`, `VLEN * 4`, or `VLEN * 8`, reducing the number of instructions needed to calculate VLEN multiples. However, this approach is *NOT* always interchangeable, as it changes the state of `VTYPE` and `VL`, which can alter the behavior of vector instructions, potentially causing incorrect code generation if applied after a vsetvli insertion. Therefore, we limit its use to the prologue/epilogue for now, as there are no vector operations within the prologue/epilogue sequence. With further analysis, we may extend this approach beyond the prologue/epilogue in the future, but starting here should be a good first step. This feature is gurded by the `+prefer-vsetvli-over-read-vlenb` feature, which is disabled by default for now.