[llvm] [RISCV][MRI] Account for fixed registers when determining callee saved regs (PR #115756)

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

> > The behavior differs from GCC for AArch64, ARM, Hexagon, SPARC, M68k and RISC-V.
> > In LLVM, different targets have different interfaces to check whether the register was `-ffixed-XXX`. I am proposing that each target migrate to this common interface in follow up patches.
> > This patch adds the interface, and migrates RISC-V and M68k to that interface, since they are already using the interface.
> 
> How will this interact with AArch64 or other targets calling setCalleeSavedRegisters which will cause `IsUpdatedCSRsInitialized` to get set preventing reaching the new code you added in `getCalleeSavedRegs`

If a target calls `setCalleeSavedRegs`, then its saying "give me full control of what registers are callee saved regs".  If a target calls `setCalleeSavedRegs`, then it is on the target to determine which registers are fixed, in the same way its the target to figure out what registers are callee saved regs.

If a target calls `getCalleeSavedRegs`, it is prevented from reaching the existing `getTargetRegisterInfo()->getCalleeSavedRegs()` code. How is this any different?



https://github.com/llvm/llvm-project/pull/115756