cfc5baf6e6
This Change adds support for two SiFive vendor attributes in clang: - "SiFive-CLIC-preemptible" - "SiFive-CLIC-stack-swap" These can be given together, and can be combined with "machine", but cannot be combined with any other interrupt attribute values. These are handled primarily in RISCVFrameLowering: - "SiFive-CLIC-stack-swap" entails swapping `sp` with `sf.mscratchcsw` at function entry and exit, which holds the trap stack pointer. - "SiFive-CLIC-preemptible" entails saving `mcause` and `mepc` before re-enabling interrupts using `mstatus`. To save these, `s0` and `s1` are first spilled to the stack, and then the values are read into these registers. If these registers are used in the function, their values will be spilled a second time onto the stack with the generic callee-saved-register handling. At the end of the function interrupts are disabled again before `mepc` and `mcause` are restored. This Change also adds support for the following two experimental extensions, which only contain CSRs: - XSfsclic - for SiFive's CLIC Supervisor-Mode CSRs - XSfmclic - for SiFive's CLIC Machine-Mode CSRs The latter is needed for interrupt support. The CFI information for this implementation is not correct, but I'd prefer to correct this in a follow-up. While it's unlikely anyone wants to unwind through a handler, the CFI information is also used by debuggers so it would be good to get it right. Co-authored-by: Ana Pazos <apazos@quicinc.com>
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