478b4b012f
This patch reworks how VG is handled around streaming mode changes. Previously, for functions with streaming mode changes, we would: - Save the incoming VG in the prologue - Emit `.cfi_offset vg, <offset>` and `.cfi_restore vg` around streaming mode changes Additionally, for locally streaming functions, we would: - Also save the streaming VG in the prologue - Emit `.cfi_offset vg, <incoming VG offset>` in the prologue - Emit `.cfi_offset vg, <streaming VG offset>` and `.cfi_restore vg` around streaming mode changes In both cases, this ends up doing more than necessary and would be hard for an unwinder to parse, as using `.cfi_offset` in this way does not follow the semantics of the underlying DWARF CFI opcodes. So the new scheme in this patch is to: In functions with streaming mode changes (inc locally streaming) - Save the incoming VG in the prologue - Emit `.cfi_offset vg, <offset>` in the prologue (not at streaming mode changes) - Emit `.cfi_restore vg` after the saved VG has been deallocated - This will be in the function epilogue, where VG is always the same as the entry VG - Explicitly reference the incoming VG expressions for SVE callee-saves in functions with streaming mode changes - Ensure the CFA is not described in terms of VG in functions with streaming mode changes A more in-depth discussion of this scheme is available in: https://gist.github.com/MacDue/b7a5c45d131d2440858165bfc903e97b But the TLDR is that following this scheme, SME unwinding can be implemented with minimal changes to existing unwinders. All unwinders need to do is initialize VG to `CNTD` at the start of unwinding, then everything else is handled by standard opcodes (which don't need changes to handle VG).
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