[AArch64][SME] Use entry pstate.sm for conditional streaming-mode changes (#152169)

We only do conditional streaming mode changes in two cases:

- Around calls in streaming-compatible functions that don't have a
streaming body
- At the entry/exit of streaming-compatible functions with a streaming
body

In both cases, the condition depends on the entry pstate.sm value. Given
this, we don't need to emit calls to __arm_sme_state at every mode
change.

This patch handles this by placing a "AArch64ISD::ENTRY_PSTATE_SM" node
in the entry block and copying the result to a register. The register is
then used whenever we need to emit a conditional streaming mode change.
The "ENTRY_PSTATE_SM" node expands to a call to "__arm_sme_state" only
if (after SelectionDAG) the function is determined to have
streaming-mode changes.

This has two main advantages:

1. It allows back-to-back conditional smstart/stop pairs to be folded
2. It has the correct behaviour for EH landing pads
- These are entered with pstate.sm = 0, and should switch mode based on
the entry pstate.sm
   - Note: This is not fully implemented yet

0 files changed, 0 insertions, 0 deletions