[llvm] [AArch64][SME] Conditionally do smstart/smstop (PR #77113)

[Sander de Smalen via llvm-commits]

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@@ -65,6 +65,16 @@ SMEAttrs::SMEAttrs(const AttributeList &Attrs) {
 std::optional<bool>
 SMEAttrs::requiresSMChange(const SMEAttrs &Callee,
                            bool BodyOverridesInterface) const {
+  // If the caller has a streaming body and streaming compatible interface,
+  // we will have already conditionally enabled streaming mode on function
+  // entry. We need to disable streaming mode when a callee does not have A
+  // streaming interface, body, or streaming compatible interface.
+  if (hasStreamingBody() && hasStreamingCompatibleInterface())
+    return (!Callee.hasStreamingInterfaceOrBody() &&
+            !Callee.hasStreamingCompatibleInterface())
+               ? std::optional<bool>(false)
+               : std::nullopt;
+
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sdesmalen-arm wrote:

The three states are to describe:
* `std::nullopt` = No change needed
* `std::optional<bool>(true)` = Change needed, smstart
* `std::optional<bool>(false)` = Change needed, smstop
I do agree the interface is confusing and could be better named.

https://github.com/llvm/llvm-project/pull/77113