Preserve SoftDevice memory side effects under nRF52 LTO

src/helpers/nrf52/SoftDeviceSvcCompat.h

@@ -0,0 +1,28 @@
+#pragma once
+
+#include <stdint.h>
+
+// Nordic's SDK SVC wrappers contain assembly without a memory clobber. With
+// whole-program optimization, GCC can infer that the wrappers neither read
+// nor write the structures passed to the SoftDevice. That assumption is false
+// for GAP pairing keys, GATT notifications, and many other supervisor calls.
+// Keep each SVC opaque to its callers and declare its memory side effects.
+// This header is force-included in nRF52 C and C++ compilation, including the
+// framework, before nrf_svc.h defines SVCALL.
+#if defined(NRF52_PLATFORM) && defined(__GNUC__) \
+    && !defined(SVCALL_AS_NORMAL_FUNCTION) && !defined(SVCALL)
+#if __GNUC__ >= 9
+#define MESHCORE_SVC_ATTRIBUTES __attribute__((naked, unused, noipa))
+#else
+#define MESHCORE_SVC_ATTRIBUTES __attribute__((naked, unused, noinline))
+#endif
+
+#define SVCALL(number, return_type, signature)                         \
+  _Pragma("GCC diagnostic push")                                     \
+  _Pragma("GCC diagnostic ignored \"-Wreturn-type\"")                 \
+  MESHCORE_SVC_ATTRIBUTES static return_type signature {              \
+    __asm volatile ("svc %0\n" "bx r14" : : "I" ((uint16_t)(number)) \
+                    : "r0", "memory");                             \
+  }                                                                 \
+  _Pragma("GCC diagnostic pop")
+#endif