build(android): add opt-in flag to disable SVE dispatch on Apple Silicon emulators

plugin-nativeprocessor/platforms/android/cpp/CMakeLists.txt

@@ -54,6 +54,15 @@ if(DEFINED WITH_QRCODE)
     target_link_libraries(${TARGET} ZXing::ZXing)
 endif(DEFINED WITH_QRCODE)
 
+# Development only. Hides SVE from the CPU detection done inside the library so that
+# KleidiCV uses its NEON code paths: Android emulators on Apple Silicon advertise SVE2
+# in HWCAP although the host CPU cannot execute a single SVE instruction.
+# See hwcap_sve.cpp
+option(DISABLE_SVE "hide SVE from CPU detection, for Android emulators on Apple Silicon" OFF)
+if(DISABLE_SVE)
+    add_definitions(-DDISABLE_SVE)
+endif(DISABLE_SVE)
+
 target_link_libraries(${TARGET} log jnigraphics z)
 
 # Specifies libraries CMake should link to your target library. You

plugin-nativeprocessor/platforms/android/cpp/hwcap_sve.cpp

@@ -0,0 +1,63 @@
+// Development only: compiled in when the `DISABLE_SVE` CMake option is set
+// (`--gradleArgs=-PdisableSVE` on the build command line). Toggling the flag alone may
+// not be enough, the CLI reuses the prebuilt plugin aar until a plugin source changes.
+//
+// Android emulators running on Apple Silicon advertise SVE, SVE2 and friends in HWCAP
+// even though the host CPU implements none of them. KleidiCV, which OpenCV uses as its
+// arm64 HAL, reads HWCAP in its static initializers and picks its SVE2 kernels
+// accordingly, so the first SVE instruction executed (`addvl`) raises SIGILL and kills
+// the app. It shows up as a crash in `nativeCrop` through `cv::remap`.
+//
+// Defining `getauxval` here makes the linker bind every call coming from OpenCV and
+// KleidiCV to this version instead of libc's, and hiding the SVE bits makes KleidiCV
+// fall back to its NEON implementations. Never enable this for a release build: it
+// would give up KleidiCV's SVE2 code paths on the devices that really support them.
+#if defined(DISABLE_SVE) && defined(__aarch64__)
+
+#include <asm/hwcap.h>
+#include <dlfcn.h>
+#include <sys/auxv.h>
+
+namespace {
+
+constexpr unsigned long kSveHwcapMask = HWCAP_SVE;
+
+constexpr unsigned long kSveHwcap2Mask = HWCAP2_SVE2 | HWCAP2_SVEAES | HWCAP2_SVEPMULL |
+                                         HWCAP2_SVEBITPERM | HWCAP2_SVESHA3 | HWCAP2_SVESM4 |
+                                         HWCAP2_SVEI8MM | HWCAP2_SVEF32MM | HWCAP2_SVEF64MM |
+                                         HWCAP2_SVEBF16 | HWCAP2_SVE_EBF16 | HWCAP2_SVE2P1 |
+                                         HWCAP2_SVE_B16B16;
+
+using GetAuxValFn = unsigned long (*)(unsigned long);
+
+} // namespace
+
+extern "C" __attribute__((visibility("hidden"))) unsigned long getauxval(unsigned long type) {
+    static GetAuxValFn realGetAuxVal = nullptr;
+    static bool resolving = false;
+    if (realGetAuxVal == nullptr) {
+        if (resolving) {
+            // `dlsym` called us back before we could resolve: report no CPU feature at all
+            return 0;
+        }
+        resolving = true;
+        auto resolved = reinterpret_cast<GetAuxValFn>(dlsym(RTLD_DEFAULT, "getauxval"));
+        resolving = false;
+        if (resolved == nullptr || resolved == &getauxval) {
+            // never call ourselves: this function is hidden so `dlsym` should not see it,
+            // but a mistake here would recurse until the stack blows up
+            return 0;
+        }
+        realGetAuxVal = resolved;
+    }
+    const unsigned long value = realGetAuxVal(type);
+    if (type == AT_HWCAP) {
+        return value & ~kSveHwcapMask;
+    }
+    if (type == AT_HWCAP2) {
+        return value & ~kSveHwcap2Mask;
+    }
+    return value;
+}
+
+#endif // DISABLE_SVE && __aarch64__

plugin-nativeprocessor/platforms/android/include.gradle

@@ -8,11 +8,15 @@ android {
                 cmake {
                     cppFlags "-frtti -fexceptions"
                     abiFilters 'armeabi-v7a', "arm64-v8a", 'x86', 'x86_64'
+                    def cmakeArguments = ["-DANDROID_SUPPORT_FLEXIBLE_PAGE_SIZES=ON", "-DANDROID_TOOLCHAIN=clang", "-DANDROID_STL=c++_static"]
                     if (project.hasProperty("withQRCode")) {
-                        arguments "-DANDROID_SUPPORT_FLEXIBLE_PAGE_SIZES=ON", "-DANDROID_TOOLCHAIN=clang", "-DANDROID_STL=c++_static", "-DWITH_QRCODE=1"
-                    } else {
-                        arguments "-DANDROID_SUPPORT_FLEXIBLE_PAGE_SIZES=ON", "-DANDROID_TOOLCHAIN=clang", "-DANDROID_STL=c++_static"
+                        cmakeArguments << "-DWITH_QRCODE=1"
                     }
+                    // development only, see plugin-nativeprocessor/platforms/android/cpp/hwcap_sve.cpp
+                    // always passed explicitly: a value left in the CMake cache would survive
+                    // the flag being dropped and silently disable SVE in a release build
+                    cmakeArguments << ("-DDISABLE_SVE=" + (project.hasProperty("disableSVE") ? "1" : "0"))
+                    arguments(*cmakeArguments)
                 }
             }
         }