Import of the watch repository from Pebble

src/fw/libgcc_override.c

@@ -0,0 +1,43 @@
+/*
+ * Copyright 2024 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "util/attributes.h"
+
+#include <stdint.h>
+
+
+// Override libgcc's table-driven __builtin_popcount implementation
+#ifdef __arm__
+EXTERNALLY_VISIBLE int __popcountsi2(uint32_t val) {
+  // Adapted from http://www.sciencezero.org/index.php?title=ARM%3a_Count_ones_%28bit_count%29
+  uint32_t tmp;
+  __asm("and  %[tmp], %[val], #0xaaaaaaaa\n\t"
+        "sub  %[val], %[val], %[tmp], lsr #1\n\t"
+
+        "and  %[tmp], %[val], #0xcccccccc\n\t"
+        "and  %[val], %[val], #0x33333333\n\t"
+        "add  %[val], %[val], %[tmp], lsr #2\n\t"
+
+        "add  %[val], %[val], %[val], lsr #4\n\t"
+        "and  %[val], %[val], #0x0f0f0f0f\n\t"
+
+        "add  %[val], %[val], %[val], lsr #8\n\t"
+        "add  %[val], %[val], %[val], lsr #16\n\t"
+        "and  %[val], %[val], #63\n\t"
+        : [val] "+l" (val), [tmp] "=&l" (tmp));
+  return val;
+}
+#endif  // __arm__