[builtins][arm64] Implement __init_cpu_features_resolver on Apple platforms (#75636)

This is a re-land of #73685

Author: jroelofs

compiler-rt/lib/builtins/cpu_model/aarch64.c

@@ -123,7 +123,9 @@ struct {
 
 // The formatter wants to re-order these includes, but doing so is incorrect:
 // clang-format off
-#if defined(__FreeBSD__)
+#if defined(__APPLE__)
+#include "aarch64/fmv/apple.inc"
+#elif defined(__FreeBSD__)
 #include "aarch64/fmv/mrs.inc"
 #include "aarch64/fmv/freebsd.inc"
 #elif defined(__Fuchsia__)

compiler-rt/lib/builtins/cpu_model/aarch64/fmv/apple.inc

@@ -0,0 +1,69 @@
+#include <TargetConditionals.h>
+#if TARGET_OS_OSX || TARGET_OS_IPHONE
+#include <dispatch/dispatch.h>
+#include <sys/sysctl.h>
+
+static bool isKnownAndSupported(const char *name) {
+  int32_t val = 0;
+  size_t size = sizeof(val);
+  if (sysctlbyname(name, &val, &size, NULL, 0))
+    return false;
+  return val;
+}
+
+void __init_cpu_features_resolver(void) {
+  // On Darwin platforms, this may be called concurrently by multiple threads
+  // because the resolvers that use it are called lazily at runtime (unlike on
+  // ELF platforms, where IFuncs are resolved serially at load time).  This
+  // function's effect on __aarch64_cpu_features should be idempotent, but even
+  // so we need dispatch_once to resolve the race condition.  Dispatch is
+  // available through libSystem, which we need anyway for the sysctl, so this
+  // does not add a new dependency.
+
+  static dispatch_once_t onceToken = 0;
+  dispatch_once(&onceToken, ^{
+    // https://developer.apple.com/documentation/kernel/1387446-sysctlbyname/determining_instruction_set_characteristics
+    static struct {
+      const char *sysctl_name;
+      enum CPUFeatures feature;
+    } features[] = {
+        {"hw.optional.arm.FEAT_FlagM", FEAT_FLAGM},
+        {"hw.optional.arm.FEAT_FlagM2", FEAT_FLAGM2},
+        {"hw.optional.arm.FEAT_FHM", FEAT_FP16FML},
+        {"hw.optional.arm.FEAT_DotProd", FEAT_DOTPROD},
+        {"hw.optional.arm.FEAT_RDM", FEAT_RDM},
+        {"hw.optional.arm.FEAT_LSE", FEAT_LSE},
+        {"hw.optional.floatingpoint", FEAT_FP},
+        {"hw.optional.AdvSIMD", FEAT_SIMD},
+        {"hw.optional.armv8_crc32", FEAT_CRC},
+        {"hw.optional.arm.FEAT_SHA1", FEAT_SHA1},
+        {"hw.optional.arm.FEAT_SHA256", FEAT_SHA2},
+        {"hw.optional.arm.FEAT_SHA3", FEAT_SHA3},
+        {"hw.optional.arm.FEAT_AES", FEAT_AES},
+        {"hw.optional.arm.FEAT_PMULL", FEAT_PMULL},
+        {"hw.optional.arm.FEAT_FP16", FEAT_FP16},
+        {"hw.optional.arm.FEAT_DIT", FEAT_DIT},
+        {"hw.optional.arm.FEAT_DPB", FEAT_DPB},
+        {"hw.optional.arm.FEAT_DPB2", FEAT_DPB2},
+        {"hw.optional.arm.FEAT_JSCVT", FEAT_JSCVT},
+        {"hw.optional.arm.FEAT_FCMA", FEAT_FCMA},
+        {"hw.optional.arm.FEAT_LRCPC", FEAT_RCPC},
+        {"hw.optional.arm.FEAT_LRCPC2", FEAT_RCPC2},
+        {"hw.optional.arm.FEAT_FRINTTS", FEAT_FRINTTS},
+        {"hw.optional.arm.FEAT_I8MM", FEAT_I8MM},
+        {"hw.optional.arm.FEAT_BF16", FEAT_BF16},
+        {"hw.optional.arm.FEAT_SB", FEAT_SB},
+        {"hw.optional.arm.FEAT_SPECRES", FEAT_PREDRES},
+        {"hw.optional.arm.FEAT_SSBS", FEAT_SSBS2},
+        {"hw.optional.arm.FEAT_BTI", FEAT_BTI},
+    };
+
+    for (size_t I = 0, E = sizeof(features) / sizeof(features[0]); I != E; ++I)
+      if (isKnownAndSupported(features[I].sysctl_name))
+        __aarch64_cpu_features.features |= (1ULL << features[I].feature);
+
+    __aarch64_cpu_features.features |= (1ULL << FEAT_INIT);
+  });
+}
+
+#endif // TARGET_OS_OSX || TARGET_OS_IPHONE