[llvm-branch-commits] [compiler-rt] [clang-tools-extra] [llvm] [clang] [builtins][arm64] Build __init_cpu_features_resolver on Apple platforms (PR #73685)

[Ahmed Bougacha via llvm-branch-commits]

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@@ -1259,6 +1270,72 @@ struct {
   // As features grows new fields could be added
 } __aarch64_cpu_features __attribute__((visibility("hidden"), nocommon));
 
+#if defined(__APPLE__)
+#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.armv8_2_sha3", FEAT_SHA3},
----------------
ahmedbougacha wrote:

I think you want `FEAT_SHA3` instead of `armv8_2_sha3` (and technically, sha512 also, but that shouldn't matter; crypto's a mess).  I thought we had one for crc32 as well, but looks like we don't.  I assume FP/SIMD are unneeded but harmless.

Otherwise, sysctl `FEAT_SPECRES` should map to enum `PREDRES`, and DIT, DPB/DPB2 are missing (DIT is vaguely plausibly useful, DPB not really.)  Rest LGTM!

https://github.com/llvm/llvm-project/pull/73685