[compiler-rt] [builtins] Refactor cpu_model support to reduce #if nesting. NFCI (PR #75635)

[Jon Roelofs via llvm-commits]

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@@ -0,0 +1,28 @@
+#include <string.h>
+#include <sys/system_properties.h>
+
+static void __isExynos9810(void) {
+  char arch[PROP_VALUE_MAX];
+  return (__system_property_get("ro.arch", arch) > 0 &&
+    strncmp(arch, "exynos9810", sizeof("exynos9810") - 1) == 0;
+}
+
+static void CONSTRUCTOR_ATTRIBUTE init_have_lse_atomics(void) {
+  unsigned long hwcap = getauxval(AT_HWCAP);
+  _Bool result = (hwcap & HWCAP_ATOMICS) != 0;
+  if (result) {
+    // Some cores in the Exynos 9810 CPU are ARMv8.2 and others are ARMv8.0;
+    // only the former support LSE atomics.  However, the kernel in the
+    // initial Android 8.0 release of Galaxy S9/S9+ devices incorrectly
+    // reported the feature as being supported.
+    //
+    // The kernel appears to have been corrected to mark it unsupported as of
+    // the Android 9.0 release on those devices, and this issue has not been
+    // observed anywhere else. Thus, this workaround may be removed if
+    // compiler-rt ever drops support for Android 8.0.
+    char arch[PROP_VALUE_MAX];
+    if (__isExynos9810())
+      result = false;
+  }
+  __aarch64_have_lse_atomics = result;
+}
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jroelofs wrote:

- [ ] FIXME: needs EOF newline

https://github.com/llvm/llvm-project/pull/75635