Fix PAL_GetLogicalProcessorCacheSizeFromOS on mac (#24777)

* Fix PAL_GetLogicalProcessorCacheSizeFromOS on mac

In a previous PR
(https://github.com/dotnet/coreclr/commit/ed52a006c01a582d4d34add40c318d6f324b99ba#diff-8447e54277bb962d167a77bb260760d7R1879),
GetCacheSizePerLogicalCpu was changed to no longer rely on cpuid on
amd64 systems; instead it uses GetLogicalProcessorCacheSizeFromOS().
Unfortunately that function consisted of a number of `#if`s, none of
which were active on macs, and we just returned 0. This caused us to
default to a gen0size of only 0.25MB, causing many GCs.

Fixed by adding a new case that uses `sysctlbyname`.

Fix #24658

* Fixes from code review

* Check for function sysctlbyname instead of header

1 files changed, 16 insertions, 0 deletions

diff --git a/src/pal/src/misc/sysinfo.cpp b/src/pal/src/misc/sysinfo.cpp
index d0e8ca64dc..1e911023cf 100644
--- a/src/pal/src/misc/sysinfo.cpp
+++ b/src/pal/src/misc/sysinfo.cpp
@@ -529,5 +529,21 @@ PAL_GetLogicalProcessorCacheSizeFromOS()
     }
 #endif
 
+#if HAVE_SYSCTLBYNAME
+    if (cacheSize == 0)
+    {
+        int64_t cacheSizeFromSysctl = 0;
+        size_t sz = sizeof(cacheSizeFromSysctl);
+        const bool success = sysctlbyname("hw.l3cachesize", &cacheSizeFromSysctl, &sz, nullptr, 0) == 0
+            || sysctlbyname("hw.l2cachesize", &cacheSizeFromSysctl, &sz, nullptr, 0) == 0
+            || sysctlbyname("hw.l1dcachesize", &cacheSizeFromSysctl, &sz, nullptr, 0) == 0;
+        if (success)
+        {
+            assert(cacheSizeFromSysctl > 0);
+            cacheSize = (size_t) cacheSizeFromSysctl;
+        }
+    }
+#endif
+
     return cacheSize;
 }