Bug 1730222 - Support PHC on macOS when running on ARM-based machines r=glandium

This also tightens the check for PHC-type crashes in the exception handler
so that we don't accidentally try to interpret unrelated crash types as
potential PHC ones.

Differential Revision: https://phabricator.services.mozilla.com/D126365

memory/replace/phc/PHC.cpp

@@ -55,12 +55,16 @@
 //
 // The basic cost of PHC's operation is as follows.
 //
-// - The physical memory cost is 64 * 4 KiB = 256 KiB per process (assuming 4
-//   KiB pages) plus some metadata (including stack traces) for each page.
+// - The physical memory cost is 64 pages plus some metadata (including stack
+//   traces) for each page. This amounts to 256 KiB per process on
+//   architectures with 4 KiB pages and 1024 KiB on macOS/AArch64 which uses
+//   16 KiB pages.
 //
 // - The virtual memory cost is the physical memory cost plus the guard pages:
-//   another 64 * 4 KiB = 256 KiB per process. PHC is currently only enabled on
-//   64-bit platforms so the impact of the virtual memory usage is negligible.
+//   another 64 pages. This amounts to another 256 KiB per process on
+//   architectures with 4 KiB pages and 1024 KiB on macOS/AArch64 which uses
+//   16 KiB pages. PHC is currently only enabled on 64-bit platforms so the
+//   impact of the virtual memory usage is negligible.
 //
 // - Every allocation requires a size check and a decrement-and-check of an
 //   atomic counter. When the counter reaches zero a PHC allocation can occur,
@@ -283,8 +287,16 @@ using Time = uint64_t;   // A moment in time.
 using Delay = uint32_t;  // A time duration.
 
 // PHC only runs if the page size is 4 KiB; anything more is uncommon and would
-// use too much memory. So we hardwire this size.
-static const size_t kPageSize = 4096;
+// use too much memory. So we hardwire this size for all platforms but macOS
+// on ARM processors. For the latter we make an exception because the minimum
+// page size supported is 16KiB so there's no way to go below that.
+static const size_t kPageSize =
+#if defined(XP_MACOSX) && defined(__aarch64__)
+    16384
+#else
+    4096
+#endif
+    ;
 
 // There are two kinds of page.
 // - Allocation pages, from which allocations are made.

toolkit/crashreporter/breakpad-client/mac/handler/exception_handler.cc

@@ -413,10 +413,10 @@ bool ExceptionHandler::WriteMinidumpForChild(mach_port_t child,
 }
 
 #ifdef MOZ_PHC
-static void GetPHCAddrInfo(int64_t exception_subcode,
+static void GetPHCAddrInfo(int exception_type, int64_t exception_subcode,
                            mozilla::phc::AddrInfo* addr_info) {
   // Is this a crash involving a PHC allocation?
-  if (exception_subcode) {
+  if (exception_type == EXC_BAD_ACCESS) {
     // `exception_subcode` is only non-zero when it's a bad access, in which
     // case it holds the address of the bad access.
     char* addr = reinterpret_cast<char*>(exception_subcode);
@@ -443,7 +443,7 @@ bool ExceptionHandler::WriteMinidumpWithException(
 
     mozilla::phc::AddrInfo addr_info;
 #ifdef MOZ_PHC
-    GetPHCAddrInfo(exception_subcode, &addr_info);
+    GetPHCAddrInfo(exception_type, exception_subcode, &addr_info);
 #endif
 
   if (directCallback_) {