[scudo] Small cleanup of memory tagging code part 2.

Make the systemSupportsMemoryTagging() function return even
on system that don't support memory tagging. This avoids the
need to always check if memory tagging is supported before
calling the function.

Modify iterateOverChunks() to call useMemoryTagging<>(Options) to
determine if mte is supported. This already uses the cached check
of systemSupportsMemoryTagging() rather than directly calling
that function.

Updated the code that calls systemSupportsMemoryTagging().

Author: cferris1000

compiler-rt/lib/scudo/standalone/combined.h

@@ -171,8 +171,7 @@ class Allocator {
       Primary.Options.set(OptionBit::DeallocTypeMismatch);
     if (getFlags()->delete_size_mismatch)
       Primary.Options.set(OptionBit::DeleteSizeMismatch);
-    if (allocatorSupportsMemoryTagging<AllocatorConfig>() &&
-        systemSupportsMemoryTagging())
+    if (systemSupportsMemoryTagging())
       Primary.Options.set(OptionBit::UseMemoryTagging);
 
     QuarantineMaxChunkSize =
@@ -689,16 +688,15 @@ class Allocator {
       Base = untagPointer(Base);
     const uptr From = Base;
     const uptr To = Base + Size;
-    bool MayHaveTaggedPrimary =
-        allocatorSupportsMemoryTagging<AllocatorConfig>() &&
-        systemSupportsMemoryTagging();
+    const Options Options = Primary.Options.load();
+    bool MayHaveTaggedPrimary = useMemoryTagging<AllocatorConfig>(Options);
     auto Lambda = [this, From, To, MayHaveTaggedPrimary, Callback,
                    Arg](uptr Block) {
       if (Block < From || Block >= To)
         return;
       uptr Chunk;
       Chunk::UnpackedHeader Header;
-      if (MayHaveTaggedPrimary) {
+      if (UNLIKELY(MayHaveTaggedPrimary)) {
         // A chunk header can either have a zero tag (tagged primary) or the
         // header tag (secondary, or untagged primary). We don't know which so
         // try both.

compiler-rt/lib/scudo/standalone/memtag.h

@@ -261,8 +261,8 @@ inline uptr loadTag(uptr Ptr) {
 
 #else
 
-inline NORETURN bool systemSupportsMemoryTagging() {
-  UNREACHABLE("memory tagging not supported");
+inline bool systemSupportsMemoryTagging() {
+  return false;
 }
 
 inline NORETURN bool systemDetectsMemoryTagFaultsTestOnly() {

compiler-rt/lib/scudo/standalone/tests/memtag_test.cpp

@@ -28,7 +28,6 @@ TEST(MemtagBasicDeathTest, Unsupported) {
   EXPECT_DEATH(untagPointer((uptr)0), "not supported");
   EXPECT_DEATH(extractTag((uptr)0), "not supported");
 
-  EXPECT_DEATH(systemSupportsMemoryTagging(), "not supported");
   EXPECT_DEATH(systemDetectsMemoryTagFaultsTestOnly(), "not supported");
   EXPECT_DEATH(enableSystemMemoryTaggingTestOnly(), "not supported");
 

compiler-rt/lib/scudo/standalone/tests/secondary_test.cpp

@@ -27,9 +27,7 @@
 const scudo::uptr PageSize = scudo::getPageSizeCached();
 
 template <typename Config> static scudo::Options getOptionsForConfig() {
-  if (!Config::getMaySupportMemoryTagging() ||
-      !scudo::archSupportsMemoryTagging() ||
-      !scudo::systemSupportsMemoryTagging())
+  if (!scudo::systemSupportsMemoryTagging())
     return {};
   scudo::AtomicOptions AO;
   AO.set(scudo::OptionBit::UseMemoryTagging);