arch-arm: Do not compute purifyTaggedAddr in checkPermissions (#1739)

purifyTaggedAddr is known to be an expensive computation regardless of
the memoization we do, as it sits in the critical path from a host
performance point of view (instruction fetch).
In checkPermissions64 we compute it without really needing the tag
purification. The only place where it is used is to check for
PCAlignment, but the alignment checks the 3LSBs whereas a potential tag
would be stored in the most significant ones

Change-Id: I9f39db658c3575dcbacb5351813ff9bb3775046d

Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>

src/arch/arm/mmu.cc

@@ -507,9 +507,6 @@ MMU::checkPermissions64(TlbEntry *te, const RequestPtr &req, Mode mode,
     }
 
     Addr vaddr_tainted = req->getVaddr();
-    Addr vaddr = purifyTaggedAddr(vaddr_tainted, tc, state.exceptionLevel,
-        static_cast<TCR>(state.ttbcr), mode==Execute, state);
-
     Request::Flags flags = req->getFlags();
     bool is_fetch  = (mode == Execute);
     // Cache clean operations require read permissions to the specified VA
@@ -534,7 +531,7 @@ MMU::checkPermissions64(TlbEntry *te, const RequestPtr &req, Mode mode,
     // strongly ordered memory
     if (!is_fetch) {
         if (te->mtype != TlbEntry::MemoryType::Normal) {
-            if (vaddr & mask(flags & AlignmentMask)) {
+            if (vaddr_tainted & mask(flags & AlignmentMask)) {
                 stats.alignFaults++;
                 return std::make_shared<DataAbort>(
                     vaddr_tainted,