[sanitizer] Minor refactor of some ReservedAddressRange functions

[cryptoad]

Summary

Some of the functions had spurious conditional statements and checks, and some
intermediary variables that I feel made the code more complicated than it needs
to be. Also, when unmapping the whole range, the range size would be 0, but
the base was set to the address of the end of the range, which sounds prone to
error. I think nulling out the base in this scenario is a better way to go.

lib/sanitizer_common/sanitizer_fuchsia.cc

if (status != ZX_OK) {
     Report("ERROR: %s failed to deallocate 0x%zx (%zd) bytes at address %p\n",
            SanitizerToolName, size, size, addr);
     CHECK("unable to unmap" && 0);
   }
 
   DecreaseTotalMmap(size);
 }
 
-void ReservedAddressRange::Unmap(uptr fixed_addr, uptr size) {
-  uptr offset = fixed_addr - reinterpret_cast<uptr>(base_);
-  uptr addr = reinterpret_cast<uptr>(base_) + offset;
-  void *addr_as_void = reinterpret_cast<void *>(addr);
-  uptr base_as_uptr = reinterpret_cast<uptr>(base_);
-  // Only unmap at the beginning or end of the range.
-  CHECK((addr_as_void == base_) || (addr + size == base_as_uptr + size_));
+void ReservedAddressRange::Unmap(uptr addr, uptr size) {
   CHECK_LE(size, size_);
+  if (addr == reinterpret_cast<uptr>(base_))
+    // If we unmap the whole range, just null out the base.
+    base_ = (size == size_) ? nullptr : reinterpret_cast<void*>(addr + size);
+  else
+    CHECK_EQ(addr + size, reinterpret_cast<uptr>(base_) + size_);
+  size_ -= size;
   UnmapOrDieVmar(reinterpret_cast<void *>(addr), size,
                  static_cast<zx_handle_t>(os_handle_));
-  if (addr_as_void == base_) {
-    base_ = reinterpret_cast<void *>(addr + size);
-  }
-  size_ = size_ - size;
 }
 
 // This should never be called.
 void *MmapFixedNoAccess(uptr fixed_addr, uptr size, const char *name) {
   UNIMPLEMENTED();
 }
 
 void *MmapAlignedOrDieOnFatalError(uptr size, uptr alignment,

lib/sanitizer_common/sanitizer_posix_libcdep.cc

 }
 
 uptr ReservedAddressRange::Init(uptr size, const char *name, uptr fixed_addr) {
   // We don't pass `name` along because, when you enable `decorate_proc_maps`
   // AND actually use a named mapping AND are using a sanitizer intercepting
   // `open` (e.g. TSAN, ESAN), then you'll get a failure during initialization.
   // TODO(flowerhack): Fix the implementation of GetNamedMappingFd to solve
   // this problem.
-  if (fixed_addr) {
-    base_ = MmapFixedNoAccess(fixed_addr, size);
-  } else {
-    base_ = MmapNoAccess(size);
-  }
+  base_ = fixed_addr ? MmapFixedNoAccess(fixed_addr, size) : MmapNoAccess(size);
   size_ = size;
   name_ = name;
   (void)os_handle_;  // unsupported
   return reinterpret_cast<uptr>(base_);
 }
 
 // Uses fixed_addr for now.
 // Will use offset instead once we've implemented this function for real.
 uptr ReservedAddressRange::Map(uptr fixed_addr, uptr size) {
   return reinterpret_cast<uptr>(MmapFixedOrDieOnFatalError(fixed_addr, size));
 }
 
 uptr ReservedAddressRange::MapOrDie(uptr fixed_addr, uptr size) {
   return reinterpret_cast<uptr>(MmapFixedOrDie(fixed_addr, size));
 }
 
 void ReservedAddressRange::Unmap(uptr addr, uptr size) {
-  void* addr_as_void = reinterpret_cast<void*>(addr);
-  uptr base_as_uptr = reinterpret_cast<uptr>(base_);
-  // Only unmap at the beginning or end of the range.
-  CHECK((addr_as_void == base_) || (addr + size == base_as_uptr + size_));
   CHECK_LE(size, size_);
+  if (addr == reinterpret_cast<uptr>(base_))
+    // If we unmap the whole range, just null out the base.
+    base_ = (size == size_) ? nullptr : reinterpret_cast<void*>(addr + size);
+  else
+    CHECK_EQ(addr + size, reinterpret_cast<uptr>(base_) + size_);
+  size_ -= size;
   UnmapOrDie(reinterpret_cast<void*>(addr), size);
-  if (addr_as_void == base_) {
-    base_ = reinterpret_cast<void*>(addr + size);
-  }
-  size_ = size_ - size;
 }
 
 void *MmapFixedNoAccess(uptr fixed_addr, uptr size, const char *name) {
   int fd = name ? GetNamedMappingFd(name, size) : -1;
   unsigned flags = MAP_PRIVATE | MAP_FIXED | MAP_NORESERVE;
   if (fd == -1) flags |= MAP_ANON;
 
   return (void *)internal_mmap((void *)fixed_addr, size, PROT_NONE, flags, fd,

lib/sanitizer_common/sanitizer_win.cc

uptr ReservedAddressRange::Map(uptr fixed_addr, uptr size) {
   return reinterpret_cast<uptr>(MmapFixedOrDieOnFatalError(fixed_addr, size));
 }
 
 uptr ReservedAddressRange::MapOrDie(uptr fixed_addr, uptr size) {
   return reinterpret_cast<uptr>(MmapFixedOrDie(fixed_addr, size));
 }
 
 void ReservedAddressRange::Unmap(uptr addr, uptr size) {
-  void* addr_as_void = reinterpret_cast<void*>(addr);
-  uptr base_as_uptr = reinterpret_cast<uptr>(base_);
   // Only unmap if it covers the entire range.
-  CHECK((addr == base_as_uptr) && (size == size_));
-  UnmapOrDie(addr_as_void, size);
-  if (addr_as_void == base_) {
-    base_ = reinterpret_cast<void*>(addr + size);
-  }
-  size_ = size_ - size;
+  CHECK((addr == reinterpret_cast<uptr>(base_)) && (size == size_));
+  // We unmap the whole range, just null out the base.
+  base_ = nullptr;
+  size_ = 0;
+  UnmapOrDie(reinterpret_cast<void*>(addr), size);
 }
 
 void *MmapFixedOrDieOnFatalError(uptr fixed_addr, uptr size) {
   void *p = VirtualAlloc((LPVOID)fixed_addr, size,
       MEM_COMMIT, PAGE_READWRITE);
   if (p == 0) {
     char mem_type[30];
     internal_snprintf(mem_type, sizeof(mem_type), "memory at address 0x%zx",
                       fixed_addr);
     return ReturnNullptrOnOOMOrDie(size, mem_type, "allocate");
   }
   return p;
 }
 
 void *MmapNoReserveOrDie(uptr size, const char *mem_type) {
   // FIXME: make this really NoReserve?
   return MmapOrDie(size, mem_type);
 }
 
 uptr ReservedAddressRange::Init(uptr size, const char *name, uptr fixed_addr) {
-  if (fixed_addr) {
-    base_ = MmapFixedNoAccess(fixed_addr, size, name);
-  } else {
-    base_ = MmapNoAccess(size);
-  }
+  base_ = fixed_addr ? MmapFixedNoAccess(fixed_addr, size) : MmapNoAccess(size);
   size_ = size;
   name_ = name;
   (void)os_handle_;  // unsupported
   return reinterpret_cast<uptr>(base_);
 }
 
 
 void *MmapFixedNoAccess(uptr fixed_addr, uptr size, const char *name) {