Fix the condition for watchpoint matching on arm64

The language from the ARM manual previously quoted in the comment in
fact only pertains to Memory Copy and Memory Set instructions. According
to the section describing the behavior for "other instructions", the
recorded address can be > the watchpoint address. Therefore, remove the
check that the watchpoint address >= the recorded address, and update
the comment for accuracy.

Fixes #3736

src/AddressSpace.cc

@@ -1295,20 +1295,18 @@ bool AddressSpace::notify_watchpoint_fired(uintptr_t debug_status,
          watchpoint_triggered(debug_status,
                               it.second.debug_regs_for_exec_read);
     } else {
-      // The reported address may not match our watchpoint exactly.
-      // The ARM manual says:
-      //   The address recorded is within an address range of the size defined by the
-      //   DCZID_EL0.BS field. The start of the range is aligned to the size defined
-      //   by the DCZID_EL0.BS field and its end is not greater than the address that
-      //   triggered the watchpoint.
-      // So we construct a range spanning the whole block, then test that the range
-      // intersects a watchpoint range *and* that hit_addr is not past the first byte
-      // of the watched region.
+      // The reported address may not match our watchpoint exactly. The ARM
+      // manual specifies a number of conditions that apply to the address for
+      // various types of instructions (see D2.10.5, "Determining the memory
+      // location that caused a Watchpoint exception"), but at a minimum it is
+      // required that the reported address is in the same block as one of the
+      // addresses monitored by the watchpoint, where the block size is defined
+      // by DCZID_EL0.BS. So we construct a range spanning the whole block, then
+      // test that the range intersects a watchpoint range.
       auto block_size = dczid_el0_block_size();
       auto slop = hit_addr.as_int() % block_size;
       auto hit_range = MemoryRange(hit_addr - slop, block_size);
-      watchpoint_in_range = it.first.intersects(hit_range) &&
-        it.first.start() >= hit_addr;
+      watchpoint_in_range = it.first.intersects(hit_range);
     }
     if (write_triggered || read_triggered || exec_triggered || watchpoint_in_range) {
       it.second.changed = true;

src/test/watchpoint_unaligned.c

@@ -20,11 +20,7 @@ int main(void) {
   uintptr_t aligned_addr = ((uintptr_t)m | 0xff) + 1;
   test(aligned_addr - 1, aligned_addr - 1);
   test(aligned_addr + 16, aligned_addr + 15);
-
-  /* FIXME: Currently fails on arm64. */
-  if (0) {
-    test(aligned_addr + 15, aligned_addr + 16);
-  }
+  test(aligned_addr + 15, aligned_addr + 16);
 
   atomic_puts("EXIT-SUCCESS");
   return 0;

src/test/watchpoint_unaligned.py

@@ -11,6 +11,7 @@ def test():
         expect_gdb(f'watchpoint {wp}')
         send_gdb(f'delete {wp}')
 
+test()
 test()
 test()
 ok()