[Lldb-commits] [PATCH] D24610: LLDB Arm Watchpoints: Use single hardware watchpoint slot to watch multiple bytes where possible

[Pavel Labath via lldb-commits]

labath added inline comments.

================
Comment at: packages/Python/lldbsuite/test/functionalities/watchpoint/multi_watchpoint_slots/main.c:23
@@ +22,3 @@
+    {
+        printf("About to write byteArray[%d] ...\n", i); // About to write byteArray
+
----------------
zturner wrote:
> What's up with all the double spaced source code?  Is this intentional?
Indeed. The spaces seem superfluous.

================
Comment at: source/Plugins/Process/Linux/NativeRegisterContextLinux_arm.cpp:513-521
@@ -513,1 +512,11 @@
+
+  // Find out how many bytes we need to watch after 4-byte alignment boundary.
+  uint8_t watch_size = (addr & 0x03) + size;
+
+  // We cannot watch zero or more than 4 bytes after 4-byte alignment boundary.
+  if (size == 0 || watch_size > 4)
+    return LLDB_INVALID_INDEX32;
+
+  // Strip away last two bits of address for byte/half-word/word selection.
+  addr &= ~((lldb::addr_t)3);
 
----------------
zturner wrote:
> This block of code is a bit confusing to me.  Is this equivalent to:
> 
> ```
> lldb::addr_t start = llvm::alignDown(addr, 4);
> lldb::addr_t end = addr + size;
> if (start == end || (end-start)>4)
>   return LLDB_INVALID_INDEX32;
> ```
I am not sure this is much clearer, especially, as we will later need a separate varaible for `end-start` anyway.

+1 for `llvm::alignDown` though.

================
Comment at: source/Plugins/Process/Linux/NativeRegisterContextLinux_arm.cpp:552
@@ -559,1 +551,3 @@
 
+  uint8_t current_watch_size, new_watch_size;
+  // Calculate overall size width to be watched by current hardware watchpoint slot.
----------------
Looks much better. Any reason for not using `NextPowerOf2` ? Among other things, it is self-documenting, so you do not need the comment above that.

================
Comment at: source/Plugins/Process/Linux/NativeThreadLinux.cpp:202
@@ +201,3 @@
+  // Invalidate watchpoint index map to re-sync watchpoint registers.
+  m_watchpoint_index_map.clear();
+
----------------
If you add this, then the comment below becomes obsolete.

Seems like a pretty elegant solution to the incremental watchpoint update problem. I am wondering whether we need to do it on every resume though. I think it should be enough to do it when a watchpoint gets deleted (`NTL::RemoveWatchpoint`). Also, we should throw out the implementation of `NativeRegisterContextLinux_arm::ClearHardwareWatchpoint` -- it's no longer necessary, and it's not even correct anymore.


https://reviews.llvm.org/D24610