[Lldb-commits] [lldb] [lldb][debugserver] Expedite the stopped frame's stack memory in jThreadsInfo (PR #212706)

[Yao Qi via lldb-commits]

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@@ -2733,6 +2733,76 @@ static void ReadStackMemory(nub_process_t pid, nub_thread_t tid,
   }
 }
 
+// The size of each per-side stack window we expedite.  512 is sized to cover
+// the common case (locals and params sit within a few hundred bytes) while
+// bounding the per-frame cost to 1K bytes regardless of frame size.
+static const nub_size_t k_expedite_stack_window = 512;
+
+// A single contiguous chunk of expedited memory.
+struct ExpeditedMemory {
+  nub_addr_t addr;
+  std::vector<uint8_t> bytes;
+};
+
+// Read the innermost frame's stack memory so that examining its local variables
+// at a public stop is served from the expedited cache instead of generating one
+// memory-read packet.
+//
+// We produce either:
+//
+//   - one chunk [$sp, $fp) for a small frame ($fp - $sp <=
+//   2*k_expedite_stack_window), which covers the whole frame with no gap, or
+//
+//   - two chunks [$sp, $sp + k_expedite_stack_window) and [$fp -
+//   k_expedite_stack_window, $fp) for a large frame, covering the params near
+//   $sp and the locals near $fp while leaving the (rarely-interesting) middle
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qiyao wrote:

I pushed in a new commit to change the expedited windows slightly.
- If `fp` is valid, expedite two windows:  `[fp + 16, fp + 16 + k_expedite_stack_arg_size)`, skip chunk `[fp, fp+16)` which is already expedited in backtrace chain.  This windows covers function args, which was the oversight in this PR.  The second window is `[max(sp, sp - (k_expedite_stack_window - k_expedite_stack_arg_size)), fp)`.
- If `fp` isn't valid, the window is `[sp, sp + k_expedite_stack_window)`.

`k_expedite_stack_window` is still 1k, not changed.  The new constant `k_expedite_stack_arg_size` is 160, which is got from analyzing several program's dwarf info, and 160 is the optimal value.

https://github.com/llvm/llvm-project/pull/212706