[Lldb-commits] [lldb] [lldb] Add support for large watchpoints in lldb (PR #79962)

[Jonas Devlieghere via lldb-commits]

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@@ -0,0 +1,146 @@
+//===-- WatchpointAlgorithms.cpp ------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "lldb/Breakpoint/WatchpointAlgorithms.h"
+#include "lldb/Breakpoint/WatchpointResource.h"
+#include "lldb/Target/Process.h"
+#include "lldb/Utility/ArchSpec.h"
+
+#include <utility>
+#include <vector>
+
+using namespace lldb;
+using namespace lldb_private;
+
+std::vector<WatchpointResourceSP>
+WatchpointAlgorithms::AtomizeWatchpointRequest(
+    addr_t addr, size_t size, bool read, bool write,
+    WatchpointHardwareFeature supported_features, ArchSpec &arch) {
+
+  std::vector<std::pair<addr_t, size_t>> entries;
+
+  if (supported_features &
+      WatchpointHardwareFeature::eWatchpointHardwareArmMASK) {
+    entries =
+        PowerOf2Watchpoints(addr, size,
+                            /* min_byte_size */ 1,
+                            /* max_byte_size */ INT32_MAX,
+                            /* address_byte_size */ arch.GetAddressByteSize());
+  } else {
+    // As a fallback, assume we can watch any power-of-2
+    // number of bytes up through the size of an address in the target.
+    entries =
+        PowerOf2Watchpoints(addr, size,
+                            /* min_byte_size */ 1,
+                            /* max_byte_size */ arch.GetAddressByteSize(),
+                            /* address_byte_size */ arch.GetAddressByteSize());
+  }
+
+  std::vector<WatchpointResourceSP> resources;
+  for (std::pair<addr_t, size_t> &ent : entries) {
+    addr_t addr = std::get<0>(ent);
+    size_t size = std::get<1>(ent);
+    WatchpointResourceSP wp_res_sp =
+        std::make_shared<WatchpointResource>(addr, size, read, write);
+    resources.push_back(wp_res_sp);
+  }
+
+  return resources;
+}
+
+/// Convert a user's watchpoint request (\a user_addr and \a user_size)
+/// into hardware watchpoints, for a target that can watch a power-of-2
+/// region of memory (1, 2, 4, 8, etc), aligned to that same power-of-2
+/// memory address.
+///
+/// If a user asks to watch 4 bytes at address 0x1002 (0x1002-0x1005
+/// inclusive) we can implement this with two 2-byte watchpoints
+/// (0x1002 and 0x1004) or with an 8-byte watchpoint at 0x1000.
+/// A 4-byte watchpoint at 0x1002 would not be properly 4 byte aligned.
+///
+/// If a user asks to watch 16 bytes at 0x1000, and this target supports
+/// 8-byte watchpoints, we can implement this with two 8-byte watchpoints
+/// at 0x1000 and 0x1008.
+
+std::vector<std::pair<addr_t, size_t>>
+WatchpointAlgorithms::PowerOf2Watchpoints(addr_t user_addr, size_t user_size,
+                                          size_t min_byte_size,
+                                          size_t max_byte_size,
+                                          uint32_t address_byte_size) {
+
+  // Can't watch zero bytes.
+  if (user_size == 0)
+    return {};
+
+  // The aligned watch region will be less than/equal to the size of
+  // an address in this target.
+  const int address_bit_size = address_byte_size * 8;
+
+  size_t aligned_size = std::max(user_size, min_byte_size);
+  /// Round up \a user_size to the next power-of-2 size
+  /// user_size == 8   -> aligned_size == 8
+  /// user_size == 9   -> aligned_size == 16
+  /// user_size == 15  -> aligned_size == 16
+  /// user_size == 192 -> aligned_size == 256
+  /// Could be `std::bit_ceil(aligned_size)` when we build with C++20?
+
+  aligned_size = 1ULL << (address_bit_size - __builtin_clzll(aligned_size - 1));
+
+  addr_t aligned_start = user_addr & ~(aligned_size - 1);
+
+  // Does this power-of-2 memory range, aligned to power-of-2 that the
+  // hardware can watch, completely cover the requested region.
+  if (aligned_size <= max_byte_size &&
+      aligned_start + aligned_size >= user_addr + user_size)
+    return {{aligned_start, aligned_size}};
+
+  // If the maximum region we can watch is larger than the aligned
+  // size, try increasing the region size by one power of 2 and see
+  // if aligning to that amount can cover the requested region.
+  //
+  // Increasing the aligned_size repeatedly instead of splitting the
+  // watchpoint can result in us watching large regions of memory
+  // unintentionally when we could use small two watchpoints.  e.g.
+  //    user_addr 0x3ff8 user_size 32
+  // can be watched with four 8-byte watchpoints or if it's done with one
+  // MASK watchpoint, it would need to be a 32KB watchpoint (a 16KB
+  // watchpoint at 0x0 only covers 0x0000-0x4000).  A user request
+  // at the end of a power-of-2 region can lead to these undesirably
+  // large watchpoints and many false positive hits to ignore.
+  if (max_byte_size >= (aligned_size << 1)) {
+    aligned_size <<= 1;
+    aligned_start = user_addr & ~(aligned_size - 1);
+    if (aligned_size <= max_byte_size &&
+        aligned_start + aligned_size >= user_addr + user_size)
+      return {{aligned_start, aligned_size}};
+
+    // Go back to our original aligned size, to try the multiple
+    // watchpoint approach.
+    aligned_size >>= 1;
+  }
+
+  // We need to split the user's watchpoint into two or more watchpoints
+  // that can be monitored by hardware, because of alignment and/or size
+  // reasons.
+  aligned_size = std::min(aligned_size, max_byte_size);
+  aligned_start = user_addr & ~(aligned_size - 1);
+
+  std::vector<std::pair<addr_t, size_t>> result;
+  addr_t current_address = aligned_start;
+  const addr_t user_end_address = user_addr + user_size;
+  while (current_address + aligned_size < user_end_address) {
+    result.push_back({current_address, aligned_size});
+    current_address += aligned_size;
+  }
+
+  if (current_address < user_end_address) {
+    result.push_back({current_address, aligned_size});
+  }
----------------
JDevlieghere wrote:

Nit: no braces around single-line `if`s.

https://github.com/llvm/llvm-project/pull/79962