[clang-tools-extra] [compiler-rt] [lldb] [llvm] [Memprof] Adds the option to collect AccessCountHistograms for memprof. (PR #94264)

[Matthew Weingarten via cfe-commits]

================
@@ -96,19 +102,63 @@ llvm::SmallVector<SegmentEntry> readSegmentEntries(const 
char *Ptr) {
 }
 
 llvm::SmallVector<std::pair<uint64_t, MemInfoBlock>>
-readMemInfoBlocks(const char *Ptr) {
+readMemInfoBlocksV3(const char *Ptr) {
   using namespace support;
 
   const uint64_t NumItemsToRead =
-      endian::readNext<uint64_t, llvm::endianness::little>(Ptr);
+      endian::readNext<uint64_t, llvm::endianness::little, unaligned>(Ptr);
+
   llvm::SmallVector<std::pair<uint64_t, MemInfoBlock>> Items;
   for (uint64_t I = 0; I < NumItemsToRead; I++) {
     const uint64_t Id =
-        endian::readNext<uint64_t, llvm::endianness::little>(Ptr);
-    const MemInfoBlock MIB = *reinterpret_cast<const MemInfoBlock *>(Ptr);
+        endian::readNext<uint64_t, llvm::endianness::little, unaligned>(Ptr);
+
+    // We cheat a bit here and remove the const from cast to set the
+    // Histogram Pointer to newly allocated buffer. We also cheat, since V3 and
+    // V4 do not have the same fields. V3 is missing AccessHistogramSize and
+    // AccessHistogram. This means we read "dirty" data in here, but it should
+    // not segfault, since there will be callstack data placed after this in 
the
+    // binary format.
+    MemInfoBlock MIB = *reinterpret_cast<const MemInfoBlock *>(Ptr);
+    // Overwrite dirty data.
----------------
mattweingarten wrote:

My thought process is that the MemInfoBlock MIB is a allocated on the stack, 
since its a local variable and not a pointer. Meaning the reinterpret_cast 
copies the information from the data-buffer  into the stack variable. Correct 
me if I am wrong

https://github.com/llvm/llvm-project/pull/94264
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