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diff --git a/lib/fuzzer/dataflow/DataFlow.cpp b/lib/fuzzer/dataflow/DataFlow.cpp
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+++ b/lib/fuzzer/dataflow/DataFlow.cpp
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+/*===- DataFlow.cpp - a standalone DataFlow tracer                  -------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+// An experimental data-flow tracer for fuzz targets.
+// It is based on DFSan and SanitizerCoverage.
+// https://clang.llvm.org/docs/DataFlowSanitizer.html
+// https://clang.llvm.org/docs/SanitizerCoverage.html#tracing-data-flow
+//
+// It executes the fuzz target on the given input while monitoring the
+// data flow for every instrumented comparison instruction.
+//
+// The output shows which functions depend on which bytes of the input.
+//
+// Build:
+//   1. Compile this file with -fsanitize=dataflow
+//   2. Build the fuzz target with -g -fsanitize=dataflow
+//       -fsanitize-coverage=trace-pc-guard,pc-table,func,trace-cmp
+//   3. Link those together with -fsanitize=dataflow
+//
+//  -fsanitize-coverage=trace-cmp inserts callbacks around every comparison
+//  instruction, DFSan modifies the calls to pass the data flow labels.
+//  The callbacks update the data flow label for the current function.
+//  See e.g. __dfsw___sanitizer_cov_trace_cmp1 below.
+//
+//  -fsanitize-coverage=trace-pc-guard,pc-table,func instruments function
+//  entries so that the comparison callback knows that current function.
+//
+//
+// Run:
+//   # Collect data flow for INPUT_FILE, write to OUTPUT_FILE (default: stdout)
+//   ./a.out INPUT_FILE [OUTPUT_FILE]
+//
+//   # Print all instrumented functions. llvm-symbolizer must be present in PATH
+//   ./a.out
+//
+// Example output:
+// ===============
+//  F0 11111111111111
+//  F1 10000000000000
+//  ===============
+// "FN xxxxxxxxxx": tells what bytes of the input does the function N depend on.
+//    The byte string is LEN+1 bytes. The last byte is set if the function
+//    depends on the input length.
+//===----------------------------------------------------------------------===*/
+
+#include <assert.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdint.h>
+#include <string.h>
+
+#include <execinfo.h>  // backtrace_symbols_fd
+
+#include <sanitizer/dfsan_interface.h>
+
+extern "C" {
+extern int LLVMFuzzerTestOneInput(const unsigned char *Data, size_t Size);
+__attribute__((weak)) extern int LLVMFuzzerInitialize(int *argc, char ***argv);
+} // extern "C"
+
+static size_t InputLen;
+static size_t NumFuncs;
+static const uintptr_t *FuncsBeg;
+static __thread size_t CurrentFunc;
+static dfsan_label *FuncLabels;  // Array of NumFuncs elements.
+static char *PrintableStringForLabel;  // InputLen + 2 bytes.
+static bool LabelSeen[1 << 8 * sizeof(dfsan_label)];
+
+// Prints all instrumented functions.
+static int PrintFunctions() {
+  // We don't have the symbolizer integrated with dfsan yet.
+  // So use backtrace_symbols_fd and pipe it through llvm-symbolizer.
+  // TODO(kcc): this is pretty ugly and may break in lots of ways.
+  //      We'll need to make a proper in-process symbolizer work with DFSan.
+  FILE *Pipe = popen("sed 's/(+/ /g; s/).*//g' "
+                     "| llvm-symbolizer "
+                     "| grep 'dfs\\$' "
+                     "| sed 's/dfs\\$//g'", "w");
+  for (size_t I = 0; I < NumFuncs; I++) {
+    uintptr_t PC = FuncsBeg[I * 2];
+    void *const Buf[1] = {(void*)PC};
+    backtrace_symbols_fd(Buf, 1, fileno(Pipe));
+  }
+  pclose(Pipe);
+  return 0;
+}
+
+extern "C"
+void SetBytesForLabel(dfsan_label L, char *Bytes) {
+  if (LabelSeen[L])
+    return;
+  LabelSeen[L] = true;
+  assert(L);
+  if (L <= InputLen + 1) {
+    Bytes[L - 1] = '1';
+  } else {
+    auto *DLI = dfsan_get_label_info(L);
+    SetBytesForLabel(DLI->l1, Bytes);
+    SetBytesForLabel(DLI->l2, Bytes);
+  }
+}
+
+static char *GetPrintableStringForLabel(dfsan_label L) {
+  memset(PrintableStringForLabel, '0', InputLen + 1);
+  PrintableStringForLabel[InputLen + 1] = 0;
+  memset(LabelSeen, 0, sizeof(LabelSeen));
+  SetBytesForLabel(L, PrintableStringForLabel);
+  return PrintableStringForLabel;
+}
+
+static void PrintDataFlow(FILE *Out) {
+  for (size_t I = 0; I < NumFuncs; I++)
+    if (FuncLabels[I])
+      fprintf(Out, "F%zd %s\n", I, GetPrintableStringForLabel(FuncLabels[I]));
+}
+
+int main(int argc, char **argv) {
+  if (LLVMFuzzerInitialize)
+    LLVMFuzzerInitialize(&argc, &argv);
+  if (argc == 1)
+    return PrintFunctions();
+  assert(argc == 4 || argc == 5);
+  size_t Beg = atoi(argv[1]);
+  size_t End = atoi(argv[2]);
+  assert(Beg < End);
+
+  const char *Input = argv[3];
+  fprintf(stderr, "INFO: reading '%s'\n", Input);
+  FILE *In = fopen(Input, "r");
+  assert(In);
+  fseek(In, 0, SEEK_END);
+  InputLen = ftell(In);
+  fseek(In, 0, SEEK_SET);
+  unsigned char *Buf = (unsigned char*)malloc(InputLen);
+  size_t NumBytesRead = fread(Buf, 1, InputLen, In);
+  assert(NumBytesRead == InputLen);
+  PrintableStringForLabel = (char*)malloc(InputLen + 2);
+  fclose(In);
+
+  fprintf(stderr, "INFO: running '%s'\n", Input);
+  for (size_t I = 1; I <= InputLen; I++) {
+    dfsan_label L = dfsan_create_label("", nullptr);
+    assert(L == I);
+    size_t Idx = I - 1;
+    if (Idx >= Beg && Idx < End)
+      dfsan_set_label(L, Buf + Idx, 1);
+  }
+  dfsan_label SizeL = dfsan_create_label("", nullptr);
+  assert(SizeL == InputLen + 1);
+  dfsan_set_label(SizeL, &InputLen, sizeof(InputLen));
+
+  LLVMFuzzerTestOneInput(Buf, InputLen);
+  free(Buf);
+
+  bool OutIsStdout = argc == 4;
+  fprintf(stderr, "INFO: writing dataflow to %s\n",
+          OutIsStdout ? "<stdout>" : argv[4]);
+  FILE *Out = OutIsStdout ? stdout : fopen(argv[4], "w");
+  PrintDataFlow(Out);
+  if (!OutIsStdout) fclose(Out);
+}
+
+extern "C" {
+
+void __sanitizer_cov_trace_pc_guard_init(uint32_t *start,
+                                         uint32_t *stop) {
+  assert(NumFuncs == 0 && "This tool does not support DSOs");
+  assert(start < stop && "The code is not instrumented for coverage");
+  if (start == stop || *start) return;  // Initialize only once.
+  for (uint32_t *x = start; x < stop; x++)
+    *x = ++NumFuncs;  // The first index is 1.
+  FuncLabels = (dfsan_label*)calloc(NumFuncs, sizeof(dfsan_label));
+  fprintf(stderr, "INFO: %zd instrumented function(s) observed\n", NumFuncs);
+}
+
+void __sanitizer_cov_pcs_init(const uintptr_t *pcs_beg,
+                              const uintptr_t *pcs_end) {
+  assert(NumFuncs == (pcs_end - pcs_beg) / 2);
+  FuncsBeg = pcs_beg;
+}
+
+void __sanitizer_cov_trace_pc_indir(uint64_t x){}  // unused.
+
+void __sanitizer_cov_trace_pc_guard(uint32_t *guard){
+  uint32_t FuncNum = *guard - 1;  // Guards start from 1.
+  assert(FuncNum < NumFuncs);
+  CurrentFunc = FuncNum;
+}
+
+void __dfsw___sanitizer_cov_trace_switch(uint64_t Val, uint64_t *Cases,
+                                         dfsan_label L1, dfsan_label UnusedL) {
+  assert(CurrentFunc < NumFuncs);
+  FuncLabels[CurrentFunc] = dfsan_union(FuncLabels[CurrentFunc], L1);
+}
+
+#define HOOK(Name, Type)                                                       \
+  void Name(Type Arg1, Type Arg2, dfsan_label L1, dfsan_label L2) {            \
+    assert(CurrentFunc < NumFuncs);                                            \
+    FuncLabels[CurrentFunc] =                                                  \
+        dfsan_union(FuncLabels[CurrentFunc], dfsan_union(L1, L2));             \
+  }
+
+HOOK(__dfsw___sanitizer_cov_trace_const_cmp1, uint8_t)
+HOOK(__dfsw___sanitizer_cov_trace_const_cmp2, uint16_t)
+HOOK(__dfsw___sanitizer_cov_trace_const_cmp4, uint32_t)
+HOOK(__dfsw___sanitizer_cov_trace_const_cmp8, uint64_t)
+HOOK(__dfsw___sanitizer_cov_trace_cmp1, uint8_t)
+HOOK(__dfsw___sanitizer_cov_trace_cmp2, uint16_t)
+HOOK(__dfsw___sanitizer_cov_trace_cmp4, uint32_t)
+HOOK(__dfsw___sanitizer_cov_trace_cmp8, uint64_t)
+
+} // extern "C"