Experimantal dfsan mode "fast16labels=1"

[kcc]

Summary

dfsan mode "fast16labels=1".
In this mode the labels are treated as 16-bit bit masks.

[pcc]

LGTM

Maybe worth documenting which of the dfsan_* functions will work in this mode.

[kcc]

All other dfsan functions will actually continue working - most of them just won't be needed.

compiler-rt/trunk/lib/dfsan/dfsan.cc

if (label == kInitializingLabel) {
     Die();
   }
 }
 
 // Resolves the union of two unequal labels.  Nonequality is a precondition for
 // this function (the instrumentation pass inlines the equality test).
 extern "C" SANITIZER_INTERFACE_ATTRIBUTE
 dfsan_label __dfsan_union(dfsan_label l1, dfsan_label l2) {
+  if (flags().fast16labels)
+    return l1 | l2;
   DCHECK_NE(l1, l2);
 
   if (l1 == 0)
     return l2;
   if (l2 == 0)
     return l1;
 
   if (l1 > l2)

compiler-rt/trunk/lib/dfsan/dfsan_flags.inc

DFSAN_FLAG(
     bool, strict_data_dependencies, true,
     "Whether to propagate labels only when there is an obvious data dependency"
     "(e.g., when comparing strings, ignore the fact that the output of the"
     "comparison might be data-dependent on the content of the strings). This"
     "applies only to the custom functions defined in 'custom.c'.")
 DFSAN_FLAG(const char *, dump_labels_at_exit, "", "The path of the file where "
                                                   "to dump the labels when the "
                                                   "program terminates.")
+DFSAN_FLAG(bool, fast16labels, false,
+    "Enables experimental mode where DFSan supports only 16 power-of-2 labels "
+    "(1, 2, 4, 8, ... 32768) and the label union is computed as a bit-wise OR."
+)

compiler-rt/trunk/test/dfsan/fast16labels.c

+// RUN: %clang_dfsan %s -o %t && DFSAN_OPTIONS=fast16labels=1 %run %t
+//
+// Tests DFSAN_OPTIONS=fast16labels=1
+//
+#include <sanitizer/dfsan_interface.h>
+
+#include <assert.h>
+#include <stdio.h>
+
+int foo(int a, int b) {
+  return a + b;
+}
+
+int main() {
+  int a = 10;
+  int b = 20;
+  dfsan_set_label(8, &a, sizeof(a));
+  dfsan_set_label(512, &b, sizeof(b));
+  int c = foo(a, b);
+  printf("A: 0x%x\n", dfsan_get_label(a));
+  printf("B: 0x%x\n", dfsan_get_label(b));
+  dfsan_label l = dfsan_get_label(c);
+  printf("C: 0x%x\n", l);
+  assert(l == 520);  // OR of the other two labels.
+}