[compiler-rt] add check-cmp flag for nsan

compiler-rt/lib/nsan/nsan.cpp

@@ -25,7 +25,7 @@
 //          on the runtime configuration. The middle part indicates the type of
 //          the application value, the suffix (f,d,l) indicates the type of the
 //          shadow, and depends on the instrumentation configuration.
-//        * __nsan_fcmp_fail_* emits a warning for an fcmp instruction whose
+//        * __nsan_fcmp_fail_* emits a warning for a fcmp instruction whose
 //          corresponding shadow fcmp result differs.
 //
 //===----------------------------------------------------------------------===//
@@ -682,7 +682,7 @@ void fCmpFailFT(const FT Lhs, const FT Rhs, ShadowFT LhsShadow,
   if (flags().enable_warning_stats)
     nsan_stats->AddWarning(CheckTypeT::kFcmp, pc, bp, 0.0);
 
-  if (flags().disable_warnings)
+  if (flags().disable_warnings || !flags().check_cmp)
     return;
 
   // FIXME: ideally we would print the shadow value as FP128. Right now because

compiler-rt/lib/nsan/nsan_flags.inc

@@ -49,4 +49,7 @@ NSAN_FLAG(bool, enable_loadtracking_stats, false,
 NSAN_FLAG(bool, poison_in_free, true, "")
 NSAN_FLAG(bool, print_stats_on_exit, false, "If true, print stats on exit.")
 NSAN_FLAG(bool, check_nan, false,
-          "If true, check the floating-point number is nan")
+          "If true, check the floating-point number is nan")
+NSAN_FLAG(bool, check_cmp, true,
+          "If true, emit a warning for a fcmp instruction whose "
+          "corresponding shadow fcmp result differs.")

compiler-rt/test/nsan/fcmp.cpp

@@ -0,0 +1,28 @@
+// RUN: %clangxx_nsan -O2 -g %s -o %t
+// RUN: env NSAN_OPTIONS=check_cmp=true,halt_on_error=0 %run %t 2>&1 | FileCheck %s -check-prefix=CMP_ENABLE
+// RUN: env NSAN_OPTIONS=check_cmp=false,halt_on_error=0 %run %t 2>&1 | FileCheck %s -check-prefix=CMP_DISABLE
+
+#include <cmath>
+#include <cstdio>
+
+// 0.6/0.2 is slightly below 3, so the comparison will fail after a certain
+// threshold that depends on the precision of the computation.
+__attribute__((noinline))  // To check call stack reporting.
+bool DoCmp(double a, double b, double c, double threshold) {
+  return c - a / b < threshold;
+  // CMP_ENABLE: WARNING: NumericalStabilitySanitizer: floating-point comparison results depend on precision
+  // CMP_ENABLE: double    {{ *}}precision dec (native): {{.*}}<{{.*}}
+  // CMP_ENABLE: __float128{{ *}}precision dec (shadow): {{.*}}<{{.*}}
+  // CMP_ENABLE: {{#0 .*in DoCmp}}
+}
+
+int main() {
+  double threshold = 1.0;
+  for (int i = 0; i < 60; ++i) {
+    threshold /= 2;
+    // CMP_DISABLE: value at threshold {{.*}}
+    printf("value at threshold %.20f: %i\n", threshold,
+           DoCmp(0.6, 0.2, 3.0, threshold));
+  }
+  return 0;
+}