[dfsan] Make warn_unimplemented off by default

[stephan.yichao.zhao]

Summary

Because almost all internal use cases need to turn warn_unimplemented off.

[browneee]

This makes issues silent by default.

Instead, I think we should reduce noise by changing the tool to only report each unimplemented function once.

[stephan.yichao.zhao]

In D106594#2898097, @browneee wrote:

This makes issues silent by default.

This is a trade off with setting this environment variable or flag explicitly.
No use cases turn this flag on for the time being.

Instead, I think we should reduce noise by changing the tool to only report each unimplemented function once.

I am making another change to report only one warning.

[stephan.yichao.zhao]

https://github.com/llvm/llvm-project/commit/a806f933a2bf7db444c5687d8d4e07910150ed9c#diff-16afbead86e233f55f648488eae9df30bfd3467147380dfcdeb5a4df0a733905

compiler-rt/lib/dfsan/dfsan_flags.inc

 //===-- dfsan_flags.inc -----------------------------------------*- C++ -*-===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 //
 // DFSan runtime flags.
 //
 //===----------------------------------------------------------------------===//
 #ifndef DFSAN_FLAG
 # error "Define DFSAN_FLAG prior to including this file!"
 #endif
 
 // DFSAN_FLAG(Type, Name, DefaultValue, Description)
 // See COMMON_FLAG in sanitizer_flags.inc for more details.
 
-DFSAN_FLAG(bool, warn_unimplemented, true,
+DFSAN_FLAG(bool, warn_unimplemented, false,
            "Whether to warn on unimplemented functions.")
 DFSAN_FLAG(bool, warn_nonzero_labels, false,
            "Whether to warn on unimplemented functions.")
 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(

compiler-rt/test/dfsan/flags.c

-// RUN: %clang_dfsan %s -fsanitize-ignorelist=%S/Inputs/flags_abilist.txt -mllvm -dfsan-debug-nonzero-labels -o %t && %run %t 2>&1 | FileCheck %s
+// RUN: %clang_dfsan %s -fsanitize-ignorelist=%S/Inputs/flags_abilist.txt -mllvm -dfsan-debug-nonzero-labels -o %t && DFSAN_OPTIONS=warn_unimplemented=1 %run %t 2>&1 | FileCheck %s
 // RUN: %clang_dfsan %s -fsanitize-ignorelist=%S/Inputs/flags_abilist.txt -mllvm -dfsan-debug-nonzero-labels -o %t && DFSAN_OPTIONS=warn_unimplemented=0 %run %t 2>&1 | count 0
 // RUN: %clang_dfsan %s -fsanitize-ignorelist=%S/Inputs/flags_abilist.txt -mllvm -dfsan-debug-nonzero-labels -o %t && DFSAN_OPTIONS=warn_nonzero_labels=1 %run %t 2>&1 | FileCheck --check-prefix=CHECK-NONZERO %s
 //
 // REQUIRES: x86_64-target-arch
 
 // Tests that flags work correctly.
 
 #include <sanitizer/dfsan_interface.h>