Warning: unsequenced modification and access to 'range'

[dmax621]

• What is the issue you have?
  Compiler warning about this line in version 3.2.0
  

  json/include/nlohmann/detail/input/lexer.hpp

  Line 197 in 9f3857e

  if (JSON_LIKELY(*range <= current and current <= *(++range)))

  
  That line is (almost) unchanged in the current version
  

  json/include/nlohmann/detail/input/lexer.hpp

  Line 206 in ffe0e3d

  if (JSON_HEDLEY_LIKELY(*range <= current and current <= *(++range)))

• What is the expected behavior?
  No warnings :)

• And what is the actual behavior instead?

warning: unsequenced modification and access to 'range' [-Wunsequenced]
    if (JSON_LIKELY(*range <= current and current <= *(++range)))
                     ^                                   ~
1 warning generated.

• Which compiler and operating system are you using? Is it a supported compiler?
  clang, probably version 3.4.0
  CentOS 6 (probably, or possibly 7).
• Did you use a released version of the library or the version from the develop branch?
  Version 3.2.0
• If you experience a compilation error: can you compile and run the unit tests?
  Probably not. The warning was reported by our CI system, which runs tests on whichever of many servers is available, and they have different configurations, in particular different compiler & OS versions. I can't tell which specific machine reported this, and I can't reproduce it on my development system (CentOS 7, clang 3.4.2).

The offending line actually looks perfectly OK to me -- and should be a sequence point -- although not for overloaded operator&&, but this instance has a bool on each side (unless there's some bizarre overloading of operator<= going on).

It could be related to clang not quite treating and the same as &&. Or it could be related to the JSON_LIKELY() macro, and misbehaviour of __builtin_expect(x) when x has side-effects.

Anyway, I have fixed this locally by explicitly fetching the range elements in a well-defined sequence (low first, then high) before using them in the comparison (which is then guaranteed to have no side-effects).

@@ class lexer
-        for (auto range = ranges.begin(); range != ranges.end(); ++range)
+        for (auto range = ranges.begin(); range != ranges.end(); )
         {
+            // Fetch range elements in a well-defined order
+            const int low = *range++;
+            const int high = *range++;
 
             get();
-            if (JSON_LIKELY(*range <= current and current <= *(++range)))
+            if (JSON_LIKELY(low <= current and current <= high))

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[nlohmann]

I cannot reproduce this warning.

[nlohmann]

@dmax621 Is the issue still visible to you?

[dmax621]

As I mentioned in my original report, I couldn't reproduce this on my development system in the first place. The original message appeared in a log from our CI system, and I wasn't entirely certain which node the tests had been run on, or what the specific node configuration was. I tend to regard it as most likely a compiler bug, possibly due to clang not quite treating and the same as &&, or some misbehaviour of __builtin_expect(x) when x has side-effects (there was a clang bug of this kind at some point). But I'd be inclined to change the source to the fix shown anyway, as a matter of preferring a policy of not using expressions with side-effects as arguments to macros, just in case the macro doesn't handle them correctly.

[nlohmann]

Thanks for the response!

[mprygh]

I am seeing this same warning.
It does not appear to matter, just seems to be a warning.
clang version 3.8.0-2ubuntu4 (tags/RELEASE_380/final)
From json-3.7.3.tar.gz
I am using the single_include.
Ubuntu16
CXXFLAGS=-g -std=c++14 -Wall -pedantic

./json.hpp:7336:71: warning: unsequenced modification and access to 'range'
[-Wunsequenced]
if (JSON_HEDLEY_LIKELY(*range <= current and current <= *(++range)))
~~~~~ ^
./json.hpp:1117:58: note: expanded from macro 'JSON_HEDLEY_LIKELY'

define JSON_HEDLEY_LIKELY(expr) __builtin_expect(!!(expr), 1)

                                                     ^~~~

[mprygh]

I upgraded to clang version 9, and the warning went away.

FYI: clang 3.8 is the default for Ubuntu 16.

[mprygh]

Also FYI:
clang version 9, with -std=c++17, spews a ton of errors.
gcc -std=c++17 works fine.

[nlohmann]

Can you please share the warnings/errors for Clang9 with C++17?

[mprygh]

(Sorry, was on vacation)

I could not reproduce the errors at first. Even on original clang 3.8.
But I have changed the code, in particular some includes.

Some interesting findings!
It turns out:, if you do a "using namespace std" BEFORE you include your library, it fails.
If I do the using after your library it is fine.
That is on clang 3.8. Error was same as above.

I still cannot reproduce the version 9, c++17 errors.
But I would guess it is something similar. Might be boost or some other thing.
Sorry I did not save those.