Silence clang warning -Wtautological-pointer-compare

[beutlich]

See this CI run compared to the previous one.

Close #2109.

[HansOlsson]

I'm investigating this would cause null-pointer warnings in our tool-chains - and if so if that can be avoided.

[HansOlsson]

Investigating more I don't think that this is the correct solution, and would either just disable the check - or see if it can be weakened in some other way.

The reason is that as far as I understand:

• These attributes/SAL only generate warnings, not hard errors, if used incorrectly. Thus removing the "tautological" check just removes defence in depth, and if someone calls with a null-pointer we have changed the code from doing nothing to crashing.
• These attributes/SAL are optional - so we cannot be sure that users of these will even have warnings enabled.
• Not all compilers support these attributes/SAL.

Note that Visual Studio C++ (2019 at least) has considered this - and I would say that most of these functions are part of the trust boundary:

"If a function appears at a trust boundary, we recommend that you use the Pre_defensive annotation. The "defensive" modifier modifies certain annotations to indicate that, at the point of call, the interface should be checked strictly, but in the implementation body it should assume that incorrect parameters might be passed. In that case, In Pre_defensive is preferred at a trust boundary to indicate that although a caller will get an error if it attempts to pass NULL, the function body will be analyzed as if the parameter might be NULL, and any attempts to de-reference the pointer without first checking it for NULL will be flagged."

[beutlich]

OK, I updated the code to only silence the clang warning while not changing any nullptr checks. See here for the CI run.

[HansOlsson]

Looks good now.

[HansOlsson]

Hmm...

Actually it seems that disabling "-Wtautological-pointer-compare" only disables the warnings, but clang (and gcc) still optimizes away the null-pointer tests, which is kind of dangerous.
It is necessary to add "-fno-delete-null-pointer-checks" to get reliable results.

[beutlich]

I added -fno-delete-null-pointer-checks here such that this optimization is disabled whenever new binaries are built.

[sjoelund]

I added a commit with the compiler flag added for the autotools project (only added if the flag exists for the compiler)