Mercury Bugs - mercury
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IDProjectCategoryView StatusDate SubmittedLast Update
0000467mercuryFeature Requestpublic2018-08-18 16:152018-09-07 10:23
Reporterwangp 
Assigned To 
PrioritynormalSeverityminorReproducibilityalways
StatusnewResolutionopen 
PlatformOSOS Version
Product Version 
Target VersionFixed in Version 
Summary0000467: compiling with -Wuninitialized or -Wmaybe-uninitialized
DescriptionThe high-level C backend generates code like this:

    if (succeeded)
      U_4 = UPrime_5;
    else
    {
      mercury__require__error_1_p_0((MR_String) "uint.det_from_int: cannot convert int to uint");
    }
    return U_4;

gcc does not know that the function call will not return so it MAY warn that U_4 is never initialised. mgnuc just happens to suppress the warnings on x86 and x86-64 for an unrelated reason:

case "$FULLARCH" in
    i*86-*|x86_64*)
        # The use of stack_pointer in the ASM_JUMP macro defined in
        # runtime/mercury_goto.h causes lots of warnings about using possibly
        # uninitialized variables; there's no easy way to suppress them except
        # by disabling the warning.
        case "$COMPILER" in
            gcc)
                CHECK_OPTS="$CHECK_OPTS -Wno-uninitialized"
                ;;
        esac
        ;;

Ideally we would ONLY disable uninitialized variable warnings in the low-level C grades where it is unavoidable.

In high-level C grades, I think we should declare the C function corresponding to an `erroneous' Mercury procedure as not returning, e.g. using the macro MR_NO_RETURN or similar. MR_NO_RETURN has implementations for gcc/clang/msvc. C11 also adds _Noreturn.

Alternatively, we could add assert(0) after calls to functions that will not return, but presumably that would increase code size.
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Notes
(0001007)
juliensf   
2018-08-19 15:21   
No objections from me to using MR_NO_RETURN on erroneous procedures. (We probably can't rely on C11 just yet, although MR_NO_RETURN could be updated to use _Noreturn if the C implementation is C11.)
(0001010)
zs   
2018-09-07 10:23   
I started work on adding a new function flag that will tell mlds_to_c_func.m
whether it should emit MR_NO_RETURN for a function being declared or defined.

Issue History
Date ModifiedUsernameFieldChange
2018-08-18 16:15wangpNew Issue
2018-08-19 15:21juliensfNote Added: 0001007
2018-09-07 10:23zsNote Added: 0001010