10.10 Certain Changes We Don't Want to Make

This section lists changes that people frequently request, but which we do not make because we think GCC is better without them.

• Checking the number and type of arguments to a function which has an old-fashioned definition and no prototype.

  Such a feature would work only occasionally—only for calls that appear in the same file as the called function, following the definition. The only way to check all calls reliably is to add a prototype for the function. But adding a prototype eliminates the motivation for this feature. So the feature is not worthwhile.

• Warning about using an expression whose type is signed as a shift count.

  Shift count operands are probably signed more often than unsigned. Warning about this would cause far more annoyance than good.

• Warning about assigning a signed value to an unsigned variable.

  Such assignments must be very common; warning about them would cause more annoyance than good.

• Warning when a non-void function value is ignored.

  C contains many standard functions that return a value that most programs choose to ignore. One obvious example is printf. Warning about this practice only leads the defensive programmer to clutter programs with dozens of casts to void. Such casts are required so frequently that they become visual noise. Writing those casts becomes so automatic that they no longer convey useful information about the intentions of the programmer. For functions where the return value should never be ignored, use the warn_unused_result function attribute (see section Declaring Attributes of Functions).

• Making ‘-fshort-enums’ the default.

  This would cause storage layout to be incompatible with most other C compilers. And it doesn't seem very important, given that you can get the same result in other ways. The case where it matters most is when the enumeration-valued object is inside a structure, and in that case you can specify a field width explicitly.

• Making bit-fields unsigned by default on particular machines where “the ABI standard” says to do so.

  The ISO C standard leaves it up to the implementation whether a bit-field declared plain int is signed or not. This in effect creates two alternative dialects of C.

  The GNU C compiler supports both dialects; you can specify the signed dialect with ‘-fsigned-bitfields’ and the unsigned dialect with ‘-funsigned-bitfields’. However, this leaves open the question of which dialect to use by default.

  Currently, the preferred dialect makes plain bit-fields signed, because this is simplest. Since int is the same as signed int in every other context, it is cleanest for them to be the same in bit-fields as well.

  Some computer manufacturers have published Application Binary Interface standards which specify that plain bit-fields should be unsigned. It is a mistake, however, to say anything about this issue in an ABI. This is because the handling of plain bit-fields distinguishes two dialects of C. Both dialects are meaningful on every type of machine. Whether a particular object file was compiled using signed bit-fields or unsigned is of no concern to other object files, even if they access the same bit-fields in the same data structures.

  A given program is written in one or the other of these two dialects. The program stands a chance to work on most any machine if it is compiled with the proper dialect. It is unlikely to work at all if compiled with the wrong dialect.

  Many users appreciate the GNU C compiler because it provides an environment that is uniform across machines. These users would be inconvenienced if the compiler treated plain bit-fields differently on certain machines.

  Occasionally users write programs intended only for a particular machine type. On these occasions, the users would benefit if the GNU C compiler were to support by default the same dialect as the other compilers on that machine. But such applications are rare. And users writing a program to run on more than one type of machine cannot possibly benefit from this kind of compatibility.

  This is why GCC does and will treat plain bit-fields in the same fashion on all types of machines (by default).

  There are some arguments for making bit-fields unsigned by default on all machines. If, for example, this becomes a universal de facto standard, it would make sense for GCC to go along with it. This is something to be considered in the future.

  (Of course, users strongly concerned about portability should indicate explicitly in each bit-field whether it is signed or not. In this way, they write programs which have the same meaning in both C dialects.)

• Undefining __STDC__ when ‘-ansi’ is not used.

  Currently, GCC defines __STDC__ unconditionally. This provides good results in practice.

  Programmers normally use conditionals on __STDC__ to ask whether it is safe to use certain features of ISO C, such as function prototypes or ISO token concatenation. Since plain gcc supports all the features of ISO C, the correct answer to these questions is “yes”.

  Some users try to use __STDC__ to check for the availability of certain library facilities. This is actually incorrect usage in an ISO C program, because the ISO C standard says that a conforming freestanding implementation should define __STDC__ even though it does not have the library facilities. ‘gcc -ansi -pedantic’ is a conforming freestanding implementation, and it is therefore required to define __STDC__, even though it does not come with an ISO C library.

  Sometimes people say that defining __STDC__ in a compiler that does not completely conform to the ISO C standard somehow violates the standard. This is illogical. The standard is a standard for compilers that claim to support ISO C, such as ‘gcc -ansi’—not for other compilers such as plain gcc. Whatever the ISO C standard says is relevant to the design of plain gcc without ‘-ansi’ only for pragmatic reasons, not as a requirement.

  GCC normally defines __STDC__ to be 1, and in addition defines __STRICT_ANSI__ if you specify the ‘-ansi’ option, or a ‘-std’ option for strict conformance to some version of ISO C. On some hosts, system include files use a different convention, where __STDC__ is normally 0, but is 1 if the user specifies strict conformance to the C Standard. GCC follows the host convention when processing system include files, but when processing user files it follows the usual GNU C convention.

• Undefining __STDC__ in C++.

  Programs written to compile with C++-to-C translators get the value of __STDC__ that goes with the C compiler that is subsequently used. These programs must test __STDC__ to determine what kind of C preprocessor that compiler uses: whether they should concatenate tokens in the ISO C fashion or in the traditional fashion.

  These programs work properly with GNU C++ if __STDC__ is defined. They would not work otherwise.

  In addition, many header files are written to provide prototypes in ISO C but not in traditional C. Many of these header files can work without change in C++ provided __STDC__ is defined. If __STDC__ is not defined, they will all fail, and will all need to be changed to test explicitly for C++ as well.

• Deleting “empty” loops.

  Historically, GCC has not deleted “empty” loops under the assumption that the most likely reason you would put one in a program is to have a delay, so deleting them will not make real programs run any faster.

  However, the rationale here is that optimization of a nonempty loop cannot produce an empty one. This held for carefully written C compiled with less powerful optimizers but is not always the case for carefully written C++ or with more powerful optimizers.

  Thus GCC will remove operations from loops whenever it can determine those operations are not externally visible (apart from the time taken to execute them, of course). As GCC improves, it will remove the loop itself. Indeed, with ‘-funroll-loops’ small loops can already be removed, so leaving an empty non-unrolled loop is both sub-optimal and inconsistent.

  Be aware of this when performing timing tests, for instance the following loop can be completely removed, provided some_expression can provably not change any global state.

  { int sum = 0; int ix; for (ix = 0; ix != 10000; ix++) sum += some_expression; }

  Even though sum is accumulated in the loop, no use is made of that summation, so the accumulation can be removed.

• Making side effects happen in the same order as in some other compiler.

  It is never safe to depend on the order of evaluation of side effects. For example, a function call like this may very well behave differently from one compiler to another:

  void func (int, int); int i = 2; func (i++, i++);

  There is no guarantee (in either the C or the C++ standard language definitions) that the increments will be evaluated in any particular order. Either increment might happen first. func might get the arguments ‘2, 3’, or it might get ‘3, 2’, or even ‘2, 2’.

• Making certain warnings into errors by default.

  Some ISO C testsuites report failure when the compiler does not produce an error message for a certain program.

  ISO C requires a “diagnostic” message for certain kinds of invalid programs, but a warning is defined by GCC to count as a diagnostic. If GCC produces a warning but not an error, that is correct ISO C support. If testsuites call this “failure”, they should be run with the GCC option ‘-pedantic-errors’, which will turn these warnings into errors.

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