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Using and Porting the GNU Compiler Collection (GCC)

7.12: 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 about unreachable code.
It's very common to have unreachable code in machine-generated programs. For example, this happens normally in some files of GNU C itself.
Warning when a non-void function value is ignored.
Coming as I do from a Lisp background, I balk at the idea that there is something dangerous about discarding a value. There are functions that return values which some callers may find useful; it makes no sense to clutter the program with a cast to void whenever the value isn't useful.
Assuming (for optimization) that the address of an external symbol is never zero.
This assumption is false on certain systems when `#pragma weak' is used.
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 bitfields unsigned by default on particular machines where ``the ABI standard'' says to do so.
The ANSI C standard leaves it up to the implementation whether a bitfield 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 bitfields 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 bitfields as well.

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

There are some arguments for making bitfields 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 bitfield 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__ as long as you don't use `-traditional'. This provides good results in practice.

Programmers normally use conditionals on __STDC__ to ask whether it is safe to use certain features of ANSI C, such as function prototypes or ANSI token concatenation. Since plain `gcc' supports all the features of ANSI 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 ANSI C program, because the ANSI 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 ANSI C library.

Sometimes people say that defining __STDC__ in a compiler that does not completely conform to the ANSI C standard somehow violates the standard. This is illogical. The standard is a standard for compilers that claim to support ANSI C, such as `gcc -ansi'---not for other compilers such as plain `gcc'. Whatever the ANSI 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. 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 ANSI 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 ANSI 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, which holds for C but is not always the case for C++.

Moreover, with `-funroll-loops' small ``empty'' loops are already removed, so the current behavior is both sub-optimal and inconsistent and will change in the future.
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'.
Not allowing structures with volatile fields in registers.
Strictly speaking, there is no prohibition in the ANSI C standard against allowing structures with volatile fields in registers, but it does not seem to make any sense and is probably not what you wanted to do. So the compiler will give an error message in this case.

Using and Porting the GNU Compiler Collection (GCC)