Is the GCC option -Wdeclaration-after-statement purely stylistic? - c

Question

Is the option -Wdeclaration-after-statement stylistic only? By that I mean, if I macro'd all cases in my C code where a variable was defined and I initialized them in them in the same fashion migrating from this older style C90 to the newer C99 style, would that code be byte-for-byte the same?
Here is how the option -Wdeclaration-after-statement is documented (from man gcc):
Warn when a declaration is found after a statement in a block. This construct, known from C++, was introduced with ISO C99 and is by default allowed in GCC. It is not supported by ISO C90.
And it allows you to take code like
int a;
{
a = 42;
printf( "%d", a );
}
and turn it into
int a = 42;
printf( "%d", a );
This is a follow-up to my question here.

Answer 1

I may be confused here, but I think that we are missing something.
Prior to C99, all variable declarations had to occur before any statements in a block. It did not matter where you assigned it a value (except maybe in generated assembly code).
int a;
do_something();
a = 7;
do_something_else();
What you could not do in C but has always been perfectly legal in C++ is mix declarations and statements:
do_something();
int a = 7;
do_something_else();
With the advent of C99 you can now do the same thing in C as you can in C++, and declare a variable anywhere in a block, even after non-declaration statements.
Ultimately it was a design decision based on making it easier to write a compiler that leaked into the language specification. Compilers are a little more sophisticated now (and much larger).

Answer 2

Statement in this context either refers to a full expression ending with ; or alternatively a compound statement, { }. These terms are taken from the formal C grammar.
Declaration after statement does not apply to the code in your question. Because... you placed the declaration before a (compound) statement. Your two examples have nothing to do with this gcc setting, so it would appear that you have misunderstood what it does.
Rather, the relevant example would be:
{
int a = 42;
}
versus
{
puts("hello");
int a = 42;
}
The former example is fine in any C version. The latter is fine in standard C, but not in the deprecated C90 standard. So the only purpose of the warning nowadays would be to give a diagnostic message for standard C programs where a certain coding style is enforced.
The vast majority of programmers out there should not use this warning and stick to standard C as defined in ISO 9899:2018.
EDIT:
This is merely about whether or not you can rewrite code from a C90 to C99 conventions and obtain binary compatibility
Yes you can rewrite to C99 style without any consequences. None of these affect how the variable is treated in the "abstract machine" as C calls it. Here are some examples, 2 relevant and 1 irrelevant:
void relevant_c99_example (void) {
if ( 1 ) {
int a = 42;
printf( "%d", a );
}
else {
int a = 42;
printf( "%d", a );
}
}
void relevant_c90_example (void) {
int a = 42;
if ( 1 ) {
printf( "%d", a );
}
else {
printf( "%d", a );
}
}
void irrelevant_example (void) {
int a;
if ( 1 ) {
a = 42;
printf( "%d", a );
}
else {
a = 42;
printf( "%d", a );
}
}
https://godbolt.org/z/1oqvoesx7
These 3 produce 100% identical machine code when tested on both gcc and clang for x86_64. They even produce 100% identical machine code when optimizations are disabled! Which one shouldn't expect, since that isn't how you benchmark C code correctly. But it happens to be the case here.

Answer 3

No, it's not. For example, the above two snippets will compile byte-for-byte the same. So will the foo and bar below, but baz will not. Here is a link to GodBolt. This demonstrates that hoisting the initialization to the declaration may NOT produce the same code
void foo () {
int a;
{
if ( 1 ) {
a = 42;
printf( "%d", a );
}
else {
a = 42;
printf( "%d", a );
}
}
}
void bar () {
int a = 42;
{
if ( 1 ) {
printf( "%d", a );
}
else {
printf( "%d", a );
}
}
}
void baz () {
int a;
{
if ( rand() > 0 ) {
a = 42;
printf( "%d", a );
}
else {
a = 42;
printf( "%d", a );
}
}
}