分类： C/C++
2011-05-06 11:37:25
I have seen some commits that "fix" gcc 3.3 alias warnings, that does not
give me warm fuzzy feelings (the commits that is), and I have alse seen a
lot of confusion about aliasing (and ISO C in general) on different
mailing lists, so I have tried to explain some of the issues that I know
have/will bite us.
Apologies if this is too elementary...
ALIASING
========
What is aliasing?
=================
The hardware-centric view of pointers is that they can point at any
memory,
so a write through a pointer may change any variable in a program:
int i = 23;
*f = 5;
/* We don't know what value i has at this point. */
We cannot know what value i has, since the pointers &i and f may point
at the same address (that is what ISO C means when it say that &i and f
may alias).
This prevents many types of optimizations, and it makes a real difference
since most pointers in real life cannot point on the same position. ISO C
improves the situation (for the compiler) by roughly saying "pointers of
different types cannot point to the same address".
int
foo(float *f) {
int i = 23;
*f = 5.0;
/* A float* cannot point on the same address as int*. */
return i * 2;
}
So the compiler may optimize this to
int
foo(float *f) {
*f = 5.0;
return 46;
}
The ISO specification does not really prevent pointers to point to the
same address -- it specifies that the result is undefined when you
dereference a pointer that points to an object of a different
(incompatible) type. So the following example is OK:
int i = 23, *tmp;
tmp = (int*)f;
*tmp = 5;
/* We don't know what value i has at this point. */
But note the important difference that we are actually writing the memory
position as an "int" and not as a "float".
There exist an important exception to the rule above: char* may alias all
types (too much code would break if ISO had prevented this...)
There are cases where you wish to access the same memory as different
types:
float *f = 2.718;
printf("The memory word has value 0x%08x\n", *((int*)f));
You cannot do that in ISO C, but gcc has an extension in that it
considers memory in unions as having multiple types, so the following
will work in gcc (but is not guaranteed to work in other compilers!)
union {
int i;
float f;
} u;
u.f = 2.718;
printf("The memory word has value 0x%08x\n", u.i);
One bieffect of this is that gcc may miss optimization opportunities
when you use union-heavy constructs.
What the standard says [*]
==========================
The aliasing rules are stated in clause 6.5 (Expressions):
7 An object shall have its stored value accessed only by an lvalue
expression that has one of the following types: {footnote 73}
a type compatible with the effective type of the object,
a qualified version of a type compatible with the effective type of
the object,
a type that is the signed or unsigned type corresponding to the
effective type of the object,
a type that is the signed or unsigned type corresponding to a
qualified version of the effective type of the object,
an aggregate or union type that includes one of the aforementioned
types among its members (including, recursively, a member of a
subaggregate or contained union), or
a character type. (万能类型 ：）
{footnote 73} The intent of this list is to specify those circumstances
in which an object may or may not be aliased.
The gcc warnings
================
gcc may warn for some constructs that break the aliasing rules, but not
all of them (or not even most of them!), so a warning-free source code
does not give you any guarantee.
The most common warning you will see is probably "dereferencing type-
punned pointer will break strict-aliasing rules". The place where it
warns is in general not wrong -- what gcc tries to tell you is that you
will break the aliasing rules when you dereference the pointer later
(unless you cast it back to its original type first). This warning
should be interpreted as saying that your interfaces are badly designed,
and the correct way to avoid the warning is to redesign them in a way
where you do not need to cast between conflicting types. (Even if you
often can make this warning go away by changing void** to void*...)
POINTER CASTS AND ALIGNMENT
===========================
The problem
===========
Many architectures requires that pointers are correctly aligned when
accessing objects bigger than a byte. There are however many places
in system code where you receive unaligned data (e.g. the network stacks)
so you need to fix it up:
char* data;
struct foo_header *tmp, header;
tmp = data + offset;
memcpy(&header, tmp, sizeof(header));
if (header.len < FOO)
[...]
But this does not work... The reason is that the behavior is undefined
when you assign an unaligned value to a pointer that points to a type
that need to be aligned. What happens in the example above is that gcc
notices that tmp and header must be aligned, so it may use an inlined
memcpy that uses instructions that assumes aligned data.
The correct way to fix this is not to use the foo_header pointer
char* data;
struct foo_header header;
memcpy(&header, data + offset, sizeof(header));
if (header.len < FOO)
[...]
The original example above might look silly, but this has bitten us a
couple of times already...
What the standard says [*]
==========================
The pointer alignment requirements are stated in clause 6.3.2.3
(Pointers):
7 A pointer to an object or incomplete type may be converted to a pointer
to a different object or incomplete type. If the resulting pointer is
not correctly aligned {footnote 57} for the pointed-to type, the
behavior is undefined. Otherwise, when converted back again, the result
shall compare equal to the original pointer. [...]
{footnote 57} In general, the concept "correctly aligned" is transitive:
if a pointer to type A is correctly aligned for a pointer to type B,
which in turn is correctly aligned for a pointer to type C, then a
pointer to type A is correctly aligned for a pointer to type C.
CONCLUSION
==========
ISO C is not your grandfather's C, and it is wrong to think of it as a
high-level machine language...
Pointer casts are evil (both explicit and implicit casts), and you
should think twice before adding a pointer cast to the code...
[*] The standard references are from ISO/IEC 9899:1999, but the older
ANSI/ISO standard says essentially the same thing.
