mirror of
https://github.com/pineappleEA/pineapple-src.git
synced 2024-12-09 05:58:24 -05:00
48 lines
1.9 KiB
Plaintext
Executable File
48 lines
1.9 KiB
Plaintext
Executable File
Undefined Behavior
|
|
------------------
|
|
In the C language, some operations are undefined, like signed integer overflow,
|
|
dereferencing freed pointers, accessing outside allocated space, ...
|
|
|
|
Undefined Behavior must not occur in a C program, it is not safe even if the
|
|
output of undefined operations is unused. The unsafety may seem nit picking
|
|
but Optimizing compilers have in fact optimized code on the assumption that
|
|
no undefined Behavior occurs.
|
|
Optimizing code based on wrong assumptions can and has in some cases lead to
|
|
effects beyond the output of computations.
|
|
|
|
|
|
The signed integer overflow problem in speed critical code
|
|
----------------------------------------------------------
|
|
Code which is highly optimized and works with signed integers sometimes has the
|
|
problem that some (invalid) inputs can trigger overflows (undefined behavior).
|
|
In these cases, often the output of the computation does not matter (as it is
|
|
from invalid input).
|
|
In some cases the input can be checked easily in others checking the input is
|
|
computationally too intensive.
|
|
In these remaining cases a unsigned type can be used instead of a signed type.
|
|
unsigned overflows are defined in C.
|
|
|
|
SUINT
|
|
-----
|
|
As we have above established there is a need to use "unsigned" sometimes in
|
|
computations which work with signed integers (which overflow).
|
|
Using "unsigned" for signed integers has the very significant potential to
|
|
cause confusion
|
|
as in
|
|
unsigned a,b,c;
|
|
...
|
|
a+b*c;
|
|
The reader does not expect b to be semantically -5 here and if the code is
|
|
changed by maybe adding a cast, a division or other the signedness will almost
|
|
certainly be mistaken.
|
|
To avoid this confusion a new type was introduced, "SUINT" is the C "unsigned"
|
|
type but it holds a signed "int".
|
|
to use the same example
|
|
SUINT a,b,c;
|
|
...
|
|
a+b*c;
|
|
here the reader knows that a,b,c are meant to be signed integers but for C
|
|
standard compliance / to avoid undefined behavior they are stored in unsigned
|
|
ints.
|
|
|