On Tue, 3 Jan 2012 11:45:21 -0800 (PST), Monkey wrote:
> -O3 causes a segfault on Linux in the same place - I wonder if it has
> something do with optimising out some instructions which don't appear
> to be used. In the mask section I make use of int* and float* pointing
> to the same address, and it seemed to be around this point that
> multiblend was failing. -O2, though, seems okay on Linux.
This sounds like a pointer aliasing problem. See the man page for gcc:
-fstrict-aliasing
Allow the compiler to assume the strictest aliasing rules
applicable to the language being compiled. For C (and C++), this
activates optimizations based on the type of expressions. In
particular, an object of one type is assumed never to reside at the
same address as an object of a different type, unless the types are
almost the same. For example, an "unsigned int" can alias an
"int", but not a "void*" or a "double". A character type may alias
any other type.
Pay special attention to code like this:
union a_union {
int i;
double d;
};
int f() {
union a_union t;
t.d = 3.0;
return t.i;
}
The practice of reading from a different union member than the one
most recently written to (called "type-punning") is common. Even
with -fstrict-aliasing, type-punning is allowed, provided the
memory is accessed through the union type. So, the code above will
work as expected. However, this code might not:
int f() {
union a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}
Similarly, access by taking the address, casting the resulting
pointer and dereferencing the result has undefined behavior, even
if the cast uses a union type, e.g.:
int f() {
double d = 3.0;
return ((union a_union *) &d)->i;
}
The -fstrict-aliasing option is enabled at levels -O2, -O3, -Os.
--
Robert Krawitz <[email protected]>
Tall Clubs International -- http://www.tall.org/ or 1-888-IM-TALL-2
Member of the League for Programming Freedom -- http://ProgFree.org
Project lead for Gutenprint -- http://gimp-print.sourceforge.net
"Linux doesn't dictate how I work, I dictate how Linux works."
--Eric Crampton
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