Paul Schlie wrote:
On Fri, Jan 26, 2007 at 06:57:43PM -0500, Paul Schlie wrote:
Robert Dewar wrote:
People always say this, but they don't really realize what they are
saying. This would mean you could not put variables in registers, and
would essentially totally disable optimization.
- can you provide an example of a single threaded program where the
assignment of variable to a machine register validly changes its
observable logical results?
If the program has a hash table that stores pointers to objects, and
the hash function depends on pointer addresses, then the choice to
allocate some objects in registers rather than in stack frames will change
the addresses. If the algorithm depends on the order of hash traversal,
then -O2 will change its behavior.
- if the compiler chooses to alias an object's logical storage location
utilizing a register, and that object's logical address is well specified
by a pointer whose value is itself subsequently utilized; it shouldn't have
any logical effect on that object's logical pointer's value; as it's the
responsibility of the compiler to preserve the semantics specified by the
program.
(however as you appear to be describing an algorithm attempting to rely on
the implicit addresses of object storage locations resulting from an assumed
calling or allocation convention; and as such assumptions are well beyond
the scope of most typical language specifications; it' not clear that such
an algorithm should ever be presumed to reliably work regardless of any
applied optimizations?)
Isn't that the gist of the entire overflow wraps issue? Signed overflow
is undefined in C and always has been. It' not clear that any program
that relies on it should ever be presumed to reliably work regardless of
any applied optimization.
Best to use a language that has no undefined behaviors if you don't want
optimizations to change program behavior.
Yes I know that for one reason or another, many people will not be using
such a language. So we try to do our best with making gcc a useful
compiler.
David Daney
