On Tue, Mar 6, 2012 at 6:55 PM, Aldy Hernandez <al...@redhat.com> wrote:
> On 02/29/12 03:22, Richard Guenther wrote:
>> So fixing up individual passes is easier - I can only think of PRE being
>> problematic right now, I am not aware that any other pass moves loads
>> or stores. So I'd simply pre-compute the stmt bit in PRE and adjust
>> if (gimple_has_volatile_ops (stmt)
>> || stmt_could_throw_p (stmt))
>> in compute_avail accordingly.
> Initially I thought PRE would be problematic for transactions, but perhaps
> it isn't. As I understand, for PRE we hoist loads/computations that are
> mostly redundant, but will be performed on every path:
> if (flag)
> a = b + c;
> d = b + c; <-- [b + c] always computed
> Even if we hoist [b + c] before the flag, [b + c] will be computed on every
> path out of "if (flag)...". So... we can allow this transformation within
> transactions, right?
Note that partial PRE (enabled at -O3) can insert expressions into paths
that did _not_ execute the expression. For regular PRE you are right.