http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35305
--- Comment #4 from davidxl <xinliangli at gmail dot com> 2012-10-30 18:57:18
UTC ---
The suggested transformation can be useful in some cases, but won't be general
enough. The listed example is an extreme case. For instance, the second a+b
instance does not have to be in the hot trace but still still hot enough to be
PREed. Or there is no one dominating traces available -- the expression is
available in all incoming paths except for one rare path.
switch (a)
{
case 1:
g[1] = a + b;
break;
case 2:
g[2] = a + b;
break;
case 3:
g[3] = a + b;
...
default:
g[0] = 0;
}
switch (b)
{
case 1:
... a + b; // partially redundant
case 2:
... a + b; // redundant
default:
// does nothing
break;
}
Regarding handling dereferences, the availability and down-safety analysis
needs to be extended to to recognize safe speculation candidates.
Example 1: dereference of same pointer fully available --
int g1, g2;
struct A
{
int a;
int b;
};
void foo(struct A* ap, int k,int m)
{
if (__builtin_expect (k, 1))
g1 = ap->b;
else
g2 = ap->a;
if (__builtin_expect (m, 1))
{
g2 = ap->b; // Good safe speculative PRE candidate
}
}
Example 2: deference of ap fully anticipated
int g1, g2;
struct A
{
int a;
int b;
};
void foo(struct A* ap, int k,int m)
{
if (__builtin_expect (k, 1))
g1 = ap->b;
if (__builtin_expect (m, 1))
g2 = ap->b; // Safe to speculatively hoist across the branch
into the else of the previous branch
else
g1 = ap->a;
}
David
(In reply to comment #3)
> Wouldn't this be a candidate for forming a superblock from hot traces of
> a function? Thus in the testcase
>
> if (k && m)
> {
> g1 = a + b;
> g2 = a + b;
> }
> else
> {
> ... old code
> }
>
> which would also handle the case where we cannot speculatively move code
> (like dereferences)?
