On Mon, Oct 23, 2023 at 2:56 PM Hanke Zhang wrote:
>
> Richard Biener 于2023年10月23日周一 20:32写道：
> >
> > On Mon, Oct 23, 2023 at 12:50 PM Hanke Zhang wrote:
> > >
> > > Hi Richard:
> > >
> > > Thanks for your advice. But when I try a simpler example like the one
> > > below before looking at the code, GCC still does nothing.
> > >
> > > int main() {
> > > int width;
> > > scanf("%d", &width);
> > > int sum = 0;
> > > for (int i = 0; i < width; i++) sum += i;
> > > printf("%d\n", sum);
> > > }
> > >
> > > I tried O3 and LTO, but still the same. So I'd like to ask why, or am
> > > I doing something wrong?
> >
> > -fdump-tree-sccp-details-scev reveals
> >
> > (set_scalar_evolution
> > instantiated_below = 5
> > (scalar = sum_9)
> > (scalar_evolution = {0, +, {1, +, 1}_1}_1))
> > )
> > (chrec_apply
> > (varying_loop = 1)
> > (chrec = {0, +, {1, +, 1}_1}_1)
> > (x = (unsigned int) width.0_12 + 4294967295)
> > (res = scev_not_known))
> >
> > so we don't know how to apply a variable number of iterations to
> > the affine expression {0, +, {1, +, 1}_1}_1, that is, we do not
> > know how to compute the final value of the reduction.
> >
> > For a constant, say width == 100 we do:
> >
> > (set_scalar_evolution
> > instantiated_below = 2
> > (scalar = sum_6)
> > (scalar_evolution = {0, +, {1, +, 1}_1}_1))
> > )
> > (chrec_apply
> > (varying_loop = 1)
> > (chrec = {0, +, {1, +, 1}_1}_1)
> > (x = 99)
> > (res = 4950))
>
> Yeah, I also found this result in previous experiments. But what
> confuses me is that if the 'width' can't be inferred to INTEGER_CST,
> there's nothing we can do, right?
>
> Because in my case, the variables corresponding to 'width' are almost
> all undetermined values, such as 'width = rand()'. That said, I can
> hardly get any optimizations in my cases, right?
I think the result would be (width * (width - 1)) / 2, chrec_apply is where
"pattern matching" of known series is done.
Richard.
>
> Thanks
> Hanke Zhang
>
>
> >
> > Richard.
> >
> > >
> > > Thanks
> > > Hanke Zhang
> > >
> > > Richard Biener 于2023年10月19日周四 20:00写道：
> > > >
> > > > On Tue, Oct 17, 2023 at 2:39 PM Hanke Zhang
> > > > wrote:
> > > > >
> > > > > Hi Richard
> > > > > I get it, thank you again.
> > > > >
> > > > > And I got another problem, so I'd like ask it by the way. Can the left
> > > > > shift of the induction variable in a loop be optimized as a constant?
> > > > > Like the code below:
> > > > >
> > > > > int ans = 0;
> > > > > int width = rand() % 16;
> > > > > for (int j = 0; j < width; j++)
> > > > > ans += 1 << (j + width)
> > > > >
> > > > > into:
> > > > >
> > > > > int width = rand() % 16;
> > > > > ans = (1 << (2 * width) - (1 << width));
> > > > >
> > > > > I came across a more complex version of that and found that gcc
> > > > > doesn't seem to handle it, so wanted to write a pass myself to
> > > > > optimize it.
> > > > >
> > > > > I got two questions here. Does GCC have such optimizations? If I want
> > > > > to do my own optimization, where should I put it? Put it behind the
> > > > > pass_iv_optimize?
> > > >
> > > > GCC has the final value replacement pass (pass_scev_cprop) doing these
> > > > kind of transforms. Since 'ans' does not have an affine evolution this
> > > > case would need to be pattern matched (there are some existing pattern
> > > > matchings in the pass).
> > > >
> > > > > Thanks
> > > > > Hanke Zhang
> > > > >
> > > > > Richard Biener 于2023年10月17日周二 20:00写道：
> > > > > >
> > > > > > On Tue, Oct 17, 2023 at 1:54 PM Hanke Zhang
> > > > > > wrote:
> > > > > > >
> > > > > > > Richard Biener 于2023年10月17日周二
> > > > > > > 17:26写道：
> > > > > > > >
> > > > > > > > On Thu, Oct 12, 2023 at 2:18 PM Hanke Zhang via Gcc
> > > > > > > > wrote:
> > > > > > > > >
> > > > > > > > > Hi, I'm recently working on vectorization of GCC. I'm stuck
> > > > > > > > > in a small
> > > > > > > > > problem and would like to ask for advice.
> > > > > > > > >
> > > > > > > > > For example, for the following code:
> > > > > > > > >
> > > > > > > > > int main() {
> > > > > > > > > int size = 1000;
> > > > > > > > > int *foo = malloc(sizeof(int) * size);
> > > > > > > > > int c1 = rand(), t1 = rand();
> > > > > > > > >
> > > > > > > > > for (int i = 0; i < size; i++) {
> > > > > > > > > if (foo[i] & c1) {
> > > > > > > > > foo[i] = t1;
> > > > > > > > > }
> > > > > > > > > }
> > > > > > > > >
> > > > > > > > > // prevents the loop above from being optimized
> > > > > > > > > for (int i = 0; i < size; i++) {
> > > > > > > > > printf("%d", foo[i]);
> > > > > > > > > }
> > > > > > > > > }
> > > > > > > > >
> > > > > > > > > First of all, the if statement block in the loop will be
> > > > > > > > > converted to
> > > > > > > > > a MASK_STORE through if-conversion optimization. But after
> > > > > > > > > tree-vector, it will still become a branched form. The part
> > > > > > > > > of the
> > > > > > > > > final disassembly structur
