On Fri, Oct 13, 2023 at 10:16 PM Hanke Zhang via Gcc <gcc@gcc.gnu.org> wrote:
>
> Hi, I'm working on optimizing if-conversion for my own business
> recently. I got a problem here.
>
> I tried to optimize it in such a case, for example, when a conditional
> statement block has only if statement and no else statement, the
> source C code looks like this:
>
> int* foo; // assume this has been initialized
> int c = rand(), t = rand(), size = 10000000;
> for (int i = 0; i < size; i++) {
> if (foo[i] & (1 << c)) foo[i] ^= (1 << t);
> }
>
> Part of its corresponding gimple is optimized like this before if-conversion:
>
> <bb 5>:
> # i_71 = PHI <i_39(12), 0(9)>
> # ivtmp_9 = PHI <ivtmp_73(12), 10000000(9)>
> _5 = (long unsigned int) i_71;
> _6 = _5 * 4;
> _7 = foo_23 + _6;
> _8 = *_7;
> shifttmp_75 = _8 & shifttmp_76;
> if (shifttmp_75 != 0)
> goto <bb 6>; [50.00%]
> else
> goto <bb 14>; [50.00%]
>
> <bb 14> [local count: 531502205]:
> goto <bb 7>; [100.00%]
>
> <bb 6> [local count: 531502204]:
> _12 = _8 ^ _11;
> *_7 = _12;
>
> <bb 7> [local count: 1063004409]:
> i_39 = i_71 + 1;
> ivtmp_73 = ivtmp_9 - 1;
> if (ivtmp_73 != 0)
> goto <bb 12>; [99.00%]
> else
> goto <bb 8>; [1.00%]
>
> I want to add some statements to gimple to make it like adding an else
> block to the source code.
>
> // What I expected:
> int* foo; // assume this has been initialized
> int c = rand(), t = rand(), size = 10000000;
> for (int i = 0; i < size; i++) {
> if (foo[i] & (1 << c)) foo[i] ^= (1 << t);
> + else foo[i] = foo[i]; // I want to add a statment here !!!!!
> }
>
> And of course I can't change the source code for real, so I can only
> add a pass in front of if-conversion to modify the gimple.
>
> For the example above, I know that I have to add them in the block
> '<bb14>', but what confuses me is that I don't know what kind of
> statement to add to be legal due to my poor experience.
>
> I try to add something like this below, but the compile error just
> happened. So I'm here for help. What kind of statements should I add
> here?
>
> <bb 14> [local count: 531502205]:
> + *_7 = *_7
> goto <bb 7>; [100.00%]
>
> Finally, The reason I did this was to avoid MASK_STORE generation,
> because it might add an if branch in the final assembly which I don't
> like it to be. And after such a modification, if-conversion should
> have been changed it to the form of a ternary expression, which would
> reduce the occurrence of branches after final vectorization and
> produce more efficient code.
>
> Or there if is a better way to get rid of MASK_STORE, please tell me
> about that. :)
So there are 2 issues with this transformation which you need to take
into account.
1) C11/C++11 threading model (-fallow-store-data-races is needed)
2) foo could be read only and cause a trap if written to. if the
branch is never taken there would be no writes
Thanks,
Andrew Pinski
>
> Thanks
> Hanke Zhang
