On 10/14/23 09:49, Andrew Pinski via Gcc wrote:
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
Right. See ifcvt_memrefs_wont_trap. That probably could be extended
to capture additional cases. But I'm not sure that'll be sufficient for
Hanke's problem.
Jeff
