Question about gimple code during optimizing if-conversion

[Jeff Law]

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