[llvm-dev] Comparing Clang and GCC: only clang stores updated value in each iteration.

[Jonas Paulsson via llvm-dev]

Hi Philip and Eli,


> I think your example may be a bit over reduced.  Unless I'm misreading 
> this, a starts at 1, is incremented one each iteration, and then is 
> tested against zero.  The only way this loop can exit is if a has 
> wrapped around and C++ states that signed integers are assumed to not 
> overflow.  We can/should be replacing the whole loop with an unreachable.
>
> Do we still fail to optimize if either a) you use an unsigned which 
> has defined overflow or b) you use a non-zero exit test? That is, 
> change the example to something like:
> int a = 1;
> void b() {
>   do
>     if (a)
>       a++;
>   while (a != 500);
> }

Yes, both if I change 'a' to unsigned, or replace the exit test with 
500, clang stores in each iteration while gcc does not.

 > (Eli) Your testcase is a bit weird because the condition of the while 
loop is the same as the condition of the if statement.  Is that really 
what the original loop looks like?

No, not really, the reduced one just shows the difference between gcc 
and clang. There were some variations to this, but I chose this since it 
gave a very small output. Sorry if it was confusing.

>
> If so, then yes, this is probably a case where the aggressive LoopPRE 
> mentioned in the other thread that Eli linked to would be useful.  
> Once we'd done the PRE, then everything else should collapse.
Thanks for the link, it's good to know this issue is recognized. If I 
understand it correctly, the reason clang is storing in each iteration 
is due to concurrency. As a newbie I wonder how this works in practice 
since even if the value is stored in each iteration two threads could 
still do this simultaneously if not some sort of atomic operation is 
doing it, right? What happens here is that the value of 'a' is loaded 
once before the loop, then incremented and stored in each iteration. How 
does that help with multiple threads compared to storing it after the loop?

Is there an option to change this behavior in gcc or clang? It seems 
that gcc is assuming a single thread, while clang is not. It would be 
nice to have the same setting here when comparing them. Or am I missing 
something?

Thanks

Jonas

>
>>
>> bin/clang -O3 -march=z13 -mllvm -unroll-count=1
>>
>>         .text
>>         .file   "testfun.i"
>>         .globl  b                       # -- Begin function b
>>         .p2align        4
>>         .type   b, at function
>> b:                                      # @b
>> # %bb.0:                                # %entry
>>         lrl     %r0, a
>> .LBB0_1:                                # %do.body
>>                                         # =>This Inner Loop Header: 
>> Depth=1
>>         cije    %r0, 0, .LBB0_3
>> # %bb.2:                                # %if.then
>>                                         #   in Loop: Header=BB0_1 
>> Depth=1
>>         ahi     %r0, 1
>>         strl    %r0, a
>> .LBB0_3:                                # %do.cond
>>                                         #   in Loop: Header=BB0_1 
>> Depth=1
>>         cijlh   %r0, 0, .LBB0_1
>> # %bb.4:                                # %do.end
>>         br      %r14
>> .Lfunc_end0:
>>         .size   b, .Lfunc_end0-b
>>                                         # -- End function
>>         .type   a, at object               # @a
>>         .data
>>         .globl  a
>>         .p2align        2
>> a:
>>         .long   1                       # 0x1
>>         .size   a, 4
>>
>>
>> gcc -O3 -march=z13:
>>
>>         .file   "testfun.i"
>>         .machinemode zarch
>>         .machine "z13"
>> .text
>>         .align  8
>> .globl b
>>         .type   b, @function
>> b:
>> .LFB0:
>>         .cfi_startproc
>>         larl    %r1,a
>>         lt      %r1,0(%r1)
>>         je      .L1
>>         larl    %r1,a
>>         mvhi    0(%r1),0
>> .L1:
>>         br      %r14
>>         .cfi_endproc
>> .LFE0:
>>         .size   b, .-b
>> .globl a
>> .data
>>         .align  4
>>         .type   a, @object
>>         .size   a, 4
>> a:
>>         .long   1
>>         .ident  "GCC: (GNU) 8.0.1 20180324 (Red Hat 8.0.1-0.20)"
>>         .section        .note.GNU-stack,"", at progbits
>>
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>