[llvm-dev] Particular type of loop optimization

[mats petersson via llvm-dev]

Just to be clear, "What are you actually trying to achieve?" was meant to
ask what is your overall goal, and given that overall goal - not the
transformation of the IR, but "why do you need to do this" - as a simple
performance enhancement, or for some other reason?

And like has been stated, alias-analysis will help here, but some cases,
the compiler will not be able to certainly say that no access from the
pointer will affect something else, meaning that the compiler NEEDS to take
the conservative route and reload the pointer - it doesn't seem to be a
-fstrict-alias flag for clang, like that of gcc.

--
Mats

On 3 February 2016 at 12:05, Gleison Souza via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Thanks Mehdi,
>
> I tried to use this, but some debug information can be lost in these
> optimizations.
> I need write in the source file to insert information before the loops,
> and in
> some cases, I'm writing after the loop header.
>
> Please, take a look:
>
> int foo1 (int *a, int *b, int n) {
>
>   int i, s= 0;
>
>   for (i = 0; i < n; i++) {
>
>     s = s * a[i];
>
>   }
>
>
>
>   for (i = 0; i < n; i++) {
>
>     b[i] = a[i] + 3;
>
>     s += a[i];
>
>    }
>
>   return s;
>
> }
>
> In this case, using the line obtained by this one in the LLVM's IR:
>
> Line = l->getStartLoc().getLine();
>
> The source file is cloned, and I'm writing my annotations inside the loop
> now.
> I can't do several modifications in the struct of code, just the
> necessary, thats the problem.
>
> Regards,
>
>     Gleison
>
> 2016-02-03 1:07 GMT-02:00 Mehdi Amini <mehdi.amini at apple.com>:
>
>>
>> On Feb 2, 2016, at 8:35 AM, Gleison Souza via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>> Dear LLVMers,
>>
>>     I am trying to implement a particular type of loop optimization, but
>> I am having problems with global variables. To solve this problem, I would
>> like to know if LLVM has some pass that moves loads outside loops. I will
>> illustrate with an example. I want to transform this code below. I am
>> writing in C for readability, but I am analysing LLVM IR:
>>
>> int *vectorE;
>>
>> void foo (int n) {
>>   int i;
>>   for (i = 0; i < n; i++)
>>     vectorE[i] = i;
>> }
>>
>> into this one:
>>
>> int *vectorE;
>>
>> void foo (int n) {
>>   int i;
>>   int* aux = vectorE;
>>   for (i = 0; i < n; i++)
>>     aux[i] = i;
>> }
>>
>>
>>
>> Have you looked at the output of clang with optimization enabled (even
>> O1)? For this C++ code the optimizer moves the access to the global in the
>> loop preheader, and then the loop itself does not access the global at all,
>> which seems to be what you’re looking for.
>>
>> Try: clang -O1 -S -o - -mllvm -print-after-all
>>
>> —
>> Mehdi
>>
>>
>>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20160203/cfeb3250/attachment.html>