[llvm-dev] Particular type of loop optimization

[Mehdi Amini via llvm-dev]

> 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


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