[llvm-dev] LoopDeletion / removal of empty loops.

[Atmn Patel via llvm-dev]

Hi Jonas,

We've also had this discussion about removing dead loops, and we've
introduced two attributes based on which LoopDeletion should know whether
or not to remove C/C++ loops. The final patch that modifies LoopDeletion is
under review at https://reviews.llvm.org/D86844. I still haven't had time
to properly address the errors in this patch (it was failing a two stage
build), but I hope to get back to it at some point over the next few weeks.

Atmn

On Sat, Dec 19, 2020, 3:12 PM Jonas Paulsson via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi,
>
> It seems that omnetpp runs ~10% faster with gcc than with clang on
> SystemZ. This is due to the small function printAddressTable which
> contains a loop with a single statement. It is run in "express-mode",
> which means that the function will contain mainly an empty loop, which
> GCC removes (or at least makes an early exit) while clang emits the loop
> to be iterated over.
>
> The loop is iterating with an std::iterator over an std::map. GCC has
> recently changed behavior to remove most (but not intentional ones)
> empty loops, and I wonder if clang should do the same? There was a
> discussion in the GCC community
> (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89713) which resulted in
> this change to assume that loops are finite based on the "forward
> progress guarantee" of the standard, IIUC.
>
> For instance, this function:
>
> #include <map>
> void fun(std::map<int, int> &M) {
>    for (std::map<int, int>::iterator I = M.begin(); I != M.end(); I++)
>      ;
> }
>
> will result in an empty function by GCC, while clang generates the empty
> loop to be iterated over.
>
> I see a comment in LoopDeletion.cpp:
>
> /// A loop is considered dead if it does not impact the observable
> behavior of
> /// the program other than finite running time. This never removes a
> loop that
> /// might be infinite (unless it is never executed), as doing so could
> change
> /// the halting/non-halting nature of a program.
>
> This loop does pass the isLoopDead() check in LoopDeletion.cpp:204, but
> the loop is not deleted since ScalarEvolution cannot resolve the trip
> count.
>
> This is of course very important for performance in general and in
> particular for the profits of loop unswitching and other passes
> producing empty loops. So I wonder if it is the case that we could start
> doing this as well in llvm?
>
> /Jonas
>
>
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