[llvm-dev] Question about Unrolling Loop with Multiple Exits

Fri Jul 16 09:29:13 PDT 2021

Hi Philip,

Thanks for your kind reply.

> A) Are you measuring on tip of tree?ï¿½ There were changes for multiple exit unrolling which landed very recently.

Yep, I am investigating benchmarks with llvm tip and I can see the llvm fails to unroll some loops with multiple exits.
> B) One of your exits does not dominate your latch.ï¿½ Those are generally hard
> C) This example does not seem to require gotos.ï¿½ I strongly suggest reducing your test cases if you want more informed commentary.ï¿½

I am looking at perlbench recently and it has `goto` statements inside loop. The example is a reduced case. When I look at the gcc's output of the example, it looks like gcc unrolls only the below `if` statement block...

    if (*s++ != '\n')
      continue;

Thanks
JinGu Kang

From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Philip Reames via llvm-dev
Sent: 16 July 2021 15:52
To: Jingu Kang <Jingu.Kang at arm.com>; llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] Question about Unrolling Loop with Multiple Exits

A) Are you measuring on tip of tree?ï¿½ There were changes for multiple exit unrolling which landed very recently.

B) One of your exits does not dominate your latch.ï¿½ Those are generally hard.ï¿½

C) This example does not seem to require gotos.ï¿½ I strongly suggest reducing your test cases if you want more informed commentary.ï¿½

Philip
On 7/16/21 7:42 AM, Jingu Kang via llvm-dev wrote:
Hi All,
ï¿½
While I am investigating benchmarks, I have found loops which llvm fails to unroll because the loops have multiple exits.
For example,
ï¿½
char *foo(void);
int boo(char *s);
ï¿½
int test(char *s, char *end, char *check1, char *check2) {
ï¿½ while (s <= end) {
ï¿½ï¿½ï¿½ if (*s++ != '\n')
ï¿½ï¿½ï¿½ï¿½ï¿½ continue;
ï¿½ï¿½ï¿½ if (check1 || check2) {
ï¿½ï¿½ï¿½ï¿½ï¿½ s = foo();
ï¿½ï¿½ï¿½ï¿½ï¿½ if (!s)
ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ goto ret1;
ï¿½ï¿½ï¿½ }ï¿½ï¿½
ï¿½ï¿½ï¿½ï¿½if (boo(s))
ï¿½ï¿½ï¿½ï¿½ï¿½ goto ret0;
ï¿½ }
ï¿½ goto ret1;
ï¿½
ret0:
ï¿½ return 0;
ret1:
ï¿½ return 1;
}
ï¿½
Above code causes below messages from LoopUnroll pass.
ï¿½
Bailout for multi-exit handling when latch exit has >1 predecessor.
Multiple exit/exiting blocks in loop and multi-exit unrolling not enabled!
ï¿½
I can see the option `unroll-runtime-multi-exit` and comments about it. I wonder there are already reviews for the work on phabriactor or some people are working on it.
If someone knows information about it, please share it.
ï¿½
Thanks
JinGu Kang
ï¿½
ï¿½
ï¿½
ï¿½
ï¿½
ï¿½

_______________________________________________

LLVM Developers mailing list

llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>

https://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/20210716/e2f0c72a/attachment.html>