llvm dev - Nov 2021 - Dumping branch weights in loop unroll phase

Hello,

I am using a cross compiler built from llvm trunk source.

Host - x86
Target - AArch64

I am seeing the same loop unroll dump before and after loop unroll pass
using flags - "-O2 -funroll-loops". Is there a way to check whether
the
loop has unrolled or not?

As I am seeing both before and after loop unroll dumps as same, is there a
way to dump branch weights? I tried flag "-stats" but I am not getting
any
warnings or actual branch weights.

Thanks and Regards,
Shubham
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Hi,

Please avoid double posting.
> On Nov 24, 2021, at 16:46, Shubham Narlawar via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> Hello,
> 
> I am using a cross compiler built from llvm trunk source.
> 
> Host - x86
> Target - AArch64
> 
> I am seeing the same loop unroll dump before and after loop unroll pass
using flags - "-O2 -funroll-loops". Is there a way to check whether
the loop has unrolled or not?
> 
How are you dumping the IR before/after unrolling? You are comparing the IR
after all optimizations, note that `-O2 already enables unrolling, so adding
`-funroll-loops` shouldn’t change much. You can disable unrolling though, using
`-fno-unroll-loops.`

To see which loops have been unrolled, you can use optimization remarks, e.g.
add ` -Rpass=loop-unroll `(https://clang.godbolt.org/z/W418qeaGc
<https://clang.godbolt.org/z/W418qeaGc>)  or `-fsave-optimization-record`.
https://llvm.org/docs/Remarks.html <https://llvm.org/docs/Remarks.html>
> As I am seeing both before and after loop unroll dumps as same, is there a
way to dump branch weights? I tried flag "-stats" but I am not getting
any warnings or actual branch weights.
You can take a look at the metadata in LLVM IR
(https://llvm.org/docs/BranchWeightMetadata.html
<https://llvm.org/docs/BranchWeightMetadata.html>) Not sure if there’s a
pass to dump them in a nice way.


Cheers,
Florian
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