llvm dev - Oct 2015 - How can I tell llvm, that a branch is preferred ?

If I read the llvm language correctly, it doesn't have a way to specify 
the preferred branch, correct ? I see nothing in the specs for
"branch"
or "switch". And __buildin_expect does nothing, that I am sure of.

Unfortunately llvm has this knack for ordering my one most crucial part 
of code exactly the opposite I want to, it does: (x86_64)

	cmpq	%r15, (%rax,%rdx)
	jne	LBB0_3
Ltmp18:
	leaq	8(%rax,%rdx), %rcx
	jmp	LBB0_4
LBB0_3:
	addq	$8, %rcx
LBB0_4:



when I want,

	cmpq	%r15, (%rax,%rdx)
	jeq	LBB0_3

	addq	$8, %rcx
	jmp	LBB0_4
LBB0_3:
	leaq	8(%rax,%rdx), %rcx
LBB0_4:

since that saves me executing a jump 99.9% of the time. Is there 
anything I can do ?

Ciao
    Nat!

Hi Nat!,

I think you can try to attach branch weight metadata to the branch or switch,
and give a higher weight to the likely taken path. In that way, LLVM should be
able to order the code correctly.

For branch weight metadata, you can refer to the following document:

http://llvm.org/docs/BranchWeightMetadata.html
<http://llvm.org/docs/BranchWeightMetadata.html>

thanks,
chen 

> On Oct 27, 2015, at 10:55 AM, Nat! via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> If I read the llvm language correctly, it doesn't have a way to specify
the preferred branch, correct ? I see nothing in the specs for
"branch" or "switch". And __buildin_expect does nothing,
that I am sure of.
> 
> Unfortunately llvm has this knack for ordering my one most crucial part of
code exactly the opposite I want to, it does: (x86_64)
> 
> 	cmpq	%r15, (%rax,%rdx)
> 	jne	LBB0_3
> Ltmp18:
> 	leaq	8(%rax,%rdx), %rcx
> 	jmp	LBB0_4
> LBB0_3:
> 	addq	$8, %rcx
> LBB0_4:
> 
> 
> 
> when I want,
> 
> 	cmpq	%r15, (%rax,%rdx)
> 	jeq	LBB0_3
> 
> 	addq	$8, %rcx
> 	jmp	LBB0_4
> LBB0_3:
> 	leaq	8(%rax,%rdx), %rcx
> LBB0_4:
> 
> since that saves me executing a jump 99.9% of the time. Is there anything I
can do ?
> 
> Ciao
>   Nat!
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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On Tue, Oct 27, 2015 at 10:55 AM, Nat! via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> If I read the llvm language correctly, it doesn't have a way to specify
the
> preferred branch, correct ? I see nothing in the specs for
"branch" or
> "switch". And __buildin_expect does nothing, that I am sure of.
>
> Unfortunately llvm has this knack for ordering my one most crucial part of
> code exactly the opposite I want to, it does: (x86_64)
>
>         cmpq    %r15, (%rax,%rdx)
>         jne     LBB0_3
> Ltmp18:
>         leaq    8(%rax,%rdx), %rcx
>         jmp     LBB0_4
> LBB0_3:
>         addq    $8, %rcx
> LBB0_4:
>
>
>
> when I want,
>
>         cmpq    %r15, (%rax,%rdx)
>         jeq     LBB0_3
>
>         addq    $8, %rcx
>         jmp     LBB0_4
> LBB0_3:
>         leaq    8(%rax,%rdx), %rcx
> LBB0_4:
>
> since that saves me executing a jump 99.9% of the time. Is there anything I
> can do ?
I haven't tried myself, but the documentation claims that
__builtin_expect should work:
http://llvm.org/docs/BranchWeightMetadata.html#builtin-expect

Can you turn your code into an example we can try?

 - Hans

On Tue, Oct 27, 2015 at 10:55 AM, Nat! via llvm-dev <llvm-dev at
lists.llvm.org
> wrote:
> If I read the llvm language correctly, it doesn't have a way to specify
> the preferred branch, correct ? I see nothing in the specs for
"branch" or
> "switch". And __buildin_expect does nothing, that I am sure of.
>
>
Well, you could generate a !prof annotation to specify branch weights that
are then used when computing branch probability and block frequencies (
http://llvm.org/docs/BranchWeightMetadata.html)

Diego.
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Thanks, sounds good. I will try out.

Ciao
   Nat!