[llvm-dev] How can I tell llvm, that a branch is preferred ?

[Chen Li via llvm-dev]

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!
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