llvm dev - Oct 2012 - [LLVMdev] Question on Fence Instruction

Hi,

I have a question with the latest released LLVM which supports Fence Instruction
as IR. Say if I intentionally place a Sequentially Consistent Fence Instruction
somewhere in the code, then would the other transformation passes that applied
later respect the Fence and do not perform any reordering across it?

Going further, if I place an SC Fence immediately after every load/store
instruction, before any opt is done, and then apply those standard compile
transformations, would the result code be SC-preserving? In other words, does
the compiler restrict any reordering for potentially shared accesses that
"protected" by these fences?

Thank you.

Yuelu
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On Tue, Oct 16, 2012 at 01:44:57PM +0000, Duan, Yue Lu
wrote:
> Hi, 
> 
> I have a question with the latest released LLVM which supports Fence
> Instruction as IR. Say if I intentionally place a Sequentially Consistent
Fence
> Instruction somewhere in the code, then would the other transformation
passes
> that applied later respect the Fence and do not perform any reordering
across
> it?
  In theory, all optimization passes should respect sc. If you find any
counter example, I think it's a bug.

HTH,
chenwj

-- 
Wei-Ren Chen (陳韋任)
Computer Systems Lab, Institute of Information Science,
Academia Sinica, Taiwan (R.O.C.)
Tel:886-2-2788-3799 #1667
Homepage: http://people.cs.nctu.edu.tw/~chenwj

Thank you very much for the quick reply. I was trying to confirm what I did was
correct. I did a test that could enable a simple way of sc-preserving
compilation by inserting fences for every load/store instruction before any
opts, applying standard opts and then removing them after assembly code
generation. It turned out that such sc-preserving compilation only caused ~4%
slowdown for 18 benchmarks on average on a Intel Xeon machine. The result
surprised me a lot because it was reported that such naive way of compilation
can cause 20% slowdown in a recent PLDI paper (they also use LLVM), so I posted
this question. I will try to examine if the generated binary code really
respects sc fences.

Yuelu
________________________________________
From: 陳韋任 (Wei-Ren Chen) [chenwj at iis.sinica.edu.tw]
Sent: Wednesday, October 17, 2012 9:00 AM
To: Duan, Yue Lu
Cc: llvmdev at cs.illinois.edu
Subject: Re: [LLVMdev] Question on Fence Instruction

On Tue, Oct 16, 2012 at 01:44:57PM +0000, Duan, Yue Lu
wrote:
> Hi,
>
> I have a question with the latest released LLVM which supports Fence
> Instruction as IR. Say if I intentionally place a Sequentially Consistent
Fence
> Instruction somewhere in the code, then would the other transformation
passes
> that applied later respect the Fence and do not perform any reordering
across
> it?
  In theory, all optimization passes should respect sc. If you find any
counter example, I think it's a bug.

HTH,
chenwj

--
Wei-Ren Chen (陳韋任)
Computer Systems Lab, Institute of Information Science,
Academia Sinica, Taiwan (R.O.C.)
Tel:886-2-2788-3799 #1667
Homepage: http://people.cs.nctu.edu.tw/~chenwj