[llvm-dev] Some questions about software pipeline in LLVM 4.0.0

[Ehsan Amiri via llvm-dev]

Hi Brendon

Certainly, there are some real copies that end up being generated, but I think it's better to exclude the copies from the schedule since most will be eliminated.

I was wondering what was the cause of the real copies that was being generated in your experience? Something that I noticed when experimenting with LLVM on our out-of-tree backend, was that there are copy instructions generated **because of** modulo scheduling.

For example before modulo scheduling I have

%vreg6<def> = PHI %vreg23, <BB#1>, %vreg17
%vreg25<def> = INSN1 %vreg1, %vreg6;
% vreg26<def> = INSN1 %vreg2, %vreg6     <-- same opcode as previous insn
% vreg17<def> = INSN2 %vreg6, %vreg5;

So for the phi node here, if we do phi elimination and register coalescing, we won't have any copy insn left. But after modulo scheduling the instructions above, now appear like this:

%vreg73<def> = PHI %vreg59, <BB#5>, %vreg62, <BB#6>;
%vreg61<def> = INSN1 %vreg1, %vreg73;
%vreg62<def> = INSN2 %vreg73, %vreg5;
%vreg64<def> = INSN1 %vreg2, %vreg73;

Now if you look right after the third insn after modulo scheduling, both vreg73 and vreg62 are live here. So when we remove the corresponding phi instruction, we end up with a copy instruction that cannot be removed by register coalescing.

IIUC, this is a byproduct of modulo scheduling. I have not really started tuning modulo scheduling for our target, so I don't know if this is a result of modulo scheduling not being tuned or not? Have you seen this type of Copy? Any insights are greatly appreciated.

Thanks
Ehsan


From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Brendon Cahoon via llvm-dev
Sent: Wednesday, May 31, 2017 8:00 PM
To: zhangqiang (CO); llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] Some questions about software pipeline in LLVM 4.0.0

Hi - I replied to the original sender only by mistake. Sorry about that.

When we started working on the pipeliner, and added it before the scheduler, we also were concerned that the scheduler or other passes would undo the work of the pipeliner. The initial thought was that we would add information (using metadata or some other way like you've suggested) to the basic block to tell the scheduler not to schedule the block.  It turns out, that for us, we never needed to do so.  It was pretty rare that the scheduler would "undo" the work of the pipeliner. Actually, in the cases that it did, it turned out to be a problem with the scheduler since it wasn't making good decisions.

In general, most of the extra copies that are added by prior to the register allocator are eliminated.  Certainly, there are some real copies that end up being generated, but I think it's better to exclude the copies from the schedule since most will be eliminated.  Otherwise, including the copies in the schedule will require resources that may never be used, which is worse in my opinion.

We decided to run the pipeliner on SSA form since the presence of the Phis helps identify recurrences and other dependences.  Without the Phis, we need another way to identify recurrences.  Also, if it's done just prior to register allocation we need to re-generate the liveness information for all the new virtual registers and CFG. Unfortunately, you're correct - there is a lot of code that deals with Phis. The code that generates the Phis in the swp kernel and epilogs is a mess and very complicated.  This portion of the pipeliner really needs some attention to reduce the complexity and improve readability.  This has been on my list for quite a while.

While I think we could move the location of the pipeliner, I don't think the extra work to do so would provide much benefit. In general, we've been able to work around the cases when extra copies or instructions are added, or when the scheduler messes up the kernel.  Also, for Hexagon, there are many passes that run after the register that deal with scheduling. If you have specific cases where you're seeing a problem, it would be interesting to take a look at them.

Thanks,
Brendon

From: zhangqiang (CO) [mailto:zhangqiang75 at huawei.com]
Sent: Thursday, May 25, 2017 3:33 AM
To: llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
Cc: bcahoon at codeaurora.org<mailto:bcahoon at codeaurora.org>
Subject: Some questions about software pipeline in LLVM 4.0.0

Hi,

I have some questions about the implementation of Software pipeline in MachinePipeliner.cpp.

First, in hexagon backend, between MachinePipeliner and regalloc pass, there're some other passes like phi eliminate, two-address, register coalescing, which may change or insert intructions like 'copy' in MBB, and swp kernel loop may be destroyed by these passes.
Why not put MachinePipeliner just before reg alloc pass like gcc's modulo scheduler does? In order to keep SSA pattern?
I found many codes to process PHI nodes in MachinePipeliner.cpp. So I think if we move MachinePipeliner just before regalloc, it will simplify the data/resource dependency graph for SMS.

Another question, in gcc, there's a flag BB_DISABLE_SCHEDULE in Basic block, which is used by SMS to prevent other schedulers from messing with the loop schedule. So, in llvm , where can I find the similar flag to prevent the machine scheduler touch the kernel loop?
I have debug some swp cases(hexagon), and find machine scheduler will re-schedule the SMS kernel loop. Why not add such a flag?


Best Regards,

Thanks
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