Dominique Torette via llvm-dev
2018-Jun-20 08:18 UTC
[llvm-dev] Node deletion during DAG Combination ?
Hi,
I'm trying to optimize the 'extract_vector_elt' for my SIMD
microcontroller.
The idea is, during DAG combination, to merge load/extract sequence into an
architecture specific node.
During Instruction Selection, this specific node will be target selected to an
architecture specific instruction.
By 'combination of DAG nodes' I understand 'replacing a set of DAG
nodes by an (usually smaller) set of DAG nodes, removing dead nodes and updating
all the chains'.
I'm using the setTargetDAGCombine(ISD::EXTRACT_VECTOR_ELT) and build the new
LOAD_VECTOR_EXTRACT node in the PerformDAGCombine().
As per the following trace this work, t59 and params from t58 are merged into
t72.
During this combination, the chain from t59 to t58 has been merged from new t72
to t57, skipping over t58.
So far, so good.
My problem is that this node t58 is not removed from DAG. One reason could be
that node t62 is still chained to t58 (due to the fact the extract_vector_elt
has no chain).
(I also have to investigate why the t57 and t58 are duplicated... This should be
another problem).
I've take a look at other DAGCombine implementation for others architecture
but I didn't found an API to explicitly remove a node and trigger overall
chain update.
Could someone point out an example of API for such node deletion during DAG
Combination?
Regards, Dominique T.
t56: ch = store<Volatile ST8[%l2]> t54:1, t55,
FrameIndex:i16<1>, undef:i16
t57: v2f32,ch = load<Volatile LD8[%l1]> t56, FrameIndex:i16<0>,
undef:i16
t58: v2f32,ch = load<Volatile LD8[%l1]> t57:1, FrameIndex:i16<0>,
undef:i16
t59: f32 = extract_vector_elt t58, Constant:i16<0>
t62: ch = llvm.clp.writeapb.f32 t58:1,
TargetConstant:i16<397>, Constant:i32<24575>, t59
Combining: t59: f32 = extract_vector_elt t58, Constant:i16<0>
... into: t72: f32 = CLPISD::LOAD_VECTOR_EXTRACT_o t57:1,
FrameIndex:i16<0>, Constant:i16<0>
Combining: t56: ch = store<Volatile ST8[%l2]> t54:1, t55,
FrameIndex:i16<1>, undef:i16
... into: t73: ch = CLPISD::STORE_VECTOR_INSERT_oo t53:1,
FrameIndex:i16<1>, t53, Constant:i16<1>
t73: ch = CLPISD::STORE_VECTOR_INSERT_oo t53:1, FrameIndex:i16<1>,
t53, Constant:i16<1>
t57: v2f32,ch = load<Volatile LD8[%l1]> t73, FrameIndex:i16<0>,
undef:i16
t58: v2f32,ch = load<Volatile LD8[%l1]> t57:1,
FrameIndex:i16<0>, undef:i16
t72: f32 = CLPISD::LOAD_VECTOR_EXTRACT_o t57:1,
FrameIndex:i16<0>, Constant:i16<0>
t62: ch = llvm.clp.writeapb.f32 t58:1,
TargetConstant:i16<397>, Constant:i32<24575>, t72
t73: ch = MOVATO_B_oo t53, TargetFrameIndex:i16<1>, t53:1
t57: v2f32,ch = LOAD_AB_o TargetFrameIndex:i16<0>, t73
t58: v2f32,ch = LOAD_AB_o TargetFrameIndex:i16<0>, t57:1
t61: i32,ch = LOAD_A_i TargetConstant:i32<24575>,
t58:1
t72: f32 = LOAD_A_o TargetFrameIndex:i16<0>, t57:1
t62: ch = WRITEAPB_A_oo t61, t72, t58:1
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Dominique Torette
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Tim Northover via llvm-dev
2018-Jun-20 09:12 UTC
[llvm-dev] Node deletion during DAG Combination ?
Hi Dominique, On Wed, 20 Jun 2018 at 09:18, Dominique Torette via llvm-dev <llvm-dev at lists.llvm.org> wrote:> The idea is, during DAG combination, to merge load/extract sequence into an architecture specific node.Interesting. My first thought would be to do that during instruction selection rather than as a combine.> My problem is that this node t58 is not removed from DAG. One reason could be that node t62 is still chained to t58 (due to the fact the extract_vector_elt has no chain).Yep, you never need to actually remove nodes from the DAG yourself, but if you're replacing one you have to make sure it's a real orphan afterwards -- no uses accessible from the root of the DAG. So in your case you need to make sure that not only all uses of the extracted element are replaced, but all users of the chain attached to the load.> Combining: t59: f32 = extract_vector_elt t58, Constant:i16<0> > ... into: t72: f32 = CLPISD::LOAD_VECTOR_EXTRACT_o t57:1, FrameIndex:i16<0>, Constant:i16<0>Here it looks like you're not creating your LOAD_VECTOR_EXTRACT to integrate into the chain in the first place. Even ignoring the replacement issue loads need to take and produce an extra chain argument to enforce correct ordering. So there are 3 steps: 1. Add the incoming chain of the load you highlighted as an operand to this new combined node. 2. Create the combined node with an output chain too (essentially just an extra MVT::Other output value). 3. Manually call SelectionDAG::ReplaceAllUsesWith to replace all uses of the load node's chain with this new one because the generic combining logic only knows about the extract being combined. It can't automatically tell you've also fiddled about with a load further up the DAG. That'll leave the old load with no uses in the DAG and it'll automatically be cleaned up later. Cheers. Tim.
Dominique Torette via llvm-dev
2018-Jun-20 14:09 UTC
[llvm-dev] Node deletion during DAG Combination ?
Tim wrote:> -----Original Message----- > From: Tim Northover [mailto:t.p.northover at gmail.com] > Sent: mercredi 20 juin 2018 11:13 > To: Dominique Torette > Cc: LLVM Developers Mailing List > Subject: Re: [llvm-dev] Node deletion during DAG Combination ? > > So there are 3 steps: > > 1. Add the incoming chain of the load you highlighted as an operand to > this new combined node. > 2. Create the combined node with an output chain too (essentially just > an extra MVT::Other output value).Steps 1 and 2 have been implemented. t72 has now Chain output. t73: ch = CLPISD::STORE_VECTOR_INSERT_oo t53:1, FrameIndex:i16<1>, t53, Constant:i16<1> t57: v2f32,ch = load<Volatile LD8[%l1]> t73, FrameIndex:i16<0>, undef:i16 t58: v2f32,ch = load<Volatile LD8[%l1]> t57:1, FrameIndex:i16<0>, undef:i16 t72: f32,ch = CLPISD::LOAD_VECTOR_EXTRACT_o t57:1, FrameIndex:i16<0>, Constant:i16<0> t62: ch = llvm.clp.writeapb.f32 t58:1, TargetConstant:i16<397>, Constant:i32<24575>, t72> 3. Manually call SelectionDAG::ReplaceAllUsesWith to replace all uses > of the load node's chain with this new one because the generic > combining logic only knows about the extract being combined. It can't > automatically tell you've also fiddled about with a load further up > the DAG.SelectionDAG::ReplaceAllUsesWith() was the API I was looking for… But I have a problem with the 3 implementations of the SelectionDAG::ReplaceAllUsesWith(). They checks somehow the congruency of the returned values types…. t58 returns first a v2f32 while t72 return first a f32 ! This raises assertion. ☹ I understand such type constraint for general substitution, but in this case the only reference to t58 is the chain from t62. ------------------------------------------------------------------------------ E-MAIL DISCLAIMER The present message may contain confidential and/or legally privileged information. If you are not the intended addressee and in case of a transmission error, please notify the sender immediately and destroy this E-mail. Disclosure, reproduction or distribution of this document and its possible attachments is strictly forbidden. SPACEBEL denies all liability for incomplete, improper, inaccurate, intercepted, (partly) destroyed, lost and/or belated transmission of the current information given that unencrypted electronic transmission cannot currently be guaranteed to be secure or error free. Upon request or in conformity with formal, contractual agreements, an originally signed hard copy will be sent to you to confirm the information contained in this E-mail. SPACEBEL denies all liability where E-mail is used for private use. SPACEBEL cannot be held responsible for possible viruses that might corrupt this message and/or your computer system. ------------------------------------------------------------------------------- -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20180620/a93ee146/attachment.html>