[llvm-dev] OPC_EmitMergeInputChains1_0 failed while lowering callseq_start

Fri Sep 8 06:27:26 PDT 2017

It seems that the checks performed at the beginning of WalkChainUsers() make the hypothesis that at least one parameters.
These  checks make use of ChainedNode->use_begin. If parameters are used this iterator points to CopyToReg node of the first parameter.
If no parameter, this iterator points to the call node and return InducesCycle error code.

Here is a source code with no parameter that reproduces the problem.

declare void @foo32_2()

define i32 @calli32_stack() {
    call void @foo32_2()
    ret i32 0
}

In using LLVM version 3.8, this problem affects most the architecture that I've investigated.

From: Dominique Torette
Sent: mercredi 6 septembre 2017 18:15
To: llvm-dev at lists.llvm.org
Cc: Roel Jordans (r.jordans at tue.nl)
Subject: OPC_EmitMergeInputChains1_0 failed while lowering callseq_start

I'm lowering the 'callseq_start' and 'callseq_end' using nodes/patterns and the SelectCodeCommon().

'callseq_end' is well morphed into an architecture specific ADJCALLSTACKUP PseudoInstruction.

ISEL: Starting pattern match on root node: t9: ch,glue = callseq_end t8, TargetConstant:i16<0>, TargetConstant:i16<0>, t8:1

  Morphed node: t9: ch,glue = ADJCALLSTACKUP TargetConstant:i16<0>, TargetConstant:i16<0>, t8, t8:1

'callseq_start' failed to be morphed into ADJCALLSTACKDOWN :-/

LLVM ERROR: Cannot select: t6: ch,glue = callseq_start t0, TargetConstant:i16<0>
  t5: i16 = TargetConstant<0>

Here is the debug trace of the matching. It should match at 280, but failed at 279 on OPC_EmitMergeInputChains1_0.

ISEL: Starting pattern match on root node: t6: ch,glue = callseq_start t0, TargetConstant:i16<0>

  Skipped scope entry (due to false predicate) at index 2, continuing at 30
  Skipped scope entry (due to false predicate) at index 31, continuing at 104
  Skipped scope entry (due to false predicate) at index 105, continuing at 183
  Skipped scope entry (due to false predicate) at index 184, continuing at 225
  Skipped scope entry (due to false predicate) at index 226, continuing at 267
  Match failed at index 279
  Continuing at 287
  Match failed at index 288
  Continuing at 311

/*267*/     /*Scope*/ 19, /*->287*/
/*268*/       OPC_CheckOpcode, TARGET_VAL(ISD::CALLSEQ_START),
/*271*/       OPC_RecordNode, // #0 = 'callseq_start' chained node
/*272*/       OPC_RecordChild1, // #1 = $amt
/*273*/       OPC_MoveChild, 1,
/*275*/       OPC_CheckOpcode, TARGET_VAL(ISD::TargetConstant),
/*278*/       OPC_MoveParent,
/*279*/       OPC_EmitMergeInputChains1_0,
/*280*/       OPC_MorphNodeTo, TARGET_VAL(CLP::ADJCALLSTACKDOWN), 0|OPFL_Chain|OPFL_GlueOutput,
                  0/*#VTs*/, 1/*#Ops*/, 1,
              // Src: (callseq_start (timm:i16):$amt) - Complexity = 6
              // Dst: (ADJCALLSTACKDOWN (timm:i16):$amt)
/*287*/     /*Scope*/ 23, /*->311*/

Using GDB, I've traced the reason of the failure:
SelectCodeCommon() failed because Case OPC_EmitMergeInputChains1_0 'Failed to merge' because HandleMergeInputChains() returns CR_InducesCycle because  WalkChainUsers() returns CR_InducesCycle;
      // Because we structurally match the load/store as a read/modify/write,
      // but the call is chained between them.  We cannot fold in this case
      // because it would induce a cycle in the graph.

What's the meaning of such status? How to fix the following pattern or the Instruction Selection Graph?

Here are the pattern/nodes definitions:

            def SDT_CLPCallSeqStart : SDCallSeqStart<[ SDTCisVT<0, i16> ]>;

def callseq_start : SDNode<"ISD::CALLSEQ_START", SDT_CLPCallSeqStart,
                           [SDNPHasChain, SDNPOutGlue, SDNPSideEffect]>;

def ADJCALLSTACKDOWN       : CLPPseudoInst<(ins i16imm:$amt), (outs), [],
                                   "# ADJCALLSTACKDOWN $amt",
                                   [(callseq_start timm:$amt)],NoItinerary>;

[cid:image001.png at 01D3289B.AE202320]

[http://www.spacebel.be/wp-content/uploads/2011/06/image-sign-sbp.jpg]

Dominique Torette
System Architect
Rue des Chasseurs Ardennais - Liège Science Park - B-4031 Angleur
Tel: +32 (0) 4 361 81 11 - Fax: +32 (0) 4 361 81 20
www.spacebel.be<http://www.spacebel.be/>

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