[LLVMdev] Assert in live update from MI scheduler.

Sergei Larin slarin at codeaurora.org
Wed Jun 13 13:15:06 PDT 2012 

Andy, 

 

  You are probably right here - look at this - before phi elimination this
code looks much more sane:

 

# *** IR Dump After Live Variable Analysis ***:

# Machine code for function push: SSA

Function Live Outs: %R0

 

BB#0: derived from LLVM BB %entry

    %vreg5<def> = IMPLICIT_DEF; IntRegs:%vreg5

    %vreg4<def> = TFRI_V4 <ga:@xx_stack>; IntRegs:%vreg4

    Successors according to CFG: BB#1

 

BB#1: derived from LLVM BB %for.cond

    Predecessors according to CFG: BB#0 BB#1

    %vreg0<def> = PHI %vreg4, <BB#0>, %vreg3, <BB#1>;
IntRegs:%vreg0,%vreg4,%vreg3

    %vreg1<def> = PHI %vreg5, <BB#0>, %vreg2, <BB#1>;
IntRegs:%vreg1,%vreg5,%vreg2

    %vreg2<def> = LDriw %vreg0<kill>, 0; mem:LD4[%stack.0.in]
IntRegs:%vreg2,%vreg0

    %vreg3<def> = ADD_ri %vreg2, 8; IntRegs:%vreg3,%vreg2

    %vreg6<def> = CMPEQri %vreg2, 0; PredRegs:%vreg6 IntRegs:%vreg2

    JMP_cNot %vreg6<kill>, <BB#1>, %PC<imp-def>; PredRegs:%vreg6

    JMP <BB#2>

    Successors according to CFG: BB#2 BB#1

 

BB#2: derived from LLVM BB %for.end

    Predecessors according to CFG: BB#1

    %vreg7<def> = LDriw %vreg1<kill>, 0; mem:LD4[%first1](tbaa=!"any
pointer") IntRegs:%vreg7,%vreg1

    STriw_GP <ga:@yy_instr>, 0, %vreg7<kill>; mem:ST4[@yy_instr](tbaa=!"any
pointer") IntRegs:%vreg7

    %vreg8<def> = IMPLICIT_DEF; IntRegs:%vreg8

    %R0<def> = COPY %vreg8<kill>; IntRegs:%vreg8

    JMPR %PC<imp-def>, %R31<imp-use>, %R0<imp-use,kill>

 

Right after the dead vreg is introduced:

 

# *** IR Dump After Eliminate PHI nodes for register allocation ***:

# Machine code for function push: Post SSA

Function Live Outs: %R0

 

BB#0: derived from LLVM BB %entry

    %vreg4<def> = TFRI_V4 <ga:@xx_stack>; IntRegs:%vreg4

    %vreg9<def> = COPY %vreg4<kill>; IntRegs:%vreg9,%vreg4

    Successors according to CFG: BB#1

 

BB#1: derived from LLVM BB %for.cond

    Predecessors according to CFG: BB#0 BB#1

    %vreg0<def> = COPY %vreg9<kill>; IntRegs:%vreg0,%vreg9

    %vreg1<def> = COPY %vreg10<kill>; IntRegs:%vreg1,%vreg10
<<<<<<<<<<<<<<<<<<<<<<<<<<< Not defined on first iteration.. 

    %vreg2<def> = LDriw %vreg0<kill>, 0; mem:LD4[%stack.0.in]
IntRegs:%vreg2,%vreg0

    %vreg3<def> = ADD_ri %vreg2, 8; IntRegs:%vreg3,%vreg2

    %vreg6<def> = CMPEQri %vreg2, 0; PredRegs:%vreg6 IntRegs:%vreg2

    %vreg9<def> = COPY %vreg3<kill>; IntRegs:%vreg9,%vreg3

    %vreg10<def> = COPY %vreg2<kill>; IntRegs:%vreg10,%vreg2

    JMP_cNot %vreg6<kill>, <BB#1>, %PC<imp-def>; PredRegs:%vreg6

    JMP <BB#2>

    Successors according to CFG: BB#2 BB#1

 

BB#2: derived from LLVM BB %for.end

    Predecessors according to CFG: BB#1

    %vreg7<def> = LDriw %vreg1<kill>, 0; mem:LD4[%first1](tbaa=!"any
pointer") IntRegs:%vreg7,%vreg1

    STriw_GP <ga:@yy_instr>, 0, %vreg7<kill>; mem:ST4[@yy_instr](tbaa=!"any
pointer") IntRegs:%vreg7

    %vreg8<def> = IMPLICIT_DEF; IntRegs:%vreg8

    %R0<def> = COPY %vreg8<kill>; IntRegs:%vreg8

    JMPR %PC<imp-def>, %R31<imp-use>, %R0<imp-use,kill>

 

# End machine code for function push.

 

So the problem is elsewhere after all. 

 

I'll keep on digging. Thanks.

 

Sergei

 

--

Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum.

 

From: Andrew Trick [mailto:atrick at apple.com] 
Sent: Wednesday, June 13, 2012 1:39 PM
To: Sergei Larin; Jakob Olesen
Cc: llvmdev at cs.uiuc.edu List
Subject: Re: [LLVMdev] Assert in live update from MI scheduler.

 

On Jun 13, 2012, at 10:49 AM, Sergei Larin <slarin at codeaurora.org> wrote:

So if this early exit is taken:

 // SSA defs do not have output/anti dependencies.
 // The current operand is a def, so we have at least one.
 if (llvm::next(MRI.def_begin(Reg)) == MRI.def_end())
   return;

we do not ever get to this point:

  VRegDefs.insert(VReg2SUnit(Reg, SU));

But later, when checking for anti dependency for another MI here:

void ScheduleDAGInstrs::addVRegUseDeps(SUnit *SU, unsigned OperIdx) {
...
 // Add antidependence to the following def of the vreg it uses.
 VReg2SUnitMap::iterator DefI = VRegDefs.find(Reg);
 if (DefI != VRegDefs.end() && DefI->SU != SU)
   DefI->SU->addPred(SDep(SU, SDep::Anti, 0, Reg));

We will never find that def in VRegDefs.find(Reg) even though it exists.

I know this has been working for a while, but I am still missing something
here.
What is this statement 

if (llvm::next(MRI.def_begin(Reg)) == MRI.def_end())

should guarantee? From it there must be more than one definition in MRI.def
for that reg for it to work... 




To connect it to the original example... When parsing (BU order) this
instruction:

SU(1):   %vreg10<def> = LDriw %vreg9<kill>, 0; mem:LD4[%stack.0.in]

The %vreg10<def> never inserted to VRegDefs, so with next instruction:

SU(0):   %vreg1<def> = COPY %vreg10<kill>; IntRegs:%vreg1,%vreg10

Anti dep on %vreg10 is never created.

 

 

Thanks for the detailed explanation! My understanding is that COPY
%vreg10<kill> is illegal because is has no reaching def on all paths (LDriw
is the only def). 

 

Now, the (llvm::next(MRI.def_begin(Reg)) == MRI.def_end()) check is not
really sufficient to test SSA in the presence of undefined operands.
However, I thought we would have an IMPLICIT_DEF of the vreg in that case,
even after the phi is removed. That right Jakob? Otherwise we I think we
should somehow mark the vreg as being undefined.

 

Anyway, I added an assert to catch this problem (see below), and it never
triggered on X86. So my guess is that you have incorrect IR coming in. Can
you check where %vreg10 is defined. Before coalescing, was it a phi with an
<undef> operand?

 

It is safe to workaround the problem by removing the early exit following
the SSA check.

 

-Andy

 

--- a/lib/CodeGen/ScheduleDAGInstrs.cpp

+++ b/lib/CodeGen/ScheduleDAGInstrs.cpp

@@ -413,9 +413,11 @@ void ScheduleDAGInstrs::addVRegDefDeps(SUnit *SU,
unsigned OperIdx) {

 

   // SSA defs do not have output/anti dependencies.

   // The current operand is a def, so we have at least one.

-  if (llvm::next(MRI.def_begin(Reg)) == MRI.def_end())

+  if (llvm::next(MRI.def_begin(Reg)) == MRI.def_end()) {

+    //!!!

+    VRegDefs.insert(VReg2SUnit(Reg, SU));

     return;

-

+  }

   // Add output dependence to the next nearest def of this vreg.

   //

   // Unless this definition is dead, the output dependence should be

@@ -479,8 +481,10 @@ void ScheduleDAGInstrs::addVRegUseDeps(SUnit *SU,
unsigned OperIdx) {

 

   // Add antidependence to the following def of the vreg it uses.

   VReg2SUnitMap::iterator DefI = VRegDefs.find(Reg);

-  if (DefI != VRegDefs.end() && DefI->SU != SU)

+  if (DefI != VRegDefs.end() && DefI->SU != SU) {

+    assert(llvm::next(MRI.def_begin(Reg)) != MRI.def_end() && "SINGLEDEF");

     DefI->SU->addPred(SDep(SU, SDep::Anti, 0, Reg));

+  }

 

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