[llvm-dev] [StructurizeCFG] Trouble with branches out of a loop

[Justin Holewinski via llvm-dev]

Hi,

I've been investigating the StructurizeCFG pass, and it looks like it has
trouble handling CFG edges that break out of a loop and go directly to the
function exit.  Am I running up against a bug in the structurizer, or a
general limitation of the algorithm used?  As an aside, is there any
documentation for the algorithm used?  Is it based on a published paper?


The input IR I have is the following:

define <4 x float> @structurizer_test(<4 x float> %inp.coerce) {
  %1 = extractelement <4 x float> %inp.coerce, i32 0
  %2 = fcmp ogt float %1, 0.000000e+00
  br i1 %2, label %.lr.ph.i, label %._crit_edge.i

.lr.ph.i:                                         ; preds = %7, %0
  %i.03.i = phi float [ %8, %7 ], [ 0.000000e+00, %0 ]
  %ret.02.i = phi <4 x float> [ %5, %7 ], [ <float 1.000000e+00, float
1.000000e+00, float 1.000000e+00, float 1.000000e+00>, %0 ]
  %3 = extractelement <4 x float> %ret.02.i, i32 0
  %4 = fadd fast float %3, 0xBFB99999A0000000
  %5 = insertelement <4 x float> %ret.02.i, float %4, i32 0
  %6 = fcmp olt float %4, 5.000000e-01
  br i1 %6, label %_Z9get_colorDv2_f.exit, label %7

; <label>:7                                       ; preds = %.lr.ph.i
  %8 = fadd fast float %i.03.i, 1.000000e+01
  %9 = fcmp olt float %8, %1
  br i1 %9, label %.lr.ph.i, label %._crit_edge.i

._crit_edge.i:                                    ; preds = %7, %0
  %ret.0.lcssa.i = phi <4 x float> [ <float 1.000000e+00, float
1.000000e+00, float 1.000000e+00, float 1.000000e+00>, %0 ], [ %5, %7 ]
  %10 = insertelement <4 x float> %ret.0.lcssa.i, float 0.000000e+00, i32 2
  br label %_Z9get_colorDv2_f.exit

_Z9get_colorDv2_f.exit:                           ; preds = %._crit_edge.i,
%.lr.ph.i
  %.0.i = phi <4 x float> [ %10, %._crit_edge.i ], [ %5, %.lr.ph.i ]
  ret <4 x float> %.0.i
}

After structurization, I have a module that has what looks like a
reasonable CFG, but bad branch conditions and PHIs:

define <4 x float> @structurizer_test(<4 x float> %inp.coerce) {
  %1 = extractelement <4 x float> %inp.coerce, i32 0
  %2 = fcmp ogt float %1, 0.000000e+00
  %3 = xor i1 %2, true
  br label %Flow

Flow:                                             ; preds = %Flow1, %0
  %4 = phi <4 x float> [ %14, %Flow1 ], [ <float 1.000000e+00, float
1.000000e+00, float 1.000000e+00, float 1.000000e+00>, %0 ]
  %5 = phi <4 x float> [ %16, %Flow1 ], [ <float 1.000000e+00, float
1.000000e+00, float 1.000000e+00, float 1.000000e+00>, %0 ]
  %6 = phi float [ %17, %Flow1 ], [ 0.000000e+00, %0 ]
  %7 = phi i1 [ %18, %Flow1 ], [ %3, %0 ]
  %8 = phi i1 [ false, %Flow1 ], [ %2, %0 ]
  br i1 %8, label %.lr.ph.i, label %Flow1

.lr.ph.i:                                         ; preds = %Flow
  %i.03.i = phi float [ %6, %Flow ]
  %ret.02.i = phi <4 x float> [ %5, %Flow ]
  %9 = extractelement <4 x float> %ret.02.i, i32 0
  %10 = fadd fast float %9, 0xBFB99999A0000000
  %11 = insertelement <4 x float> %ret.02.i, float %10, i32 0
  %12 = fcmp olt float %10, 5.000000e-01
  %13 = xor i1 %12, true
  br i1 %13, label %19, label %Flow2

Flow1:                                            ; preds = %Flow2, %Flow
  %14 = phi <4 x float> [ %23, %Flow2 ], [ %4, %Flow ]
  %15 = phi <4 x float> [ %11, %Flow2 ], [ *undef*, %Flow ]
  %16 = phi <4 x float> [ %24, %Flow2 ], [ %5, %Flow ]
  %17 = phi float [ %25, %Flow2 ], [ %6, %Flow ]
  %18 = phi i1 [ %26, %Flow2 ], [ %7, %Flow ]
  br i1 *true*, label %Flow3, label %Flow

; <label>:19                                      ; preds = %.lr.ph.i
  %20 = fadd fast float %i.03.i, 1.000000e+01
  %21 = fcmp olt float %20, %1
  %22 = xor i1 %21, true
  br label %Flow2

Flow2:                                            ; preds = %19, %.lr.ph.i
  %23 = phi <4 x float> [ %11, %19 ], [ %4, %.lr.ph.i ]
  %24 = phi <4 x float> [ %11, %19 ], [ *undef*, %.lr.ph.i ]
  %25 = phi float [ %20, %19 ], [ undef, %.lr.ph.i ]
  %26 = phi i1 [ %22, %19 ], [ %7, %.lr.ph.i ]
  br label %Flow1

Flow3:                                            ; preds = %Flow1
  br i1 %18, label %._crit_edge.i, label %_Z9get_colorDv2_f.exit

._crit_edge.i:                                    ; preds = %Flow3
  %ret.0.lcssa.i = phi <4 x float> [ %14, %Flow3 ]
  %27 = insertelement <4 x float> %ret.0.lcssa.i, float 0.000000e+00, i32 2
  br label %_Z9get_colorDv2_f.exit

_Z9get_colorDv2_f.exit:                           ; preds = %._crit_edge.i,
%Flow3
  %.0.i = phi <4 x float> [ %15, %Flow3 ], [ %27, %._crit_edge.i ]
  ret <4 x float> %.0.i
}

Note the undef values in some of the PHIs and 'i1 true' for the loop branch
condition.



-- 

Thanks,

Justin Holewinski
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