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Justin Holewinski via llvm-dev

2015-Nov-02 17:14 UTC

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

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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Tom Stellard via llvm-dev

2015-Nov-02 17:26 UTC

head link

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

+ Christian

On Mon, Nov 02, 2015 at 12:14:46PM -0500, Justin Holewinski via llvm-dev
wrote:> 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?
> 
I'm not aware of any limitations, so this is a bug.  Can you file one at
llvm.org/bugs.

-Tom
> 
> 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
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

Justin Holewinski via llvm-dev

2015-Nov-02 17:43 UTC

head link

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

https://llvm.org/bugs/show_bug.cgi?id=25378

Thanks!

On Mon, Nov 2, 2015 at 12:26 PM, Tom Stellard <tom at stellard.net> wrote:
> + Christian
>
> On Mon, Nov 02, 2015 at 12:14:46PM -0500, Justin Holewinski via llvm-dev
> wrote:
> > 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?
> >
>
> I'm not aware of any limitations, so this is a bug.  Can you file one
at
> llvm.org/bugs.
>
> -Tom
>
> >
> > 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
>
>
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>

-- 

Thanks,

Justin Holewinski
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