[llvm-dev] [StructurizeCFG] Trouble with branches out of a loop
Justin Holewinski via llvm-dev
llvm-dev at lists.llvm.org
Mon Nov 2 09:43:09 PST 2015
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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