[PATCH] Make bitcast, extractelement, and insertelement considered cheap for speculation in SimplifyCFG.

[Hal Finkel]

----- Original Message -----
> From: "Matt Arsenault" <Matthew.Arsenault at amd.com>
> To: "Nadav Rotem" <nrotem at apple.com>, reviews+D2218+public+1b17721e58b4714d at llvm-reviews.chandlerc.com
> Cc: "llvm commits" <llvm-commits at cs.uiuc.edu>
> Sent: Tuesday, November 19, 2013 1:12:01 PM
> Subject: Re: [PATCH] Make bitcast, extractelement, and insertelement considered	cheap for speculation in SimplifyCFG.
> 
> On 11/19/2013 08:42 AM, Nadav Rotem wrote:
> > Hi Matt,
> >
> > I don’t know this code very well but your patch looks okay to me.
> >   Please benchmark the LLVM test suite and make sure that this
> > change does not introduce new regressions.
> >
> > SimplifyCFG should not use TTI because this pass is executed very
> > early in the optimization pipe.  TTI should only be used by
> > “lowering” transformations in the late stages of the optimization
> > pipe.
> But it is included in SimplifyCFG, and it is used in a few places
> there.
> Are these supposed to be removed at some point?

No, TTI is used in a few other places in order to check for "free" operations. We just don't use the costs until later in the pipeline.

 -Hal

> 
> > Thanks,
> > Nadav
> >
> >
> > On Nov 18, 2013, at 7:00 PM, Matt Arsenault
> > <Matthew.Arsenault at amd.com> wrote:
> >
> >> Hi nadav,
> >>
> >> This help fold more branches into selects.
> >> On R600, vectors are cheap and anything that helps
> >> remove branches is very good.
> >>
> >> I don't know why this doesn't use the TTI for this cost
> >> calculation or if it should.
> >>
> >> http://llvm-reviews.chandlerc.com/D2218
> >>
> >> Files:
> >>   lib/Transforms/Utils/SimplifyCFG.cpp
> >>   test/Transforms/SimplifyCFG/speculate-vector-ops.ll
> >>
> >> Index: lib/Transforms/Utils/SimplifyCFG.cpp
> >> ===================================================================
> >> --- lib/Transforms/Utils/SimplifyCFG.cpp
> >> +++ lib/Transforms/Utils/SimplifyCFG.cpp
> >> @@ -224,6 +224,9 @@
> >>    case Instruction::Trunc:
> >>    case Instruction::ZExt:
> >>    case Instruction::SExt:
> >> +  case Instruction::BitCast:
> >> +  case Instruction::ExtractElement:
> >> +  case Instruction::InsertElement:
> >>      return 1; // These are all cheap.
> >>
> >>    case Instruction::Call:
> >> Index: test/Transforms/SimplifyCFG/speculate-vector-ops.ll
> >> ===================================================================
> >> --- /dev/null
> >> +++ test/Transforms/SimplifyCFG/speculate-vector-ops.ll
> >> @@ -0,0 +1,60 @@
> >> +; RUN: opt -S -simplifycfg < %s | FileCheck %s
> >> +
> >> +define i32 @speculate_vector_extract(i32 %d, <4 x i32> %v) #0 {
> >> +; CHECK-LABEL: @speculate_vector_extract(
> >> +; CHECK-NOT: br
> >> +entry:
> >> +  %conv = insertelement <4 x i32> undef, i32 %d, i32 0
> >> +  %conv2 = insertelement <4 x i32> %conv, i32 %d, i32 1
> >> +  %conv3 = insertelement <4 x i32> %conv2, i32 %d, i32 2
> >> +  %conv4 = insertelement <4 x i32> %conv3, i32 %d, i32 3
> >> +  %tmp6 = add nsw <4 x i32> %conv4, <i32 0, i32 -1, i32 -2, i32
> >> -3>
> >> +  %cmp = icmp eq <4 x i32> %tmp6, zeroinitializer
> >> +  %cmp.ext = sext <4 x i1> %cmp to <4 x i32>
> >> +  %tmp8 = extractelement <4 x i32> %cmp.ext, i32 0
> >> +  %tobool = icmp eq i32 %tmp8, 0
> >> +  br i1 %tobool, label %cond.else, label %cond.then
> >> +
> >> +return:                                           ; preds =
> >> %cond.end28
> >> +  ret i32 %cond32
> >> +
> >> +cond.then:                                        ; preds =
> >> %entry
> >> +  %tmp10 = extractelement <4 x i32> %v, i32 0
> >> +  br label %cond.end
> >> +
> >> +cond.else:                                        ; preds =
> >> %entry
> >> +  %tmp12 = extractelement <4 x i32> %v, i32 3
> >> +  br label %cond.end
> >> +
> >> +cond.end:                                         ; preds =
> >> %cond.else, %cond.then
> >> +  %cond = phi i32 [ %tmp10, %cond.then ], [ %tmp12, %cond.else ]
> >> +  %tmp14 = extractelement <4 x i32> %cmp.ext, i32 1
> >> +  %tobool15 = icmp eq i32 %tmp14, 0
> >> +  br i1 %tobool15, label %cond.else17, label %cond.then16
> >> +
> >> +cond.then16:                                      ; preds =
> >> %cond.end
> >> +  %tmp20 = extractelement <4 x i32> %v, i32 1
> >> +  br label %cond.end18
> >> +
> >> +cond.else17:                                      ; preds =
> >> %cond.end
> >> +  br label %cond.end18
> >> +
> >> +cond.end18:                                       ; preds =
> >> %cond.else17, %cond.then16
> >> +  %cond22 = phi i32 [ %tmp20, %cond.then16 ], [ %cond,
> >> %cond.else17 ]
> >> +  %tmp24 = extractelement <4 x i32> %cmp.ext, i32 2
> >> +  %tobool25 = icmp eq i32 %tmp24, 0
> >> +  br i1 %tobool25, label %cond.else27, label %cond.then26
> >> +
> >> +cond.then26:                                      ; preds =
> >> %cond.end18
> >> +  %tmp30 = extractelement <4 x i32> %v, i32 2
> >> +  br label %cond.end28
> >> +
> >> +cond.else27:                                      ; preds =
> >> %cond.end18
> >> +  br label %cond.end28
> >> +
> >> +cond.end28:                                       ; preds =
> >> %cond.else27, %cond.then26
> >> +  %cond32 = phi i32 [ %tmp30, %cond.then26 ], [ %cond22,
> >> %cond.else27 ]
> >> +  br label %return
> >> +}
> >> +
> >> +attributes #0 = { nounwind }
> >> <D2218.1.patch>
> >
> 
> 
> 
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-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory