[llvm-dev] simplifycfg not happening?

[Xiangyang Guo via llvm-dev]

Hi,

if you use opt -simplifycfg, the third BB can be eliminated.

2015-09-20 0:03 GMT-04:00 Russell Wallace via llvm-dev <
llvm-dev at lists.llvm.org>:

> The following function compiles with -O3 into the following IR.
> http://llvm.org/docs/Passes.html#simplifycfg-simplify-the-cfg says
>
>    - Eliminates a basic block that only contains an unconditional branch.
>
> but the first and third blocks in the compiled function only contain an
> unconditional branch; I would have expected them to be eliminated. What am
> I missing?
>
> double f(double *a) {
>   for (int i = 0; i < 1000; i++)
>     a[i] *= 2;
>   for (int i = 0; i < 1000; i++)
>     a[i] *= 2;
>   return a[0] + a[1];
> }
>
> ; Function Attrs: nounwind uwtable
> define double @"\01?f@@YANPEAN at Z"(double* nocapture %a) #1 {
> overflow.checked:
>   br label %vector.body, !dbg !18
>
> vector.body:                                      ; preds = %vector.body,
> %overflow.checked
>   %index = phi i64 [ 0, %overflow.checked ], [ %index.next.1, %vector.body
> ], !dbg !18
>   %0 = getelementptr inbounds double, double* %a, i64 %index, !dbg !19
>   %1 = bitcast double* %0 to <2 x double>*, !dbg !20
>   %wide.load = load <2 x double>, <2 x double>* %1, align 8, !dbg !20
>   %2 = getelementptr double, double* %0, i64 2, !dbg !20
>   %3 = bitcast double* %2 to <2 x double>*, !dbg !20
>   %wide.load8 = load <2 x double>, <2 x double>* %3, align 8, !dbg !20
>   %4 = fmul <2 x double> %wide.load, <double 2.000000e+00, double
> 2.000000e+00>, !dbg !20
>   %5 = fmul <2 x double> %wide.load8, <double 2.000000e+00, double
> 2.000000e+00>, !dbg !20
>   %6 = bitcast double* %0 to <2 x double>*, !dbg !20
>   store <2 x double> %4, <2 x double>* %6, align 8, !dbg !20
>   %7 = bitcast double* %2 to <2 x double>*, !dbg !20
>   store <2 x double> %5, <2 x double>* %7, align 8, !dbg !20
>   %index.next = or i64 %index, 4, !dbg !18
>   %8 = getelementptr inbounds double, double* %a, i64 %index.next, !dbg !19
>   %9 = bitcast double* %8 to <2 x double>*, !dbg !20
>   %wide.load.1 = load <2 x double>, <2 x double>* %9, align 8, !dbg !20
>   %10 = getelementptr double, double* %8, i64 2, !dbg !20
>   %11 = bitcast double* %10 to <2 x double>*, !dbg !20
>   %wide.load8.1 = load <2 x double>, <2 x double>* %11, align 8, !dbg !20
>   %12 = fmul <2 x double> %wide.load.1, <double 2.000000e+00, double
> 2.000000e+00>, !dbg !20
>   %13 = fmul <2 x double> %wide.load8.1, <double 2.000000e+00, double
> 2.000000e+00>, !dbg !20
>   %14 = bitcast double* %8 to <2 x double>*, !dbg !20
>   store <2 x double> %12, <2 x double>* %14, align 8, !dbg !20
>   %15 = bitcast double* %10 to <2 x double>*, !dbg !20
>   store <2 x double> %13, <2 x double>* %15, align 8, !dbg !20
>   %index.next.1 = add nsw i64 %index, 8, !dbg !18
>   %16 = icmp eq i64 %index.next.1, 1000, !dbg !18
>   br i1 %16, label %vector.body10.preheader, label %vector.body, !dbg !18,
> !llvm.loop !21
>
> vector.body10.preheader:                          ; preds = %vector.body
>   br label %vector.body10, !dbg !24
>
> vector.body10:                                    ; preds =
> %vector.body10, %vector.body10.preheader
>   %index13 = phi i64 [ 0, %vector.body10.preheader ], [ %index.next21.1,
> %vector.body10 ], !dbg !25
>   %17 = getelementptr inbounds double, double* %a, i64 %index13, !dbg !24
>   %18 = bitcast double* %17 to <2 x double>*, !dbg !26
>   %wide.load26 = load <2 x double>, <2 x double>* %18, align 8, !dbg !26
>   %19 = getelementptr double, double* %17, i64 2, !dbg !26
>   %20 = bitcast double* %19 to <2 x double>*, !dbg !26
>   %wide.load27 = load <2 x double>, <2 x double>* %20, align 8, !dbg !26
>   %21 = fmul <2 x double> %wide.load26, <double 2.000000e+00, double
> 2.000000e+00>, !dbg !26
>   %22 = fmul <2 x double> %wide.load27, <double 2.000000e+00, double
> 2.000000e+00>, !dbg !26
>   %23 = bitcast double* %17 to <2 x double>*, !dbg !26
>   store <2 x double> %21, <2 x double>* %23, align 8, !dbg !26
>   %24 = bitcast double* %19 to <2 x double>*, !dbg !26
>   store <2 x double> %22, <2 x double>* %24, align 8, !dbg !26
>   %index.next21 = or i64 %index13, 4, !dbg !25
>   %25 = getelementptr inbounds double, double* %a, i64 %index.next21, !dbg
> !24
>   %26 = bitcast double* %25 to <2 x double>*, !dbg !26
>   %wide.load26.1 = load <2 x double>, <2 x double>* %26, align 8, !dbg !26
>   %27 = getelementptr double, double* %25, i64 2, !dbg !26
>   %28 = bitcast double* %27 to <2 x double>*, !dbg !26
>   %wide.load27.1 = load <2 x double>, <2 x double>* %28, align 8, !dbg !26
>   %29 = fmul <2 x double> %wide.load26.1, <double 2.000000e+00, double
> 2.000000e+00>, !dbg !26
>   %30 = fmul <2 x double> %wide.load27.1, <double 2.000000e+00, double
> 2.000000e+00>, !dbg !26
>   %31 = bitcast double* %25 to <2 x double>*, !dbg !26
>   store <2 x double> %29, <2 x double>* %31, align 8, !dbg !26
>   %32 = bitcast double* %27 to <2 x double>*, !dbg !26
>   store <2 x double> %30, <2 x double>* %32, align 8, !dbg !26
>   %index.next21.1 = add nsw i64 %index13, 8, !dbg !25
>   %33 = icmp eq i64 %index.next21.1, 1000, !dbg !25
>   br i1 %33, label %middle.block11, label %vector.body10, !dbg !25,
> !llvm.loop !27
>
> middle.block11:                                   ; preds = %vector.body10
>   %34 = load double, double* %a, align 8, !dbg !28
>   %35 = getelementptr inbounds double, double* %a, i64 1, !dbg !29
>   %36 = load double, double* %35, align 8, !dbg !29
>   %37 = fadd double %34, %36, !dbg !30
>   ret double %37, !dbg !31
> }
>
>
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