[PATCH] [SLPVectorizer] Reorder operands of shufflevector if it can result in a vectorized code.

[Michael Zolotukhin]

Hi Karthik,

While the loads widening is a real problem, the example I wrote in previous comment doesn't need any GVN invocation at all - you can run slp (+basicaa) on it and see that SLP fails to vectorize it. To make it even clearer, we can use `double` instead of `i32`:

1. **Vectorized**:

Original code:

  double a[1000], b[1000], c[1000];
  void foo()
  {
    c[0] = a[0] + b[0];
    c[1] = a[1] + b[1];
  }

IR:

  define void @foo() #0 {
    %1 = load double* getelementptr inbounds ([1000 x double]* @a, i32 0, i64 0), align 4
    %2 = load double* getelementptr inbounds ([1000 x double]* @b, i32 0, i64 0), align 4
    %3 = fadd double %1, %2
    store double %3, double* getelementptr inbounds ([1000 x double]* @c, i32 0, i64 0), align 4
    %4 = load double* getelementptr inbounds ([1000 x double]* @a, i32 0, i64 1), align 4
    %5 = load double* getelementptr inbounds ([1000 x double]* @b, i32 0, i64 1), align 4
    %6 = fadd double %4, %5
    store double %6, double* getelementptr inbounds ([1000 x double]* @c, i32 0, i64 1), align 4
    ret void
  }

IR after SLP:

  define void @foo() #0 {
    %1 = load <2 x double>* bitcast ([1000 x double]* @a to <2 x double>*), align 16, !tbaa !2
    %2 = load <2 x double>* bitcast ([1000 x double]* @b to <2 x double>*), align 16, !tbaa !2
    %3 = fadd <2 x double> %1, %2
    store <2 x double> %3, <2 x double>* bitcast ([1000 x double]* @c to <2 x double>*), align 16, !tbaa !2
    ret void
  }



2. **Not vectorized**:

Original code:

  double a[1000], b[1000], c[1000];
  void foo()
  {
    c[0] = a[0] + b[0];
    c[1] = b[1] + a[1]; // a[1] and b[1] are swapped
  }

IR:

  define void @foo() #0 {
    %1 = load double* getelementptr inbounds ([1000 x double]* @a, i32 0, i64 0), align 4
    %2 = load double* getelementptr inbounds ([1000 x double]* @b, i32 0, i64 0), align 4
    %3 = fadd double %1, %2
    store double %3, double* getelementptr inbounds ([1000 x double]* @c, i32 0, i64 0), align 4
    %4 = load double* getelementptr inbounds ([1000 x double]* @a, i32 0, i64 1), align 4
    %5 = load double* getelementptr inbounds ([1000 x double]* @b, i32 0, i64 1), align 4
    %6 = fadd double %5, %4    ; %4 and %5 are swapped
    store double %6, double* getelementptr inbounds ([1000 x double]* @c, i32 0, i64 1), align 4
    ret void
  }

IR after SLP:

  define void @foo() #0 {
    %1 = load double* getelementptr inbounds ([1000 x double]* @a, i64 0, i64 0), align 16, !tbaa !2
    %2 = load double* getelementptr inbounds ([1000 x double]* @b, i64 0, i64 0), align 16, !tbaa !2
    %3 = load double* getelementptr inbounds ([1000 x double]* @b, i64 0, i64 1), align 8, !tbaa !2
    %4 = load double* getelementptr inbounds ([1000 x double]* @a, i64 0, i64 1), align 8, !tbaa !2
    %5 = insertelement <2 x double> undef, double %1, i32 0
    %6 = insertelement <2 x double> %5, double %3, i32 1
    %7 = insertelement <2 x double> undef, double %2, i32 0
    %8 = insertelement <2 x double> %7, double %4, i32 1
    %9 = fadd <2 x double> %6, %8
    store <2 x double> %9, <2 x double>* bitcast ([1000 x double]* @c to <2 x double>*), align 16, !tbaa !2
    ret void
  }


REPOSITORY
  rL LLVM

http://reviews.llvm.org/D6677

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