rip -O3 (#372)

Author: KristofferC

docs/src/user/advanced.md

@@ -200,90 +200,6 @@ julia> vector_hessian(cumprod, [1, 2, 3])
 Likewise, you could write a version of `vector_hessian` which supports functions of the
 form `f!(y, x)`, or perhaps an in-place Jacobian with `ForwardDiff.jacobian!`.
 
-## SIMD Vectorization
-
-Many operations on ForwardDiff's dual numbers are amenable to [SIMD
-vectorization](https://en.wikipedia.org/wiki/SIMD#Hardware). For some ForwardDiff
-benchmarks, we've seen SIMD vectorization yield [speedups of almost
-3x](https://github.com/JuliaDiff/ForwardDiff.jl/issues/98#issuecomment-253149761).
-
-To enable SIMD optimizations, start your Julia process with the `-O3` flag. This flag
-enables [LLVM's SLPVectorizerPass](http://llvm.org/docs/Vectorizers.html#the-slp-vectorizer)
-during compilation, which attempts to automatically insert SIMD instructions where possible
-for certain arithmetic operations.
-
-Here's an example of LLVM bitcode generated for an addition of two `Dual` numbers without
-SIMD instructions (i.e. not starting Julia with `-O3`):
-
-```julia
-julia> using ForwardDiff: Dual
-
-julia> a = Dual(1., 2., 3., 4.)
-Dual{Nothing}(1.0,2.0,3.0,4.0)
-
-julia> b = Dual(5., 6., 7., 8.)
-Dual{Nothing}(5.0,6.0,7.0,8.0)
-
-julia> @code_llvm a + b
-
-define void @"julia_+_70852"(%Dual* noalias sret, %Dual*, %Dual*) #0 {
-top:
-  %3 = getelementptr inbounds %Dual, %Dual* %1, i64 0, i32 1, i32 0, i64 0
-  %4 = load double, double* %3, align 8
-  %5 = getelementptr inbounds %Dual, %Dual* %2, i64 0, i32 1, i32 0, i64 0
-  %6 = load double, double* %5, align 8
-  %7 = fadd double %4, %6
-  %8 = getelementptr inbounds %Dual, %Dual* %1, i64 0, i32 1, i32 0, i64 1
-  %9 = load double, double* %8, align 8
-  %10 = getelementptr inbounds %Dual, %Dual* %2, i64 0, i32 1, i32 0, i64 1
-  %11 = load double, double* %10, align 8
-  %12 = fadd double %9, %11
-  %13 = getelementptr inbounds %Dual, %Dual* %1, i64 0, i32 1, i32 0, i64 2
-  %14 = load double, double* %13, align 8
-  %15 = getelementptr inbounds %Dual, %Dual* %2, i64 0, i32 1, i32 0, i64 2
-  %16 = load double, double* %15, align 8
-  %17 = fadd double %14, %16
-  %18 = getelementptr inbounds %Dual, %Dual* %1, i64 0, i32 0
-  %19 = load double, double* %18, align 8
-  %20 = getelementptr inbounds %Dual, %Dual* %2, i64 0, i32 0
-  %21 = load double, double* %20, align 8
-  %22 = fadd double %19, %21
-  %23 = getelementptr inbounds %Dual, %Dual* %0, i64 0, i32 0
-  store double %22, double* %23, align 8
-  %24 = getelementptr inbounds %Dual, %Dual* %0, i64 0, i32 1, i32 0, i64 0
-  store double %7, double* %24, align 8
-  %25 = getelementptr inbounds %Dual, %Dual* %0, i64 0, i32 1, i32 0, i64 1
-  store double %12, double* %25, align 8
-  %26 = getelementptr inbounds %Dual, %Dual* %0, i64 0, i32 1, i32 0, i64 2
-  store double %17, double* %26, align 8
-  ret void
-}
-```
-
-If we start up Julia with `-O3` instead, the call to `@code_llvm` will show that LLVM
-can SIMD-vectorize the addition:
-
-```julia
-julia> @code_llvm a + b
-
-define void @"julia_+_70842"(%Dual* noalias sret, %Dual*, %Dual*) #0 {
-top:
-  %3 = bitcast %Dual* %1 to <4 x double>*            # cast the Dual to a SIMD-able LLVM vector
-  %4 = load <4 x double>, <4 x double>* %3, align 8
-  %5 = bitcast %Dual* %2 to <4 x double>*
-  %6 = load <4 x double>, <4 x double>* %5, align 8
-  %7 = fadd <4 x double> %4, %6                      # SIMD add
-  %8 = bitcast %Dual* %0 to <4 x double>*
-  store <4 x double> %7, <4 x double>* %8, align 8
-  ret void
-}
-```
-
-Note that whether or not SIMD instructions can actually be used will depend on your machine
-and Julia build. For example, pre-built Julia binaries might not emit vectorized LLVM
-bitcode. To overcome this specific issue, you can [locally rebuild Julia's system
-image](http://docs.julialang.org/en/latest/devdocs/sysimg).
-
 ## Custom tags and tag checking
 
 The `Dual` type includes a "tag" parameter indicating the particular function call to
@@ -315,4 +231,4 @@ want to disable this checking.
    ```julia
    cfg = GradientConfig(nothing, x)
    gradient(f, x, cfg)
-   ```
+   ```