cmd/compile: constant folding math/bits functions

What version of Go are you using (go version)?

Current git master 577967799c22e5a443ec49f494039f80e08202fe

What did you expect to see?

There is a very easy optimization possibility of constant folding various functions from math/bits.

Would there be interest in implementing these?

Personally, I could work on this, but I'm pretty new to the compiler internals and don't have a CLA yet.

$ git diff 577967799c22e5a443ec49f494039f80e08202fe

diff --git a/src/cmd/compile/internal/ssa/gen/generic.rules b/src/cmd/compile/internal/ssa/gen/generic.rules
index 92b5b04962..7a81712ccc 100644
--- a/src/cmd/compile/internal/ssa/gen/generic.rules
+++ b/src/cmd/compile/internal/ssa/gen/generic.rules
@@ -142,6 +142,14 @@
 (Div32F (Const32F [c]) (Const32F [d])) -> (Const32F [f2i(float64(i2f32(c) / i2f32(d)))])
 (Div64F (Const64F [c]) (Const64F [d])) -> (Const64F [f2i(i2f(c) / i2f(d))])
 
+// TODO: add more functions from math.bits here
+(BitLen32 (Const32 [c])) && config.PtrSize == 4 -> (Const32 [int64(bits.Len32(uint32(c)))])
+(BitLen32 (Const32 [c])) && config.PtrSize == 8 -> (Const64 [int64(bits.Len32(uint32(c)))])
+(BitLen64 (Const64 [c])) && config.PtrSize == 4 -> (Const32 [int64(bits.Len64(uint64(c)))])
+(BitLen64 (Const64 [c])) && config.PtrSize == 8 -> (Const64 [int64(bits.Len64(uint64(c)))])
+// TODO: is there a "ConstInt" to avoid "PtrSize" checks and simplify the rules above ?
+// TODO: do we have to reimplement math.bits in the compiler because of bootstrap issues ?
+
 // Convert x * 1 to x.
 (Mul8  (Const8  [1]) x) -> x
 (Mul16 (Const16 [1]) x) -> x
diff --git a/src/cmd/compile/internal/ssa/gen/rulegen.go b/src/cmd/compile/internal/ssa/gen/rulegen.go
index c23a54d9b5..e7dc2b39af 100644
--- a/src/cmd/compile/internal/ssa/gen/rulegen.go
+++ b/src/cmd/compile/internal/ssa/gen/rulegen.go
@@ -155,10 +155,12 @@ func genRules(arch arch) {
        fmt.Fprintln(w)
        fmt.Fprintln(w, "package ssa")
        fmt.Fprintln(w, "import \"math\"")
+       fmt.Fprintln(w, "import \"math/bits\"")
        fmt.Fprintln(w, "import \"cmd/internal/obj\"")
        fmt.Fprintln(w, "import \"cmd/internal/objabi\"")
        fmt.Fprintln(w, "import \"cmd/compile/internal/types\"")
        fmt.Fprintln(w, "var _ = math.MinInt8  // in case not otherwise used")
+       fmt.Fprintln(w, "var _ = bits.UintSize // in case not otherwise used")
        fmt.Fprintln(w, "var _ = obj.ANOP      // in case not otherwise used")
        fmt.Fprintln(w, "var _ = objabi.GOROOT // in case not otherwise used")
        fmt.Fprintln(w, "var _ = types.TypeMem // in case not otherwise used")