cmd/compile: de-virtualize interface calls

With this change, code like

    h := sha1.New()
    h.Write(buf)
    sum := h.Sum()

gets compiled into static calls rather than
interface calls, because the compiler is able
to prove that 'h' is really a *sha1.digest.

The InterCall re-write rule hits a few dozen times
during make.bash, and hundreds of times during all.bash.

The most common pattern identified by the compiler
is a constructor like

    func New() Interface { return &impl{...} }

where the constructor gets inlined into the caller,
and the result is used immediately. Examples include
{sha1,md5,crc32,crc64,...}.New, base64.NewEncoder,
base64.NewDecoder, errors.New, net.Pipe, and so on.

Some existing benchmarks that change on darwin/amd64:

Crc64/ISO4KB-8        2.67µs ± 1%    2.66µs ± 0%  -0.36%  (p=0.015 n=10+10)
Crc64/ISO1KB-8         694ns ± 0%     690ns ± 1%  -0.59%  (p=0.001 n=10+10)
Adler32KB-8            473ns ± 1%     471ns ± 0%  -0.39%  (p=0.010 n=10+9)

On architectures like amd64, the reduction in code size
appears to contribute more to benchmark improvements than just
removing the indirect call, since that branch gets predicted
accurately when called in a loop.

Updates #19361

Change-Id: Ia9d30afdd5f6b4d38d38b14b88f308acae8ce7ed
Reviewed-on: https://go-review.googlesource.com/37751
Run-TryBot: Philip Hofer <phofer@umich.edu>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

1 files changed, 16 insertions, 15 deletions

diff --git a/src/cmd/compile/internal/ssa/export_test.go b/src/cmd/compile/internal/ssa/export_test.go
index 74bb08d5c2..b687076a28 100644
--- a/src/cmd/compile/internal/ssa/export_test.go
+++ b/src/cmd/compile/internal/ssa/export_test.go
@@ -97,21 +97,22 @@ func (d DummyFrontend) Warnl(_ src.XPos, msg string, args ...interface{})  { d.t
 func (d DummyFrontend) Debug_checknil() bool                               { return false }
 func (d DummyFrontend) Debug_wb() bool                                     { return false }
 
-func (d DummyFrontend) TypeBool() Type    { return TypeBool }
-func (d DummyFrontend) TypeInt8() Type    { return TypeInt8 }
-func (d DummyFrontend) TypeInt16() Type   { return TypeInt16 }
-func (d DummyFrontend) TypeInt32() Type   { return TypeInt32 }
-func (d DummyFrontend) TypeInt64() Type   { return TypeInt64 }
-func (d DummyFrontend) TypeUInt8() Type   { return TypeUInt8 }
-func (d DummyFrontend) TypeUInt16() Type  { return TypeUInt16 }
-func (d DummyFrontend) TypeUInt32() Type  { return TypeUInt32 }
-func (d DummyFrontend) TypeUInt64() Type  { return TypeUInt64 }
-func (d DummyFrontend) TypeFloat32() Type { return TypeFloat32 }
-func (d DummyFrontend) TypeFloat64() Type { return TypeFloat64 }
-func (d DummyFrontend) TypeInt() Type     { return TypeInt64 }
-func (d DummyFrontend) TypeUintptr() Type { return TypeUInt64 }
-func (d DummyFrontend) TypeString() Type  { panic("unimplemented") }
-func (d DummyFrontend) TypeBytePtr() Type { return TypeBytePtr }
+func (d DummyFrontend) TypeBool() Type                               { return TypeBool }
+func (d DummyFrontend) TypeInt8() Type                               { return TypeInt8 }
+func (d DummyFrontend) TypeInt16() Type                              { return TypeInt16 }
+func (d DummyFrontend) TypeInt32() Type                              { return TypeInt32 }
+func (d DummyFrontend) TypeInt64() Type                              { return TypeInt64 }
+func (d DummyFrontend) TypeUInt8() Type                              { return TypeUInt8 }
+func (d DummyFrontend) TypeUInt16() Type                             { return TypeUInt16 }
+func (d DummyFrontend) TypeUInt32() Type                             { return TypeUInt32 }
+func (d DummyFrontend) TypeUInt64() Type                             { return TypeUInt64 }
+func (d DummyFrontend) TypeFloat32() Type                            { return TypeFloat32 }
+func (d DummyFrontend) TypeFloat64() Type                            { return TypeFloat64 }
+func (d DummyFrontend) TypeInt() Type                                { return TypeInt64 }
+func (d DummyFrontend) TypeUintptr() Type                            { return TypeUInt64 }
+func (d DummyFrontend) TypeString() Type                             { panic("unimplemented") }
+func (d DummyFrontend) TypeBytePtr() Type                            { return TypeBytePtr }
+func (d DummyFrontend) DerefItab(sym *obj.LSym, off int64) *obj.LSym { return nil }
 
 func (d DummyFrontend) CanSSA(t Type) bool {
 	// There are no un-SSAable types in dummy land.