• Russ Cox's avatar
    math: regularize build · dd8dc6f0
    Russ Cox authored
    This will be nicer to the automatic tools.
    It requires a few more assembly stubs
    but fewer Go files.
    
    There are a few instances where it looks like
    there are new blobs of code, but they are just
    being copied out of deleted files.
    
    There is no new code here.
    
    Suppose you have a portable implementation for Sin
    and a 386-specific assembly one.  The old way to
    do this was to write three files
    
    sin_decl.go
       func Sin(x float64) float64  // declaration only
    sin_386.s
       assembly implementation
    
    sin_port.go
       func Sin(x float64) float64 { ... }  // pure-Go impl
    
    and then link in either sin_decl.go+sin_386.s or
    just sin_port.go.  The Makefile actually did the magic
    of linking in only the _port.go files for those without
    assembly and only the _decl.go files for those with
    assembly, or at least some of that magic.
    
    The biggest problem with this, beyond being hard
    to explain to the build system, is that once you do
    explain it to the build system, godoc knows which
    of sin_port.go or sin_decl.go are involved on a given
    architecture, and it (correctly) ignores the other.
    That means you have to put identical doc comments
    in both files.
    
    The new approach, which is more like what we did
    in the later packages math/big and sync/atomic,
    is to have
    
    sin.go
       func Sin(x float64) float64  // decl only
       func sin(x float64) float64 {...}  // pure-Go impl
    
    sin_386.s
       // assembly for Sin (ignores sin)
    sin_amd64.s
       // assembly for Sin: jmp sin
    sin_arm.s
       // assembly for Sin: jmp sin
    
    Once we abandon Makefiles we can put all the assembly
    stubs in one source file, so the number of files will
    actually go down.
    
    Chris asked whether the branches cost anything.
    Given that they are branching to pure-Go implementations
    that are not typically known for their speed, the single
    direct branch is not going to be noticeable.  That is,
    it's on the slow path.
    
    An alternative would have been to preserve the old
    "only write assembly files when there's an implementation"
    and still have just one copy of the declaration of Sin
    (and thus one doc comment) by doing:
    
    sin.go
       func Sin(x float64) float64 { return sin(x) }
    
    sin_decl.go
       func sin(x float64) float64 // declaration only
    sin_386.s
       // assembly for sin
    
    sin_port.go
       func sin(x float64) float64 { portable code }
    
    In this version everyone would link in sin.go and
    then either sin_decl.go+sin_386.s or sin_port.go.
    
    This has an extra function call on all paths, including
    the "fast path" to get to assembly, and it triples the
    number of Go files involved compared to what I did
    in this CL.  On the other hand you don't have to
    write assembly stubs.  After starting down this path
    I decided that the assembly stubs were the easier
    approach.
    
    As for generating the assembly stubs on the fly, much
    of the goal here is to eliminate magic from the build
    process, so that zero-configuration tools like goinstall
    or the new go tool can handle this package.
    
    R=golang-dev, r, cw, iant, r
    CC=golang-dev
    https://golang.org/cl/5488057
    dd8dc6f0
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