cmd/gc: specify that C or assembly function arguments do not escape

[rsc]

It would be nice if there were a way to tell gc that an external function (written in C
or assembly) does not let its arguments escape. Right now sometimes it is faster to
execute Go than assembly because the Go code has better escape analysis while gc assumes
the worst about assembly.

[davecheney]

Comment 1:

I regret that I can only vote once for this issue.

[DanielMorsing]

Comment 2:

Could some sort of label for arguments be worked into the upcoming TYPE change in the
assembler? Not sure what the syntax would be, but the TYPE instruction does provide a
convenient place to put this information.

[rsc]

Comment 3:

Labels changed: added go1.1.

[rsc]

Comment 4:

Labels changed: added size-l.

[davecheney]

Comment 5:

@rsc. Daniel Morsing and I played with this a few months back in the context of avoiding
arguments passed to arm functions not escape. Specifically syscall.Syscall*'s where the
[]byte passed to syscall.Read() and friends was made to not escape.
Disappointingly we couldn't detect any significant improvement in the go1 benchmarks.
@dmorsing - do you still have that CL ?

[DanielMorsing]

Comment 6:

Unfortunately I've lost the CL.
I think the advantage is limited, since the only use of external functions with pointer
parameters I have found are the syscall Read and Write functions. Most of those calls
are pointed somewhere in the heap anyway.

[rsc]

Comment 7:

Suggestion at CL 7289048.

[rsc]

Comment 8:

This issue was closed by revision fd178d6.

Status changed to Fixed.