runtime: document GODEBUG=cpu.* in runtime package

So far was only documented in doc/diagnostics.

Fixes: #54183
Change-Id: Iab67a1500547f30325453c614a2f2f6621dfadb3
Reviewed-on: https://go-review.googlesource.com/c/go/+/423275
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Martin Möhrmann <moehrmann@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>

diff --git a/src/runtime/extern.go b/src/runtime/extern.go
index 15c519d233502a65a94bf8f6c183d3e81a9ae807..8fb8720e89ce0405fadc8517401185d2a6ae4ce8 100644 (file)
--- a/src/runtime/extern.go
+++ b/src/runtime/extern.go
@@ -42,6 +42,12 @@ It is a comma-separated list of name=val pairs setting these named variables:
        clobber the memory content of an object with bad content when it frees
        the object.
 
+       cpu.*: cpu.all=off disables the use of all optional instruction set extensions.
+       cpu.extension=off disables use of instructions from the specified instruction set extension.
+       extension is the lower case name for the instruction set extension such as sse41 or avx
+       as listed in internal/cpu package. As an example cpu.avx=off disables runtime detection
+       and thereby use of AVX instructions.
+
        cgocheck: setting cgocheck=0 disables all checks for packages
        using cgo to incorrectly pass Go pointers to non-Go code.
        Setting cgocheck=1 (the default) enables relatively cheap