internal/cpu: add experiment to disable CPU features with GODEBUGCPU

Needs the go compiler to be build with GOEXPERIMENT=debugcpu to be active.

The GODEBUGCPU environment variable can be used to disable usage of
specific processor features in the Go standard library.
This is useful for testing and benchmarking different code paths that
are guarded by internal/cpu variable checks.

Use of processor features can not be enabled through GODEBUGCPU.

To disable usage of AVX and SSE41 cpu features on GOARCH amd64 use:
GODEBUGCPU=avx=0,sse41=0

The special "all" option can be used to disable all options:
GODEBUGCPU=all=0

Updates #12805
Updates #15403

Change-Id: I699c2e6f74d98472b6fb4b1e5ffbf29b15697aab
Reviewed-on: https://go-review.googlesource.com/91737
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

1 files changed, 40 insertions, 6 deletions

diff --git a/src/runtime/proc.go b/src/runtime/proc.go
index e22432f8ed..ba76f7c3e7 100644
--- a/src/runtime/proc.go
+++ b/src/runtime/proc.go
@@ -473,7 +473,41 @@ const (
 )
 
 //go:linkname internal_cpu_initialize internal/cpu.initialize
-func internal_cpu_initialize()
+func internal_cpu_initialize(env string)
+
+//go:linkname internal_cpu_debugOptions internal/cpu.debugOptions
+var internal_cpu_debugOptions bool
+
+// cpuinit extracts the environment variable GODEBUGCPU from the enviroment on
+// Linux and Darwin if the GOEXPERIMENT debugcpu was set and calls internal/cpu.initialize.
+func cpuinit() {
+	const prefix = "GODEBUGCPU="
+	var env string
+
+	if haveexperiment("debugcpu") && (GOOS == "linux" || GOOS == "darwin") {
+		internal_cpu_debugOptions = true
+
+		// Similar to goenv_unix but extracts the environment value for
+		// GODEBUGCPU directly.
+		// TODO(moehrmann): remove when general goenvs() can be called before cpuinit()
+		n := int32(0)
+		for argv_index(argv, argc+1+n) != nil {
+			n++
+		}
+
+		for i := int32(0); i < n; i++ {
+			p := argv_index(argv, argc+1+i)
+			s := *(*string)(unsafe.Pointer(&stringStruct{unsafe.Pointer(p), findnull(p)}))
+
+			if hasprefix(s, prefix) {
+				env = gostring(p)[len(prefix):]
+				break
+			}
+		}
+	}
+
+	internal_cpu_initialize(env)
+}
 
 // The bootstrap sequence is:
 //
@@ -498,11 +532,11 @@ func schedinit() {
 	stackinit()
 	mallocinit()
 	mcommoninit(_g_.m)
-	internal_cpu_initialize() // must run before alginit
-	alginit()                 // maps must not be used before this call
-	modulesinit()             // provides activeModules
-	typelinksinit()           // uses maps, activeModules
-	itabsinit()               // uses activeModules
+	cpuinit()       // must run before alginit
+	alginit()       // maps must not be used before this call
+	modulesinit()   // provides activeModules
+	typelinksinit() // uses maps, activeModules
+	itabsinit()     // uses activeModules
 
 	msigsave(_g_.m)
 	initSigmask = _g_.m.sigmask