internal/cpu: add options and warnings for required cpu features

Updates #27218

Change-Id: I8603f3a639cdd9ee201c4f1566692e5b88877fc4
Reviewed-on: https://go-review.googlesource.com/c/144107
Run-TryBot: Martin Möhrmann <martisch@uos.de>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/internal/cpu/cpu.go

@@ -155,8 +155,9 @@ var options []option
 type option struct {
 	Name      string
 	Feature   *bool
-	Specified bool // Stores if feature value was specified in GODEBUGCPU.
-	Enable    bool // Stores if feature should be enabled.
+	Specified bool // whether feature value was specified in GODEBUGCPU
+	Enable    bool // whether feature should be enabled
+	Required  bool // whether feature is mandatory and can not be disabled
 }
 
 // processOptions enables or disables CPU feature values based on the parsed env string.
@@ -196,7 +197,7 @@ field:
 		if key == "all" {
 			for i := range options {
 				options[i].Specified = true
-				options[i].Enable = enable
+				options[i].Enable = enable || options[i].Required
 			}
 			continue field
 		}
@@ -222,6 +223,11 @@ field:
 			continue
 		}
 
+		if !o.Enable && o.Required {
+			print("GODEBUGCPU: can not disable \"", o.Name, "\", required feature\n")
+			continue
+		}
+
 		*o.Feature = o.Enable
 	}
 }

src/internal/cpu/cpu_arm64.go

@@ -66,8 +66,8 @@ func doinit() {
 		{Name: "asimdfhm", Feature: &ARM64.HasASIMDFHM},
 
 		// These capabilities should always be enabled on arm64:
-		//  {Name: "fp", Feature: &ARM64.HasFP},
-		//  {Name: "asimd", Feature: &ARM64.HasASIMD},
+		{Name: "fp", Feature: &ARM64.HasFP, Required: true},
+		{Name: "asimd", Feature: &ARM64.HasASIMD, Required: true},
 	}
 
 	// HWCAP feature bits

src/internal/cpu/cpu_ppc64x.go

@@ -40,11 +40,11 @@ func doinit() {
 		{Name: "scv", Feature: &PPC64.HasSCV},
 
 		// These capabilities should always be enabled on ppc64 and ppc64le:
-		//  {Name: "vmx", Feature: &PPC64.HasVMX},
-		//  {Name: "dfp", Feature: &PPC64.HasDFP},
-		//  {Name: "vsx", Feature: &PPC64.HasVSX},
-		//  {Name: "isel", Feature: &PPC64.HasISEL},
-		//  {Name: "vcrypto", Feature: &PPC64.HasVCRYPTO},
+		{Name: "vmx", Feature: &PPC64.HasVMX, Required: true},
+		{Name: "dfp", Feature: &PPC64.HasDFP, Required: true},
+		{Name: "vsx", Feature: &PPC64.HasVSX, Required: true},
+		{Name: "isel", Feature: &PPC64.HasISEL, Required: true},
+		{Name: "vcrypto", Feature: &PPC64.HasVCRYPTO, Required: true},
 	}
 
 	// HWCAP feature bits

src/internal/cpu/cpu_test.go

@@ -52,8 +52,9 @@ func TestAllCapabilitiesDisabled(t *testing.T) {
 	}
 
 	for _, o := range Options {
-		if got := *o.Feature; got != false {
-			t.Errorf("%v: expected false, got %v", o.Name, got)
+		want := o.Required
+		if got := *o.Feature; got != want {
+			t.Errorf("%v: expected %v, got %v", o.Name, want, got)
 		}
 	}
 }

src/internal/cpu/cpu_x86.go

@@ -55,13 +55,8 @@ func doinit() {
 		{Name: "sse42", Feature: &X86.HasSSE42},
 		{Name: "ssse3", Feature: &X86.HasSSSE3},
 
-		// sse2 set as last element so it can easily be removed again. See code below.
-		{Name: "sse2", Feature: &X86.HasSSE2},
-	}
-
-	// Remove sse2 from options on amd64(p32) because SSE2 is a mandatory feature for these GOARCHs.
-	if GOARCH == "amd64" || GOARCH == "amd64p32" {
-		options = options[:len(options)-1]
+		// These capabilities should always be enabled on amd64(p32):
+		{Name: "sse2", Feature: &X86.HasSSE2, Required: GOARCH == "amd64" || GOARCH == "amd64p32"},
 	}
 
 	maxID, _, _, _ := cpuid(0, 0)