Reproduction of AVX512 opmask clobbering

README.md

Maskcheck is a repro of opmask clobbering by golang async preemption

A GNU C / Asm implementation is here: https://gist.github.com/vsivsi/8511aca1bac528f49fbb45a636afa4b5

NOTE! This must be run on an Intel processor supporting AVX512F/DQ

To test: go test -count 1 -timeout 15m -run '^TestMask$' gist.github.com/vsivsi/fff8618ace4b02eb410dd8792779bf32

This should fail with something like:

% go test -count 1 -timeout 15m -run '^TestMask$' gist.github.com/vsivsi/fff8618ace4b02eb410dd8792779bf32
--- FAIL: TestMask (0.44s)
    maskcheck_test.go:16: Failed for iteration: 0 with return value: 69247732
    maskcheck_test.go:16: Failed for iteration: 1 with return value: 42154190
    maskcheck_test.go:16: Failed for iteration: 2 with return value: 43317668
    maskcheck_test.go:16: Failed for iteration: 3 with return value: 41575878
    maskcheck_test.go:16: Failed for iteration: 4 with return value: 38750998
    maskcheck_test.go:16: Failed for iteration: 5 with return value: 42730788
    maskcheck_test.go:16: Failed for iteration: 6 with return value: 41721686
    maskcheck_test.go:16: Failed for iteration: 7 with return value: 40051735
    maskcheck_test.go:16: Failed for iteration: 8 with return value: 40951907
    maskcheck_test.go:16: Failed for iteration: 9 with return value: 41829708
    maskcheck_test.go:16: Failed for iteration: 10 with return value: 33994560
    maskcheck_test.go:16: Failed for iteration: 11 with return value: 31676633
    maskcheck_test.go:16: Failed for iteration: 12 with return value: 32128705
    maskcheck_test.go:16: Failed for iteration: 13 with return value: 41683214
    maskcheck_test.go:16: Failed for iteration: 14 with return value: 38697082
    maskcheck_test.go:16: Failed for iteration: 15 with return value: 35869348
    maskcheck_test.go:16: Failed for iteration: 16 with return value: 37629503
    maskcheck_test.go:16: Failed for iteration: 17 with return value: 32931377
    maskcheck_test.go:16: Failed for iteration: 18 with return value: 44475243
    maskcheck_test.go:16: Failed for iteration: 19 with return value: 42445948
FAIL
FAIL    gist.github.com/vsivsi/fff8618ace4b02eb410dd8792779bf32 0.744s
FAIL

Disabling async preemption rescues it: GODEBUG=asyncpreemptoff=1 go test -count 1 -timeout 15m -run '^TestMask$' gist.github.com/vsivsi/fff8618ace4b02eb410dd8792779bf32

% GODEBUG=asyncpreemptoff=1 go test -count 1 -timeout 15m -run '^TestMask$' gist.github.com/vsivsi/fff8618ace4b02eb410dd8792779bf32 
ok      gist.github.com/vsivsi/fff8618ace4b02eb410dd8792779bf32 0.858s

go.mod

module gist.github.com/vsivsi/fff8618ace4b02eb410dd8792779bf32

go 1.17

require golang.org/x/sys v0.0.0-20211029165221-6e7872819dc8

go.sum

golang.org/x/sys v0.0.0-20211029165221-6e7872819dc8 h1:M69LAlWZCshgp0QSzyDcSsSIejIEeuaCVpmwcKwyLMk=
golang.org/x/sys v0.0.0-20211029165221-6e7872819dc8/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=

maskcheck.s

// +build !gccgo,!purego

#include "textflag.h"

// func maskCheck8(x uint64) uint64
// Requires: AVX512DQ
TEXT ·maskCheck8(SB), NOSPLIT, $0-16
	MOVQ  x+0(FP), AX
	MOVB  $0xfb, CL
	KMOVB CX, K1

loop:
	KMOVB K1, DX
	CMPB  DL, CL
	JNE   fail
	DECQ  AX
	JNZ   loop

fail:
	MOVQ AX, ret+8(FP)
	RET
	

maskcheck_test.go

package maskcheck

import (
	"testing"

	"golang.org/x/sys/cpu"
)

// Stub: Check if AVX512 Opmasks are being clobbered.
func maskCheck8(x uint64) uint64

func TestMask(t *testing.T) {
	if !(cpu.X86.HasAVX512 && cpu.X86.HasAVX512DQ) {
		t.Fatalf("No AVX512F/DQ support present!")
	}
	fail := false
	for x := 0; x < 20; x++ {
		r := maskCheck8(100000000)
		if r != 0 {
			fail = true
			t.Logf("Failed for iteration: %d with return value: %d", x, r)
		}
	}
	if fail {
		t.Fail()
	}
}