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Commit 11b28309 authored by David Chase's avatar David Chase
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cmd/compile: add opcode flag hasSideEffects for do-not-remove 

Added a flag to generic and various architectures' atomic
operations that are judged to have observable side effects
and thus cannot be dead-code-eliminated.

Test requires GOMAXPROCS > 1 without preemption in loop.

Fixes #19182.

Change-Id: Id2230031abd2cca0bbb32fd68fc8a58fb912070f
Reviewed-on: https://go-review.googlesource.com/37333
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: 's avatarCherry Zhang <cherryyz@google.com>
parent b5e51943
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Showing with 90 additions and 49 deletions
src/cmd/compile/internal/ssa/deadcode.go
View file @ 11b28309
......@@ -64,7 +64,7 @@ func liveValues(f *Func, reachable []bool) []bool {
q = append(q, v)
}
for _, v := range b.Values {
if opcodeTable[v.Op].call && !live[v.ID] {
if (opcodeTable[v.Op].call || opcodeTable[v.Op].hasSideEffects) && !live[v.ID] {
live[v.ID] = true
q = append(q, v)
}
......
src/cmd/compile/internal/ssa/gen/AMD64Ops.go
View file @ 11b28309
......@@ -548,15 +548,15 @@ func init() {
// store arg0 to arg1+auxint+aux, arg2=mem.
// These ops return a tuple of <old contents of *(arg1+auxint+aux), memory>.
// Note: arg0 and arg1 are backwards compared to MOVLstore (to facilitate resultInArg0)!
{name: "XCHGL", argLength: 3, reg: gpstorexchg, asm: "XCHGL", aux: "SymOff", resultInArg0: true, faultOnNilArg1: true},
{name: "XCHGQ", argLength: 3, reg: gpstorexchg, asm: "XCHGQ", aux: "SymOff", resultInArg0: true, faultOnNilArg1: true},
{name: "XCHGL", argLength: 3, reg: gpstorexchg, asm: "XCHGL", aux: "SymOff", resultInArg0: true, faultOnNilArg1: true, hasSideEffects: true},
{name: "XCHGQ", argLength: 3, reg: gpstorexchg, asm: "XCHGQ", aux: "SymOff", resultInArg0: true, faultOnNilArg1: true, hasSideEffects: true},
// Atomic adds.
// *(arg1+auxint+aux) += arg0. arg2=mem.
// Returns a tuple of <old contents of *(arg1+auxint+aux), memory>.
// Note: arg0 and arg1 are backwards compared to MOVLstore (to facilitate resultInArg0)!
{name: "XADDLlock", argLength: 3, reg: gpstorexchg, asm: "XADDL", typ: "(UInt32,Mem)", aux: "SymOff", resultInArg0: true, clobberFlags: true, faultOnNilArg1: true},
{name: "XADDQlock", argLength: 3, reg: gpstorexchg, asm: "XADDQ", typ: "(UInt64,Mem)", aux: "SymOff", resultInArg0: true, clobberFlags: true, faultOnNilArg1: true},
{name: "XADDLlock", argLength: 3, reg: gpstorexchg, asm: "XADDL", typ: "(UInt32,Mem)", aux: "SymOff", resultInArg0: true, clobberFlags: true, faultOnNilArg1: true, hasSideEffects: true},
{name: "XADDQlock", argLength: 3, reg: gpstorexchg, asm: "XADDQ", typ: "(UInt64,Mem)", aux: "SymOff", resultInArg0: true, clobberFlags: true, faultOnNilArg1: true, hasSideEffects: true},
{name: "AddTupleFirst32", argLength: 2}, // arg0=tuple <x,y>. Returns <x+arg1,y>.
{name: "AddTupleFirst64", argLength: 2}, // arg0=tuple <x,y>. Returns <x+arg1,y>.
......@@ -579,12 +579,12 @@ func init() {
// JEQ ...
// but we can't do that because memory-using ops can't generate flags yet
// (flagalloc wants to move flag-generating instructions around).
{name: "CMPXCHGLlock", argLength: 4, reg: cmpxchg, asm: "CMPXCHGL", aux: "SymOff", clobberFlags: true, faultOnNilArg0: true},
{name: "CMPXCHGQlock", argLength: 4, reg: cmpxchg, asm: "CMPXCHGQ", aux: "SymOff", clobberFlags: true, faultOnNilArg0: true},
{name: "CMPXCHGLlock", argLength: 4, reg: cmpxchg, asm: "CMPXCHGL", aux: "SymOff", clobberFlags: true, faultOnNilArg0: true, hasSideEffects: true},
{name: "CMPXCHGQlock", argLength: 4, reg: cmpxchg, asm: "CMPXCHGQ", aux: "SymOff", clobberFlags: true, faultOnNilArg0: true, hasSideEffects: true},
// Atomic memory updates.
{name: "ANDBlock", argLength: 3, reg: gpstore, asm: "ANDB", aux: "SymOff", clobberFlags: true, faultOnNilArg0: true}, // *(arg0+auxint+aux) &= arg1
{name: "ORBlock", argLength: 3, reg: gpstore, asm: "ORB", aux: "SymOff", clobberFlags: true, faultOnNilArg0: true}, // *(arg0+auxint+aux) |= arg1
{name: "ANDBlock", argLength: 3, reg: gpstore, asm: "ANDB", aux: "SymOff", clobberFlags: true, faultOnNilArg0: true, hasSideEffects: true}, // *(arg0+auxint+aux) &= arg1
{name: "ORBlock", argLength: 3, reg: gpstore, asm: "ORB", aux: "SymOff", clobberFlags: true, faultOnNilArg0: true, hasSideEffects: true}, // *(arg0+auxint+aux) |= arg1
}
var AMD64blocks = []blockData{
......
src/cmd/compile/internal/ssa/gen/ARM64Ops.go
View file @ 11b28309
......@@ -456,16 +456,16 @@ func init() {
// atomic stores.
// store arg1 to arg0. arg2=mem. returns memory. auxint must be zero.
{name: "STLR", argLength: 3, reg: gpstore, asm: "STLR", faultOnNilArg0: true},
{name: "STLRW", argLength: 3, reg: gpstore, asm: "STLRW", faultOnNilArg0: true},
{name: "STLR", argLength: 3, reg: gpstore, asm: "STLR", faultOnNilArg0: true, hasSideEffects: true},
{name: "STLRW", argLength: 3, reg: gpstore, asm: "STLRW", faultOnNilArg0: true, hasSideEffects: true},
// atomic exchange.
// store arg1 to arg0. arg2=mem. returns <old content of *arg0, memory>. auxint must be zero.
// LDAXR (Rarg0), Rout
// STLXR Rarg1, (Rarg0), Rtmp
// CBNZ Rtmp, -2(PC)
{name: "LoweredAtomicExchange64", argLength: 3, reg: gpxchg, resultNotInArgs: true, faultOnNilArg0: true},
{name: "LoweredAtomicExchange32", argLength: 3, reg: gpxchg, resultNotInArgs: true, faultOnNilArg0: true},
{name: "LoweredAtomicExchange64", argLength: 3, reg: gpxchg, resultNotInArgs: true, faultOnNilArg0: true, hasSideEffects: true},
{name: "LoweredAtomicExchange32", argLength: 3, reg: gpxchg, resultNotInArgs: true, faultOnNilArg0: true, hasSideEffects: true},
// atomic add.
// *arg0 += arg1. arg2=mem. returns <new content of *arg0, memory>. auxint must be zero.
......@@ -473,8 +473,8 @@ func init() {
// ADD Rarg1, Rout
// STLXR Rout, (Rarg0), Rtmp
// CBNZ Rtmp, -3(PC)
{name: "LoweredAtomicAdd64", argLength: 3, reg: gpxchg, resultNotInArgs: true, faultOnNilArg0: true},
{name: "LoweredAtomicAdd32", argLength: 3, reg: gpxchg, resultNotInArgs: true, faultOnNilArg0: true},
{name: "LoweredAtomicAdd64", argLength: 3, reg: gpxchg, resultNotInArgs: true, faultOnNilArg0: true, hasSideEffects: true},
{name: "LoweredAtomicAdd32", argLength: 3, reg: gpxchg, resultNotInArgs: true, faultOnNilArg0: true, hasSideEffects: true},
// atomic compare and swap.
// arg0 = pointer, arg1 = old value, arg2 = new value, arg3 = memory. auxint must be zero.
......@@ -490,8 +490,8 @@ func init() {
// STLXR Rarg2, (Rarg0), Rtmp
// CBNZ Rtmp, -4(PC)
// CSET EQ, Rout
{name: "LoweredAtomicCas64", argLength: 4, reg: gpcas, resultNotInArgs: true, clobberFlags: true, faultOnNilArg0: true},
{name: "LoweredAtomicCas32", argLength: 4, reg: gpcas, resultNotInArgs: true, clobberFlags: true, faultOnNilArg0: true},
{name: "LoweredAtomicCas64", argLength: 4, reg: gpcas, resultNotInArgs: true, clobberFlags: true, faultOnNilArg0: true, hasSideEffects: true},
{name: "LoweredAtomicCas32", argLength: 4, reg: gpcas, resultNotInArgs: true, clobberFlags: true, faultOnNilArg0: true, hasSideEffects: true},
// atomic and/or.
// *arg0 &= (|=) arg1. arg2=mem. returns memory. auxint must be zero.
......@@ -499,8 +499,8 @@ func init() {
// AND/OR Rarg1, Rtmp
// STLXRB Rtmp, (Rarg0), Rtmp
// CBNZ Rtmp, -3(PC)
{name: "LoweredAtomicAnd8", argLength: 3, reg: gpstore, asm: "AND", faultOnNilArg0: true},
{name: "LoweredAtomicOr8", argLength: 3, reg: gpstore, asm: "ORR", faultOnNilArg0: true},
{name: "LoweredAtomicAnd8", argLength: 3, reg: gpstore, asm: "AND", faultOnNilArg0: true, hasSideEffects: true},
{name: "LoweredAtomicOr8", argLength: 3, reg: gpstore, asm: "ORR", faultOnNilArg0: true, hasSideEffects: true},
}
blocks := []blockData{
......
src/cmd/compile/internal/ssa/gen/MIPSOps.go
View file @ 11b28309
......@@ -267,8 +267,8 @@ func init() {
// SYNC
// MOVW Rarg1, (Rarg0)
// SYNC
{name: "LoweredAtomicStore", argLength: 3, reg: gpstore, faultOnNilArg0: true},
{name: "LoweredAtomicStorezero", argLength: 2, reg: gpstore0, faultOnNilArg0: true},
{name: "LoweredAtomicStore", argLength: 3, reg: gpstore, faultOnNilArg0: true, hasSideEffects: true},
{name: "LoweredAtomicStorezero", argLength: 2, reg: gpstore0, faultOnNilArg0: true, hasSideEffects: true},
// atomic exchange.
// store arg1 to arg0. arg2=mem. returns <old content of *arg0, memory>.
......@@ -278,7 +278,7 @@ func init() {
// SC Rtmp, (Rarg0)
// BEQ Rtmp, -3(PC)
// SYNC
{name: "LoweredAtomicExchange", argLength: 3, reg: gpxchg, resultNotInArgs: true, faultOnNilArg0: true},
{name: "LoweredAtomicExchange", argLength: 3, reg: gpxchg, resultNotInArgs: true, faultOnNilArg0: true, hasSideEffects: true},
// atomic add.
// *arg0 += arg1. arg2=mem. returns <new content of *arg0, memory>.
......@@ -289,8 +289,8 @@ func init() {
// BEQ Rtmp, -3(PC)
// SYNC
// ADDU Rarg1, Rout
{name: "LoweredAtomicAdd", argLength: 3, reg: gpxchg, resultNotInArgs: true, faultOnNilArg0: true},
{name: "LoweredAtomicAddconst", argLength: 2, reg: regInfo{inputs: []regMask{gpspsbg}, outputs: []regMask{gp}}, aux: "Int32", resultNotInArgs: true, faultOnNilArg0: true},
{name: "LoweredAtomicAdd", argLength: 3, reg: gpxchg, resultNotInArgs: true, faultOnNilArg0: true, hasSideEffects: true},
{name: "LoweredAtomicAddconst", argLength: 2, reg: regInfo{inputs: []regMask{gpspsbg}, outputs: []regMask{gp}}, aux: "Int32", resultNotInArgs: true, faultOnNilArg0: true, hasSideEffects: true},
// atomic compare and swap.
// arg0 = pointer, arg1 = old value, arg2 = new value, arg3 = memory.
......@@ -308,7 +308,7 @@ func init() {
// SC Rout, (Rarg0)
// BEQ Rout, -4(PC)
// SYNC
{name: "LoweredAtomicCas", argLength: 4, reg: gpcas, resultNotInArgs: true, faultOnNilArg0: true},
{name: "LoweredAtomicCas", argLength: 4, reg: gpcas, resultNotInArgs: true, faultOnNilArg0: true, hasSideEffects: true},
// atomic and/or.
// *arg0 &= (|=) arg1. arg2=mem. returns memory.
......@@ -318,8 +318,8 @@ func init() {
// SC Rtmp, (Rarg0)
// BEQ Rtmp, -3(PC)
// SYNC
{name: "LoweredAtomicAnd", argLength: 3, reg: gpstore, asm: "AND", faultOnNilArg0: true},
{name: "LoweredAtomicOr", argLength: 3, reg: gpstore, asm: "OR", faultOnNilArg0: true},
{name: "LoweredAtomicAnd", argLength: 3, reg: gpstore, asm: "AND", faultOnNilArg0: true, hasSideEffects: true},
{name: "LoweredAtomicOr", argLength: 3, reg: gpstore, asm: "OR", faultOnNilArg0: true, hasSideEffects: true},
// large or unaligned zeroing
// arg0 = address of memory to zero (in R1, changed as side effect)
......
src/cmd/compile/internal/ssa/gen/S390XOps.go
View file @ 11b28309
......@@ -429,14 +429,14 @@ func init() {
// Atomic stores. These are just normal stores.
// store arg1 to arg0+auxint+aux. arg2=mem.
{name: "MOVWatomicstore", argLength: 3, reg: gpstore, asm: "MOVW", aux: "SymOff", typ: "Mem", clobberFlags: true, faultOnNilArg0: true},
{name: "MOVDatomicstore", argLength: 3, reg: gpstore, asm: "MOVD", aux: "SymOff", typ: "Mem", clobberFlags: true, faultOnNilArg0: true},
{name: "MOVWatomicstore", argLength: 3, reg: gpstore, asm: "MOVW", aux: "SymOff", typ: "Mem", clobberFlags: true, faultOnNilArg0: true, hasSideEffects: true},
{name: "MOVDatomicstore", argLength: 3, reg: gpstore, asm: "MOVD", aux: "SymOff", typ: "Mem", clobberFlags: true, faultOnNilArg0: true, hasSideEffects: true},
// Atomic adds.
// *(arg0+auxint+aux) += arg1. arg2=mem.
// Returns a tuple of <old contents of *(arg0+auxint+aux), memory>.
{name: "LAA", argLength: 3, reg: gpstorelaa, asm: "LAA", typ: "(UInt32,Mem)", aux: "SymOff", faultOnNilArg0: true},
{name: "LAAG", argLength: 3, reg: gpstorelaa, asm: "LAAG", typ: "(UInt64,Mem)", aux: "SymOff", faultOnNilArg0: true},
{name: "LAA", argLength: 3, reg: gpstorelaa, asm: "LAA", typ: "(UInt32,Mem)", aux: "SymOff", faultOnNilArg0: true, hasSideEffects: true},
{name: "LAAG", argLength: 3, reg: gpstorelaa, asm: "LAAG", typ: "(UInt64,Mem)", aux: "SymOff", faultOnNilArg0: true, hasSideEffects: true},
{name: "AddTupleFirst32", argLength: 2}, // arg0=tuple <x,y>. Returns <x+arg1,y>.
{name: "AddTupleFirst64", argLength: 2}, // arg0=tuple <x,y>. Returns <x+arg1,y>.
......@@ -461,13 +461,13 @@ func init() {
// BEQ ...
// but we can't do that because memory-using ops can't generate flags yet
// (flagalloc wants to move flag-generating instructions around).
{name: "LoweredAtomicCas32", argLength: 4, reg: cas, asm: "CS", aux: "SymOff", clobberFlags: true, faultOnNilArg0: true},
{name: "LoweredAtomicCas64", argLength: 4, reg: cas, asm: "CSG", aux: "SymOff", clobberFlags: true, faultOnNilArg0: true},
{name: "LoweredAtomicCas32", argLength: 4, reg: cas, asm: "CS", aux: "SymOff", clobberFlags: true, faultOnNilArg0: true, hasSideEffects: true},
{name: "LoweredAtomicCas64", argLength: 4, reg: cas, asm: "CSG", aux: "SymOff", clobberFlags: true, faultOnNilArg0: true, hasSideEffects: true},
// Lowered atomic swaps, emulated using compare-and-swap.
// store arg1 to arg0+auxint+aux, arg2=mem.
{name: "LoweredAtomicExchange32", argLength: 3, reg: exchange, asm: "CS", aux: "SymOff", clobberFlags: true, faultOnNilArg0: true},
{name: "LoweredAtomicExchange64", argLength: 3, reg: exchange, asm: "CSG", aux: "SymOff", clobberFlags: true, faultOnNilArg0: true},
{name: "LoweredAtomicExchange32", argLength: 3, reg: exchange, asm: "CS", aux: "SymOff", clobberFlags: true, faultOnNilArg0: true, hasSideEffects: true},
{name: "LoweredAtomicExchange64", argLength: 3, reg: exchange, asm: "CSG", aux: "SymOff", clobberFlags: true, faultOnNilArg0: true, hasSideEffects: true},
// find leftmost one
{
......
src/cmd/compile/internal/ssa/gen/genericOps.go
View file @ 11b28309
......@@ -441,20 +441,20 @@ var genericOps = []opData{
// Atomic loads return a new memory so that the loads are properly ordered
// with respect to other loads and stores.
// TODO: use for sync/atomic at some point.
{name: "AtomicLoad32", argLength: 2, typ: "(UInt32,Mem)"}, // Load from arg0. arg1=memory. Returns loaded value and new memory.
{name: "AtomicLoad64", argLength: 2, typ: "(UInt64,Mem)"}, // Load from arg0. arg1=memory. Returns loaded value and new memory.
{name: "AtomicLoadPtr", argLength: 2, typ: "(BytePtr,Mem)"}, // Load from arg0. arg1=memory. Returns loaded value and new memory.
{name: "AtomicStore32", argLength: 3, typ: "Mem"}, // Store arg1 to *arg0. arg2=memory. Returns memory.
{name: "AtomicStore64", argLength: 3, typ: "Mem"}, // Store arg1 to *arg0. arg2=memory. Returns memory.
{name: "AtomicStorePtrNoWB", argLength: 3, typ: "Mem"}, // Store arg1 to *arg0. arg2=memory. Returns memory.
{name: "AtomicExchange32", argLength: 3, typ: "(UInt32,Mem)"}, // Store arg1 to *arg0. arg2=memory. Returns old contents of *arg0 and new memory.
{name: "AtomicExchange64", argLength: 3, typ: "(UInt64,Mem)"}, // Store arg1 to *arg0. arg2=memory. Returns old contents of *arg0 and new memory.
{name: "AtomicAdd32", argLength: 3, typ: "(UInt32,Mem)"}, // Do *arg0 += arg1. arg2=memory. Returns sum and new memory.
{name: "AtomicAdd64", argLength: 3, typ: "(UInt64,Mem)"}, // Do *arg0 += arg1. arg2=memory. Returns sum and new memory.
{name: "AtomicCompareAndSwap32", argLength: 4, typ: "(Bool,Mem)"}, // if *arg0==arg1, then set *arg0=arg2. Returns true iff store happens and new memory.
{name: "AtomicCompareAndSwap64", argLength: 4, typ: "(Bool,Mem)"}, // if *arg0==arg1, then set *arg0=arg2. Returns true iff store happens and new memory.
{name: "AtomicAnd8", argLength: 3, typ: "Mem"}, // *arg0 &= arg1. arg2=memory. Returns memory.
{name: "AtomicOr8", argLength: 3, typ: "Mem"}, // *arg0 |= arg1. arg2=memory. Returns memory.
{name: "AtomicLoad32", argLength: 2, typ: "(UInt32,Mem)"}, // Load from arg0. arg1=memory. Returns loaded value and new memory.
{name: "AtomicLoad64", argLength: 2, typ: "(UInt64,Mem)"}, // Load from arg0. arg1=memory. Returns loaded value and new memory.
{name: "AtomicLoadPtr", argLength: 2, typ: "(BytePtr,Mem)"}, // Load from arg0. arg1=memory. Returns loaded value and new memory.
{name: "AtomicStore32", argLength: 3, typ: "Mem", hasSideEffects: true}, // Store arg1 to *arg0. arg2=memory. Returns memory.
{name: "AtomicStore64", argLength: 3, typ: "Mem", hasSideEffects: true}, // Store arg1 to *arg0. arg2=memory. Returns memory.
{name: "AtomicStorePtrNoWB", argLength: 3, typ: "Mem", hasSideEffects: true}, // Store arg1 to *arg0. arg2=memory. Returns memory.
{name: "AtomicExchange32", argLength: 3, typ: "(UInt32,Mem)", hasSideEffects: true}, // Store arg1 to *arg0. arg2=memory. Returns old contents of *arg0 and new memory.
{name: "AtomicExchange64", argLength: 3, typ: "(UInt64,Mem)", hasSideEffects: true}, // Store arg1 to *arg0. arg2=memory. Returns old contents of *arg0 and new memory.
{name: "AtomicAdd32", argLength: 3, typ: "(UInt32,Mem)", hasSideEffects: true}, // Do *arg0 += arg1. arg2=memory. Returns sum and new memory.
{name: "AtomicAdd64", argLength: 3, typ: "(UInt64,Mem)", hasSideEffects: true}, // Do *arg0 += arg1. arg2=memory. Returns sum and new memory.
{name: "AtomicCompareAndSwap32", argLength: 4, typ: "(Bool,Mem)", hasSideEffects: true}, // if *arg0==arg1, then set *arg0=arg2. Returns true iff store happens and new memory.
{name: "AtomicCompareAndSwap64", argLength: 4, typ: "(Bool,Mem)", hasSideEffects: true}, // if *arg0==arg1, then set *arg0=arg2. Returns true iff store happens and new memory.
{name: "AtomicAnd8", argLength: 3, typ: "Mem", hasSideEffects: true}, // *arg0 &= arg1. arg2=memory. Returns memory.
{name: "AtomicOr8", argLength: 3, typ: "Mem", hasSideEffects: true}, // *arg0 |= arg1. arg2=memory. Returns memory.
}
// kind control successors implicit exit
......
src/cmd/compile/internal/ssa/gen/main.go
View file @ 11b28309
......@@ -52,6 +52,7 @@ type opData struct {
faultOnNilArg0 bool // this op will fault if arg0 is nil (and aux encodes a small offset)
faultOnNilArg1 bool // this op will fault if arg1 is nil (and aux encodes a small offset)
usesScratch bool // this op requires scratch memory space
hasSideEffects bool // for "reasons", not to be eliminated. E.g., atomic store, #19182.
}
type blockData struct {
......@@ -208,6 +209,9 @@ func genOp() {
if v.usesScratch {
fmt.Fprintln(w, "usesScratch: true,")
}
if v.hasSideEffects {
fmt.Fprintln(w, "hasSideEffects: true,")
}
if a.name == "generic" {
fmt.Fprintln(w, "generic:true,")
fmt.Fprintln(w, "},") // close op
......
src/cmd/compile/internal/ssa/op.go
View file @ 11b28309
......@@ -34,6 +34,7 @@ type opInfo struct {
faultOnNilArg0 bool // this op will fault if arg0 is nil (and aux encodes a small offset)
faultOnNilArg1 bool // this op will fault if arg1 is nil (and aux encodes a small offset)
usesScratch bool // this op requires scratch memory space
hasSideEffects bool // for "reasons", not to be eliminated. E.g., atomic store, #19182.
}
type inputInfo struct {
......
src/cmd/compile/internal/ssa/opGen.go
View file @ 11b28309
This diff is collapsed.
test/fixedbugs/issue19182.go 0 → 100644
View file @ 11b28309
// run
// Copyright 2017 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package main
import (
"fmt"
"runtime"
"sync/atomic"
"time"
)
var a uint64 = 0
func main() {
runtime.GOMAXPROCS(2) // With just 1, infinite loop never yields
go func() {
for {
atomic.AddUint64(&a, uint64(1))
}
}()
time.Sleep(10 * time.Millisecond) // Short sleep is enough in passing case
i, val := 0, atomic.LoadUint64(&a)
for ; val == 0 && i < 100; val, i = atomic.LoadUint64(&a), i+1 {
time.Sleep(100 * time.Millisecond)
}
if val == 0 {
fmt.Printf("Failed to observe atomic increment after %d tries\n", i)
}
}
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