编译器优化中断多线程代码

在经历了艰苦的学习之后 shared variables are currently not guarded by memory barriers ,我现在遇到了另一个问题。要么我做错了什么,要么DMD中现有的编译器优化可以通过重新排序读取多线程代码 共享 变量。

例如,当我使用 dmd -O (完全优化),编译器愉快地优化了本地变量 o 在这个代码中(其中 cas 比较和交换函数来自 core.atomic )

shared uint cnt;
void atomicInc  ( ) { uint o; do { o = cnt; } while ( !cas( &cnt, o, o + 1 ) );}

类似于这样(请参见下面的dis assembly):

shared uint cnt;
void atomicInc  ( ) { while ( !cas( &cnt, cnt, cnt + 1 ) ) { } }

在“优化”代码中 cnt 从内存中读取两次,因此运行另一个线程已修改的风险 碳纳米管 介于两者之间。优化基本上破坏了比较交换算法。

这是一个错误,还是有一个正确的方法来达到预期的结果?到目前为止,我找到的唯一解决办法是使用汇编程序实现代码。

完整测试代码和其他详细信息
为了完整性,这里有一个完整的测试代码,它显示了这两个问题(没有内存障碍和优化问题)。它在三台不同的Windows机器上为dmd 2.049和dmd2.050生成以下输出(假设Dekker的算法没有死锁,这可能会发生):

dmd -O -run optbug.d
CAS   : failed
Dekker: failed

还有里面的回路 atomicInc 通过完全优化编译为:

; cnt is stored at 447C10h
; while ( !cas( &cnt, o, o + 1 ) ) o = cnt;
; 1) prepare call cas( &cnt, o, o + 1 ): &cnt and o go to stack, o+1 to eax
402027: mov    ecx,447C10h         ; ecx = &cnt
40202C: mov    eax,[447C10h]     ; eax = o1 = cnt
402031: inc    eax                 ; eax = o1 + 1 (third parameter)
402032: push   ecx                 ; push &cnt (first parameter)
    ; next instruction pushes current value of cnt onto stack
    ; as second parameter o instead of re-using o1
402033: push   [447C10h]    
402039: call   4020BC              ; 2) call cas    
40203E: xor    al,1                ; 3) test success
402040: jne    402027              ; no success try again
; end of main loop

以下是测试代码:

import core.atomic;
import core.thread;
import std.stdio;

enum loops = 0xFFFF;
shared uint cnt;

/* *****************************************************************************
 Implement atomicOp!("+=")(cnt, 1U); with CAS. The code below doesn't work with
 the "-O" compiler flag because cnt is read twice while calling cas and another
 thread can modify cnt in between.
*/
enum threads = 8;

void atomicInc  ( ) { uint o; do { o = cnt; } while ( !cas( &cnt, o, o + 1 ) );}
void threadFunc ( ) { foreach (i; 0..loops) atomicInc; }

void testCas ( ) {
    cnt = 0;
    auto tgCas = new ThreadGroup;
    foreach (i; 0..threads) tgCas.create(&threadFunc);
    tgCas.joinAll;
    writeln( "CAS   : ", cnt == loops * threads ? "passed" : "failed" );
}

/* *****************************************************************************
 Dekker's algorithm. Fails on ia32 (other than atom) because ia32 can re-order 
 read before write. Most likely fails on many other architectures.
*/
shared bool flag1 = false;
shared bool flag2 = false;
shared bool turn2 = false;   // avoids starvation by executing 1 and 2 in turns

void dekkerInc ( ) {
    flag1 = true;
    while ( flag2 ) if ( turn2 ) {
        flag1 = false; while ( turn2 )  {  /* wait until my turn */ }
        flag1 = true;
    }
    cnt++;                   // shouldn't work without a cast
    turn2 = true; flag1 = false;
}

void dekkerDec ( ) {
    flag2 = true;
    while ( flag1 ) if ( !turn2 ) {
        flag2 = false; while ( !turn2 ) { /* wait until my turn */ }
        flag2 = true;
    }
    cnt--;                   // shouldn't work without a cast
    turn2 = false; flag2 = false;
}

void threadDekkerInc ( ) { foreach (i; 0..loops) dekkerInc; }
void threadDekkerDec ( ) { foreach (i; 0..loops) dekkerDec; }

void testDekker ( ) {
    cnt = 0;
    auto tgDekker = new ThreadGroup;
    tgDekker.create( &threadDekkerInc );
    tgDekker.create( &threadDekkerDec );
    tgDekker.joinAll;
    writeln( "Dekker: ", cnt == 0 ? "passed" : "failed" );
}

/* ************************************************************************** */
void main() {
    testCas;
    testDekker;
}