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confused at code generation for a empty loop
- From: "PRC" <panruochen at gmail dot com>
- To: gcc-help <gcc-help at gcc dot gnu dot org>
- Date: Fri, 30 Nov 2007 14:45:32 +0800
- Subject: confused at code generation for a empty loop
Here is my code:
=================================================================================
#include <pthread.h>
#include <stdio.h>
#define NUM_THREADS 1
static int flag = 0;
void *thread(void *threadid)
{
int ret = rand();
printf("thread return %d!\n", ret);
sleep(10);
flag = 1;
return (void *)ret;
}
int main (int argc, char *argv[])
{
pthread_t threads[NUM_THREADS];
int rc;
rc = pthread_create(&threads[0], NULL, thread, (void *)0);
if (rc){
printf("ERROR; return code from pthread_create() is %d\n", rc);
exit(-1);
}
while( flag == 0 ) ;
return 0;
}
=================================================================================
gcc -O2 -g -lpthread a.c
objdump -S a.out > a.S
`cat a.S` shows:
=================================================================================
if (rc){
80484d4: 85 c0 test %eax,%eax
80484d6: 75 15 jne 80484ed <main+0x4d>
80484d8: a1 68 97 04 08 mov 0x8049768,%eax
80484dd: 85 c0 test %eax,%eax
80484df: 90 nop
printf("ERROR; return code from pthread_create() is %d\n", rc);
exit(-1);
}
while( flag == 0 ) ;
80484e0: 74 fe je 80484e0 <main+0x40>
================================================================================
You can see the variable `flag` is read only once. If the value of `flag` is 0 at the first time,
the program will trap into a dead loop and never exit. And gcc knows the value of `flag` could
be modified in the routine `thread`.
But if I modify the line "while( flag == 0 ) ;" to "while(flag == 0) printf("waiting..\n");"
and recompile the source code, the output assembly code becomes:
=================================================================================
while( flag == 0 ) printf("waiting..\n");
8048510: c7 04 24 74 86 04 08 movl $0x8048674,(%esp)
8048517: e8 90 fe ff ff call 80483ac <puts@plt>
804851c: a1 b8 97 04 08 mov 0x80497b8,%eax
8048521: 85 c0 test %eax,%eax
8048523: 74 eb je 8048510 <main+0x40>
=================================================================================
`flag` is read in each loop. Then when the value of `flag` is modified, the loop terminates.
I wonder why gcc generates code for a empty loop like that. Is it a bug or for optimization in some case?
My gcc is:
Using built-in specs.
Target: i386-redhat-linux
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man --infodir=/usr/share/info --enable-shared --enable-threads=posix --enable-checking=release --with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions --enable-libgcj-multifile --enable-languages=c,c++,objc,obj-c++,java,fortran,ada --enable-java-awt=gtk --disable-dssi --with-java-home=/usr/lib/jvm/java-1.4.2-gcj-1.4.2.0/jre --with-cpu=generic --host=i386-redhat-linux
Thread model: posix
gcc version 4.1.1 20060525 (Red Hat 4.1.1-1)