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c++/2926: gcc 2.95.2 with -O3 makes a bad short to int assignment
- To: gcc-gnats at gcc dot gnu dot org
- Subject: c++/2926: gcc 2.95.2 with -O3 makes a bad short to int assignment
- From: dsagal at tower-research dot com
- Date: 24 May 2001 15:16:06 -0000
- Reply-To: dsagal at tower-research dot com
>Number: 2926
>Category: c++
>Synopsis: gcc 2.95.2 with -O3 makes a bad short to int assignment
>Confidential: no
>Severity: serious
>Priority: medium
>Responsible: unassigned
>State: open
>Class: wrong-code
>Submitter-Id: net
>Arrival-Date: Thu May 24 08:26:01 PDT 2001
>Closed-Date:
>Last-Modified:
>Originator: Dmitry Sagalovskiy
>Release: gcc version 2.95.2 19991024 (release)
>Organization:
>Environment:
sparc-sun-solaris2.6
SunOS dmitrysun 5.6 Generic_105181-23 sun4u sparc SUNW,Ultra-5_10
>Description:
In the attached code, I have a variable of type int, to which I assign the result of a call of an inline function, which returns an unsigned short. The problem is that the resulting value of the int is bigger than the largest possible value for an unsigned short.
This problem only seems to occur when I compile with -O3 optimization, it doesn't happen with -O2 or with "-O1 -finline-functions". (So the result of running the program depends on whether -O3 or -O2 optimization was used, that's why I am pretty sure it's a compiler bug.) The problem doesn't happen with gcc 2.7.2.3 when I tried it. It is also very sensitive to the code I have, i.e. removing some seemingly irrelevant variables or even changing a seemingly unrelated function from virtual to plain will make this behavious disappear.
I apologize for the lack of meaningfulness of the code. I pruned it down quite a bit, and had to change and rearrange things to simplify the code while preserving the strange behavior.
Seeing how the bug is unstable, it's not hard to change something so that it doesn't show up, but this way of fixing things just isn't satisfying :)
Thanks a lot, I would really appreciate any insight on this problem.
Dmitry
>How-To-Repeat:
g++ -o bugtest -O3 -g -Wall bugtest.cpp
./bugtest
[compare to the results when compile with -O2, for example]
>Fix:
Don't use -O3 optimization?
>Release-Note:
>Audit-Trail:
>Unformatted:
----gnatsweb-attachment----
Content-Type: text/plain; name="bugtest.ii"
Content-Disposition: inline; filename="bugtest.ii"
# 1 "bugtest.cpp"
# 1 "/usr/include/stdio.h" 1 3 4
#pragma ident "@(#)stdio.h 1.49 97/05/09 SMI"
# 1 "/usr/include/sys/feature_tests.h" 1 3 4
#pragma ident "@(#)feature_tests.h 1.13 97/06/26 SMI"
extern "C" {
}
# 17 "/usr/include/stdio.h" 2 3 4
# 1 "/usr/include/sys/va_list.h" 1 3 4
#pragma ident "@(#)va_list.h 1.6 96/01/26 SMI"
extern "C" {
# 41 "/usr/include/sys/va_list.h" 3 4
typedef void *__va_list;
}
# 18 "/usr/include/stdio.h" 2 3 4
extern "C" {
typedef unsigned int size_t;
typedef long long __longlong_t;
typedef long off_t;
typedef __longlong_t off64_t;
typedef long fpos_t;
typedef __longlong_t fpos64_t;
# 122 "/usr/include/stdio.h" 3 4
typedef struct
{
int _cnt;
unsigned char *_ptr;
unsigned char *_base;
unsigned char _flag;
unsigned char _file;
} FILE;
extern FILE __iob[20 ];
extern FILE *_lastbuf;
extern unsigned char *_bufendtab[];
extern unsigned char _sibuf[], _sobuf[];
# 222 "/usr/include/stdio.h" 3 4
extern int remove(const char *);
extern int rename(const char *, const char *);
extern FILE *tmpfile(void);
extern char *tmpnam(char *);
extern int fclose(FILE *);
extern int fflush(FILE *);
extern FILE *fopen(const char *, const char *);
extern FILE *freopen(const char *, const char *, FILE *);
extern void setbuf(FILE *, char *);
extern void setbuffer(FILE *, char *, size_t);
extern int setlinebuf(FILE *);
extern int setvbuf(FILE *, char *, int, size_t);
extern int fprintf(FILE *, const char *, ...);
extern int fscanf(FILE *, const char *, ...);
extern int printf(const char *, ...);
extern int scanf(const char *, ...);
extern int snprintf(char *, size_t, const char *, ...);
extern int sprintf(char *, const char *, ...);
extern int sscanf(const char *, const char *, ...);
extern int vfprintf(FILE *, const char *, __va_list);
extern int vprintf(const char *, __va_list);
extern int vsnprintf(char *, size_t, const char *, __va_list);
extern int vsprintf(char *, const char *, __va_list);
extern int fgetc(FILE *);
extern char *fgets(char *, int, FILE *);
extern int fputc(int, FILE *);
extern int fputs(const char *, FILE *);
extern int getc(FILE *);
extern int getchar(void);
extern char *gets(char *);
extern int putc(int, FILE *);
extern int putchar(int);
extern int puts(const char *);
extern int ungetc(int, FILE *);
extern size_t fread(void *, size_t, size_t, FILE *);
extern size_t fwrite(const void *, size_t, size_t, FILE *);
extern int fgetpos(FILE *, fpos_t *);
extern int fseek(FILE *, long, int);
extern int fsetpos(FILE *, const fpos_t *);
extern long ftell(FILE *);
extern void rewind(FILE *);
extern void clearerr(FILE *);
extern int feof(FILE *);
extern int ferror(FILE *);
extern void perror(const char *);
extern int __filbuf(FILE *);
extern int __flsbuf(int, FILE *);
extern FILE *fdopen(int, const char *);
extern char *ctermid(char *);
extern int fileno(FILE *);
# 319 "/usr/include/stdio.h" 3 4
extern FILE *popen(const char *, const char *);
extern char *cuserid(char *);
extern char *tempnam(const char *, const char *);
extern int getopt(int, char *const *, const char *);
extern int getsubopt(char **, char *const *, char **);
extern char *optarg;
extern int optind, opterr, optopt;
extern int getw(FILE *);
extern int putw(int, FILE *);
extern int pclose(FILE *);
extern int fseeko(FILE *, off_t, int);
extern off_t ftello(FILE *);
extern FILE *fopen64(const char *, const char *);
extern FILE *freopen64(const char *, const char *, FILE *);
extern FILE *tmpfile64(void);
extern int fgetpos64(FILE *, fpos64_t *);
extern int fsetpos64(FILE *, const fpos64_t *);
extern int fseeko64(FILE *, off64_t, int);
extern off64_t ftello64(FILE *);
# 467 "/usr/include/stdio.h" 3 4
# 515 "/usr/include/stdio.h" 3 4
}
# 1 "bugtest.cpp" 2
# 1 "/usr/include/string.h" 1 3 4
#pragma ident "@(#)string.h 1.19 96/03/12 SMI"
extern "C" {
extern void *memcpy(void *, const void *, size_t);
extern void *memmove(void *, const void *, size_t);
extern char *strcpy(char *, const char *);
extern char *strncpy(char *, const char *, size_t);
extern char *strcat(char *, const char *);
extern char *strncat(char *, const char *, size_t);
extern int memcmp(const void *, const void *, size_t);
extern int strcmp(const char *, const char *);
extern int strcoll(const char *, const char *);
extern int strncmp(const char *, const char *, size_t);
extern size_t strxfrm(char *, const char *, size_t);
extern void *memchr(const void *, int, size_t);
extern char *strchr(const char *, int);
extern size_t strcspn(const char *, const char *);
extern char *strpbrk(const char *, const char *);
extern char *strrchr(const char *, int);
extern size_t strspn(const char *, const char *);
extern char *strstr(const char *, const char *);
extern char *strtok(char *, const char *);
extern void *memset(void *, int, size_t);
extern char *strerror(int);
extern size_t strlen(const char *);
extern void *memccpy(void *, const void *, int, size_t);
extern char *strsignal(int);
extern int ffs(int);
extern int strcasecmp(const char *, const char *);
extern int strncasecmp(const char *, const char *, size_t);
extern char *strdup(const char *);
# 136 "/usr/include/string.h" 3 4
}
# 2 "bugtest.cpp" 2
static const int Byte1Mask = 0x000000ff;
static const int Byte2Mask = 0x0000ff00;
inline void reverseInPlace(unsigned short *pval) {
unsigned int ret, val = *pval;
ret = ((val & Byte2Mask) >> 8);
ret |= ((val & Byte1Mask) << 8);
*pval = ret;
}
struct Buffer {
union {
char buffer[80];
long _aligndummy;
};
virtual int read(void *buf, int len)
{ memcpy(buf, buffer, len); return len; }
};
unsigned short getMsgSize(const char* msgbuf, bool reversed) {
unsigned short size = *(const unsigned short*)msgbuf;
if(reversed) reverseInPlace(&size);
return size;
}
const int BIGGER_THAN_SHORT = 65536;
void readMessage(Buffer &s, char *buffer, bool reversed)
{
int numDone, numToRead;
char *ptr = buffer;
numToRead = 4;
while(numToRead > 0 && (numDone = s.read(ptr, numToRead)) > 0) {
numToRead -= numDone;
ptr += numDone;
}
if(numToRead > 0) return;
numToRead = getMsgSize(buffer, reversed);
numToRead = getMsgSize(buffer, reversed);
if(numToRead > BIGGER_THAN_SHORT) {
printf("ERROR: numToRead = %d is bigger than %d\n",
numToRead, BIGGER_THAN_SHORT);
}
printf("numToRead = 0x%X, should be 0x%X\n", numToRead,
(unsigned short)numToRead);
}
int main()
{
char buffer[80];
Buffer bsock;
bsock.buffer[0] = 0x0A;
bsock.buffer[1] = 0x00;
bsock.buffer[2] = 0x00;
bsock.buffer[3] = 0x00;
readMessage(bsock, buffer, true);
}