| Summary: | Optimization level -O3 changes the program behavior | ||
|---|---|---|---|
| Product: | gcc | Reporter: | Gopal Mor <mor_gopal> |
| Component: | c++ | Assignee: | Not yet assigned to anyone <unassigned> |
| Status: | RESOLVED DUPLICATE | ||
| Severity: | normal | CC: | adruab, Andreas.Glowatz, andreg, bobm75, boris, c.pop, cdfrey, charles, christophe.guillon, cxl, czang, davids, davmac, devin, djk, dmeggy, duraid, evgeny, ezharkov, fabdouze, fm, gcc-bugs, gcc, ghouston, gino, gopalv82, grigory_zagorodnev, hayim, horst.lehser, hurbain, ivan, ja2morri, jason.elbaum, jfran, jochang, larschri, lindsayd, linuxadmin, lucho, micis, mike, mor_gopal, noaml, pgonzalez, pierre.chatelier, pjh, rarob, rick.ju, spelis, steffen.zimmermann, steger, strasbur, sumii, thomas.anders, warp, zybi |
| Priority: | P3 | ||
| Version: | 3.4.3 | ||
| Target Milestone: | --- | ||
| Host: | Target: | ||
| Build: | Known to work: | ||
| Known to fail: | Last reconfirmed: | ||
Following details are added. // g++ version info $ g++ -v Reading specs from /usr/lib/gcc/x86_64-redhat-linux/3.4.3/specs Configured with: ../configure --prefix=/usr --mandir=/usr/share/man --infodir=/usr/share/info --enable-shared --enable-threads=posix --disable-checking --with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions --enable-java-awt=gtk --host=x86_64-redhat-linux Thread model: posix gcc version 3.4.3 20050227 (Red Hat 3.4.3-22.1) // System info $uname -a Linux NEW1_dev_opteron 2.6.9-11.EL #1 Fri May 20 18:15:25 EDT 2005 x86_64 x86_64 x86_64 GNU/Linux *** This bug has been marked as a duplicate of 21920 *** The following keywords are added to this page, so that developers facing similar problem can search (hopefully) this page using search engines. bitfield problem bug gcc g++ optimization bit field strict aliasing ISO C |
Following program generates different outputs for -O1 and -O3 optimization levels. #include <stdio.h> typedef struct _airInfo{ unsigned int craftNo : 16; unsigned int onAir:1; unsigned int numCrew:2; unsigned int numPassenger:2; unsigned int onLand:1; unsigned int fuelTank:2; unsigned int height: 8; } airInfo_t ; #pragma pack(1) typedef struct _atmosphere { int temp:24; int humidity:24; int pressure:24; int altitude:24; }atmosphere_t; typedef struct _mixedInfo { airInfo_t ai; atmosphere_t atm; }mixedInfo_t; #pragma pack() int main() { mixedInfo_t mi ; mi.ai.height = 17; mi.ai.craftNo = 1; mi.ai.onAir = 0; mi.ai.numCrew = 2; mi.ai.numPassenger = 2; mi.ai.onLand = 0; mi.ai.fuelTank = 0; printf("ai in num = %u\n", (*((unsigned int *)(&mi.ai)))) ; return 0 ; } // Output from program when compiled with -O1 $ g++ -O1 test1.cxx ; ./a.out ai in num = 286523393 // Output from program when compiled with -O3 $ g++ -O3 test1.cxx ; ./a.out ai in num = 1310720