http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50065
Bug #: 50065
Summary: -Os, -O2, -O3 optimization breaks LD/ST ordering on
32-bit SPARC
Classification: Unclassified
Product: gcc
Version: 4.6.1
Status: UNCONFIRMED
Severity: major
Priority: P3
Component: c
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: tanzhan...@gmail.com
Considering the following C code.
static inline int spinlock_trylock(char* lock)
{
int reg;
__asm__ __volatile__ ("ldstub [%1],%0" : "=r"(reg) : "r"(lock) : "memory",
"cc");
return reg;
}
static inline int spinlock_is_locked(char* lock)
{
int reg;
__asm__ __volatile__ ("ldub [%1],%0" : "=r"(reg) : "r"(lock) : "memory");
return reg;
}
static inline void spinlock_lock(char* lock)
{
while(spinlock_trylock(lock))
while(spinlock_is_locked(lock));
}
static inline void spinlock_unlock(char* lock)
{
*(volatile unsigned char*)lock = 0;
}
char remap_lock;
int remap_barrier;
void inc_remap_barrier()
{
spinlock_lock(&remap_lock);
remap_barrier++;
spinlock_unlock(&remap_lock);
}
A simple ++ counter is protected by a spinlock implemented with the ldstub
atomic instruction on SPARC.
If I use -Os/-O2/-O3 to optimize the code, e.g.
sparc-ramp-gcc -c -Os test.c -o test.o
gcc will generate the following code
00000000 <inc_remap_barrier>:
0: 03 00 00 00 sethi %hi(0), %g1
4: 10 80 00 06 b 1c <inc_remap_barrier+0x1c>
8: 82 10 60 00 mov %g1, %g1 ! 0 <inc_remap_barrier>
c: c4 08 40 00 ldub [ %g1 ], %g2
10: 80 a0 a0 00 cmp %g2, 0
14: 12 bf ff fe bne c <inc_remap_barrier+0xc>
18: 01 00 00 00 nop
1c: c4 68 40 00 ldstub [ %g1 ], %g2
20: 80 a0 a0 00 cmp %g2, 0
24: 12 bf ff fa bne c <inc_remap_barrier+0xc>
28: 05 00 00 00 sethi %hi(0), %g2
2c: c0 28 40 00 clrb [ %g1 ]
30: c6 00 a0 00 ld [ %g2 ], %g3
34: 86 00 e0 01 inc %g3
38: 81 c3 e0 08 retl
3c: c6 20 a0 00 st %g3, [ %g2 ]
instruction 2C, clrb [%g1] corresponds to inline function 'spinlock_unlock'
*(volatile unsigned char*)lock = 0;
This happens before the lock protected content 'remap_barrier++', i.e.
30: c6 00 a0 00 ld [ %g2 ], %g3
34: 86 00 e0 01 inc %g3
38: 81 c3 e0 08 retl
3c: c6 20 a0 00 st %g3, [ %g2 ] ---> use the branch delay slot
This is wrong and will cause serious lock issues under a multithreading
environment.
However, the same code works fine with -O1 and -O0
This problem happens with couple of cross GCC builds (4.3.2 / 4.6.1) at our
side with various configurations (with glibc or a bare metal setting).
It breaks with the following configurations:
1.
COLLECT_GCC=sparc-ramp-gcc
COLLECT_LTO_WRAPPER=/home/charming/toolchain/sparc-ramp/libexec/gcc/sparc-ramp-elf/4.6.1/lto-wrapper
Target: sparc-ramp-elf
Configured with: /home/charming/toolchain/.build/src/gcc-4.6.1/configure
--build=i686-build_pc-linux-gnu --host=i686-build_pc-linux-gnu
--target=sparc-ramp-elf --prefix=/home/charming/toolchain/sparc-ramp
--with-local-prefix=/home/charming/toolchain/sparc-ramp/sparc-ramp-elf/sysroot
--disable-multilib --disable-libmudflap
--with-sysroot=/home/charming/toolchain/sparc-ramp/sparc-ramp-elf/sysroot
--with-newlib --enable-threads=no --disable-shared
--with-pkgversion='crosstool-NG 1.12.0' --disable-__cxa_atexit
--with-gmp=/home/charming/toolchain/.build/sparc-ramp-elf/build/static
--with-mpfr=/home/charming/toolchain/.build/sparc-ramp-elf/build/static
--with-mpc=/home/charming/toolchain/.build/sparc-ramp-elf/build/static
--with-ppl=/home/charming/toolchain/.build/sparc-ramp-elf/build/static
--with-cloog=/home/charming/toolchain/.build/sparc-ramp-elf/build/static
--with-libelf=/home/charming/toolchain/.build/sparc-ramp-elf/build/static
--enable-lto
--with-host-libstdcxx='-L/home/charming/toolchain/.build/sparc-ramp-elf/build/static/lib
-lpwl' --enable-target-optspace --disable-libgomp --disable-libmudflap
--disable-nls --enable-languages=c --with-cpu=v8
Thread model: single
gcc version 4.6.1 (crosstool-NG 1.12.0)
2.
Using built-in specs.
COLLECT_GCC=sparc-ramp-gcc
COLLECT_LTO_WRAPPER=/home/xtan/toolchain/sparc-ramp/libexec/gcc/sparc-ramp-linux-gnu/4.6.1/lto-wrapper
Target: sparc-ramp-linux-gnu
Configured with: /home/xtan/toolchain/.build/src/gcc-4.6.1/configure
--build=x86_64-build_unknown-linux-gnu --host=x86_64-build_unknown-linux-gnu
--target=sparc-ramp-linux-gnu --prefix=/home/xtan/toolchain/sparc-ramp
--with-sysroot=/home/xtan/toolchain/sparc-ramp/sparc-ramp-linux-gnu/sysroot
--enable-languages=c,c++ --disable-multilib
--with-pkgversion=crosstool-NG-1.11.3 --enable-__cxa_atexit
--disable-libmudflap --disable-libgomp --disable-libssp
--with-gmp=/home/xtan/toolchain/.build/sparc-ramp-linux-gnu/build/static
--with-mpfr=/home/xtan/toolchain/.build/sparc-ramp-linux-gnu/build/static
--with-mpc=/home/xtan/toolchain/.build/sparc-ramp-linux-gnu/build/static
--with-ppl=/home/xtan/toolchain/.build/sparc-ramp-linux-gnu/build/static
--with-cloog=/home/xtan/toolchain/.build/sparc-ramp-linux-gnu/build/static
--with-libelf=/home/xtan/toolchain/.build/sparc-ramp-linux-gnu/build/static
--with-host-libstdcxx='-L/home/xtan/toolchain/.build/sparc-ramp-linux-gnu/build/static/lib
-lpwl' --enable-threads=posix --enable-target-optspace
--with-local-prefix=/home/xtan/toolchain/sparc-ramp/sparc-ramp-linux-gnu/sysroot
--disable-nls --enable-symvers=gnu --enable-c99 --enable-long-long
Thread model: posix
gcc version 4.6.1 (crosstool-NG-1.11.3)
