[Bug c/21297] New: buf[i+i]=0 stores buf[i] when -O2

[akr at m17n dot org]

gcc 4.0.0 optimization seems to have a problem.
In the f function in following source, buf[i+i] = '\0' stores '\0' to buf[2].
But it should store to buf[4]. 

% cat t.c
extern int printf (__const char *__restrict __format, ...);

void
f(char *buf)
{
  int i;
  for (i = 0; i < 2; i++) ;
  printf("i=%d\n", i);
  buf[i+i] = '\0';
}

int main(int argc, char **argv)
{
  char buf[] = "0123456789";
  f(buf);
  printf("%s\n", buf);
  return 0;
}
% gcc -v -O2 t.c
Using built-in specs.
Target: i686-pc-linux-gnu
Configured with: ../gcc-4.0.0/configure --prefix=/home/src/gcc
Thread model: posix
gcc version 4.0.0
 /home/src/gcc/libexec/gcc/i686-pc-linux-gnu/4.0.0/cc1 -quiet -v t.c -quiet
-dumpbase t.c -mtune=pentiumpro -auxbase t -O2 -version -o /tmp/ccKF4bLU.s
ignoring nonexistent directory
"/home/src/gcc/lib/gcc/i686-pc-linux-gnu/4.0.0/../../../../i686-pc-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/local/include
 /home/src/gcc/include
 /home/src/gcc/lib/gcc/i686-pc-linux-gnu/4.0.0/include
 /usr/include
End of search list.
GNU C version 4.0.0 (i686-pc-linux-gnu)
        compiled by GNU C version 4.0.0.
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
 as -V -Qy -o /tmp/ccm6JTQG.o /tmp/ccKF4bLU.s
GNU assembler version 2.15 (i386-linux) using BFD version 2.15
 /home/src/gcc/libexec/gcc/i686-pc-linux-gnu/4.0.0/collect2 --eh-frame-hdr -m
elf_i386 -dynamic-linker /lib/ld-linux.so.2 /usr/lib/crt1.o /usr/lib/crti.o
/home/src/gcc/lib/gcc/i686-pc-linux-gnu/4.0.0/crtbegin.o
-L/home/src/gcc/lib/gcc/i686-pc-linux-gnu/4.0.0
-L/home/src/gcc/lib/gcc/i686-pc-linux-gnu/4.0.0/../../.. /tmp/ccm6JTQG.o -lgcc
--as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s --no-as-needed
/home/src/gcc/lib/gcc/i686-pc-linux-gnu/4.0.0/crtend.o /usr/lib/crtn.o
% ./a.out 
i=2
01

The binary produces "01".  But it should be "0123".
If gcc 3.3.5 is used, the binary produces "0123".

-- 
           Summary: buf[i+i]=0 stores buf[i] when -O2
           Product: gcc
           Version: 4.0.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: akr at m17n dot org
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21297