RE: GCC 2.95.3 bug

["Balogh, Ray" <Ray dot Balogh at sycamorenet dot com>]

Dear GCC folks:

I'm having a problem with GCC 2.95.3 that appears to be a compiler bug. ?It seems to be optimizing out inlined function code with side effects, and is related to binding a non-const pointer to a const pointer reference function parameter.? The problem only happens with optimization on, and goes away with -O0.

I assume this must be known problem, but I'm having a hard time finding a patch to fix it. ?It is not an option at this time to upgrade to a newer compiler version, but we are planning to do that in the future.

Below is the test program and output.

I really appreciate any help you can give.
Thanks in advance,
--Ray

-----------------------

#include <iostream>
#include <iomanip>
#include <string.h>

typedef unsigned char byte;

inline unsigned unpack2 (const byte *&data)
{
??? unsigned val = data[0] << 8 | data[1];
??? data += 2;
??? return val;
}
inline unsigned unpack2 (const byte *&data, unsigned &count)
{
??? unsigned val = data[0] << 8 | data[1];
??? data += 2;
??? count -= 2;
??? return val;
}

/*
inline unsigned unpack2 (byte *&data)
{
??? unsigned val = data[0] << 8 | data[1];
??? data += 2;
??? return val;
}
inline unsigned unpack2 (byte *&data, unsigned &count)
{
??? unsigned val = data[0] << 8 | data[1];
??? data += 2;
??? count -= 2;
??? return val;
}
*/

void extractInfo (byte *&data, unsigned &datalen,
?????????????????????? unsigned &f1, unsigned &f2)
{
??? cout << "data = " << data << ", datalen = " << datalen << endl;
??? f1 = unpack2 (data, datalen);
??? cout << "data = " << data << ", datalen = " << datalen << endl;

??? byte *peek = data;
??? //const byte *peek = data;? // **** Adding "const" works around the compiler problem ****
??????????????????????????????? // Another work-around is to uncomment the non-const f1 of unpack2() above

??? unsigned tmp = datalen;
??? cout << "peek = " << peek << ", tmp = " << tmp << endl;
??? (void) unpack2 (peek, tmp);??? // skip over irrelevant field
??? cout << "peek = " << peek << ", tmp = " << tmp << endl;
??? (void) unpack2 (peek, tmp);??? // skip over irrelevant field
??? cout << "peek = " << peek << ", tmp = " << tmp << endl;

??? unsigned xlen = unpack2 (peek, tmp);
??? cout << "peek = " << peek << ", tmp = " << tmp << endl;

??? f2 = xlen;
}


int
main(int, char**)
{
??????? byte *x = "0123456789";
??????? unsigned len = strlen(x);
??????? unsigned fld1, fld2;

??????? extractInfo (x, len, fld1, fld2);

??????? cout << "x = " << x << endl;
??????? cout << "len = " << len << endl;
??????? cout << "fld1 = " << hex << fld1 << dec << endl;
??????? cout << "fld2 = " << hex << fld2 << dec << endl;

??????? return 0;
}

-----------------------

$ gcc-2.95.3-glibc-2.1.3/i686-unknown-linux-gnu/bin/i686-unknown-linux-gnu-g++ -O0 -W -Wall -o foo foo.cpp
$ ./foo
data = 0123456789, datalen = 10
data = 23456789, datalen = 8
peek = 23456789, tmp = 8
peek = 456789, tmp = 6
peek = 6789, tmp = 4
peek = 89, tmp = 2
x = 23456789
len = 8
fld1 = 3031
fld2 = 3637

$ gcc-2.95.3-glibc-2.1.3/i686-unknown-linux-gnu/bin/i686-unknown-linux-gnu-g++ -O2 -W -Wall -o foo foo.cpp
$ ./foo
data = 0123456789, datalen = 10
data = 23456789, datalen = 8
peek = 23456789, tmp = 8
peek = 23456789, tmp = 6
peek = 23456789, tmp = 4
peek = 23456789, tmp = 2
x = 23456789
len = 8
fld1 = 3031
fld2 = 3233