Sender: chad AT mail DOT arc DOT nasa DOT gov
Message-ID: <37A68C4F.FE44DF8C@vision.arc.nasa.gov>
Date: Tue, 03 Aug 1999 06:29:35 +0000
From: Chad Netzer <chad AT vision DOT arc DOT nasa DOT gov>
X-Mailer: Mozilla 4.61 [en] (X11; I; Linux 2.2.10 i686)
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MIME-Version: 1.0
To: pgcc AT delorie DOT com
Subject: Code generation bug in 1.1.3 when compiling Python 1.5.2
Content-Type: multipart/mixed;
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Reply-To: pgcc AT delorie DOT com

This is a multi-part message in MIME format.
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The enclosed program fails to compile correctly with PGCC 1.1.3 (the
versions that comes with Mandrake 6.0) when optimization is selected.
The resulting assembly language, when optimization is selected (-O1
for example) fails to include a vital "test" instruction after the
left shift (x << 1), which it does using an "lea" instruction, rather
than a traditional left shift operand.  Here is the important section:

  if ((long)x < 0 && (sign > 0 || (x << 1) != 0))
    goto overflow;

and here is a section of the optimized assembly:

0x8048451 <main+81>: test   %ebx,%ebx
0x8048453 <main+83>: jge    0x8048464 <main+100>
0x8048455 <main+85>: cmpl   $0x0,0xfffffffc(%ebp)
0x8048459 <main+89>: jg     0x8048470 <main+112>
0x804845b <main+91>: lea    0x0(,%ebx,2),%eax
0x8048462 <main+98>: jne    0x8048470 <main+112>

There should probably be a test between main+91 and main+98 (since lea
sets no condition codes).  This bug was discovered when trying to
compile
the Python 1.5.2 sources on a Mandrake 6.0 system; the Python regression

tests were able to determine that the optimized compiled version did not

treat -MAXINT-1 properly (as this piece of code is supposed to handle
the
special wraparound case).  Here is the relevant debugging info:

% gcc -v --save-temps -O1 pgcc_bug.c
Reading specs from /usr/lib/gcc-lib/i686-pc-linux-gnu/pgcc-2.91.66/specs

gcc version pgcc-2.91.66 19990314 (egcs-1.1.2 release)
 /usr/lib/gcc-lib/i686-pc-linux-gnu/pgcc-2.91.66/cpp -lang-c -v -undef
-D__GNUC__=2 -D__GNUC_MINOR__=91 -D__ELF__ -Dunix -Di386 -D__i386__
-Dlinux -D__ELF__ -D__unix__ -D__i386__ -D__i386__ -D__linux__ -D__unix
-D__i386 -D__linux -Asystem(posix) -D__OPTIMIZE__ -Asystem(unix)
-Acpu(i386) -Amachine(i386) -Di386 -D__i386 -D__i386__ -Di686
-Dpentiumpro -D__i686 -D__i686__ -D__pentiumpro -D__pentiumpro__
pgcc_bug.c pgcc_bug.i
GNU CPP version pgcc-2.91.66 19990314 (egcs-1.1.2 release) (i386
Linux/ELF)
#include "..." search starts here:
#include <...> search starts here:
 /usr/lib/gcc-lib/i686-pc-linux-gnu/pgcc-2.91.66/include
 /usr/include
End of search list.
 /usr/lib/gcc-lib/i686-pc-linux-gnu/pgcc-2.91.66/cc1 pgcc_bug.i -quiet
-dumpbase pgcc_bug.c -O1 -version -o pgcc_bug.s
GNU C version pgcc-2.91.66 19990314 (egcs-1.1.2 release)
(i686-pc-linux-gnu) compiled by GNU C version pgcc-2.91.66 19990314
(egcs-1.1.2 release).
pgcc_bug.c: In function `main':
pgcc_bug.c:12: warning: decimal constant is so large that it is unsigned

 as -V -Qy -o pgcc_bug.o pgcc_bug.s
GNU assembler version 2.9.1 (i386-mandrake-linux), using BFD version
2.9.1.0.25 /usr/lib/gcc-lib/i686-pc-linux-gnu/pgcc-2.91.66/collect2 -m
elf_i386 -dynamic-linker /lib/ld-linux.so.2 /usr/lib/crt1.o
/usr/lib/crti.o
/usr/lib/gcc-lib/i686-pc-linux-gnu/pgcc-2.91.66/crtbegin.o
-L/usr/lib/gcc-lib/i686-pc-linux-gnu/pgcc-2.91.66 pgcc_bug.o -lgcc -lc
-lgcc /usr/lib/gcc-lib/i686-pc-linux-gnu/pgcc-2.91.66/crtend.o
/usr/lib/crtn.o


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Content-Type: text/plain; charset=us-ascii;
 name="pgcc_bug.c"
Content-Transfer-Encoding: 7bit
Content-Disposition: inline;
 filename="pgcc_bug.c"

#include <stdlib.h>
#include <stdio.h>

main(int argc, char *argv[])
{
  unsigned long x;
  int i, sign;

  /* This is bogosity to fool the optimizer */

  sign = -1;
  x = 2147483648;

  if (!strcmp(argv[0],
	      "Never ever run this loop, just use it to insure the optimizer "
	      "cannot rely on the x and sign vars being constant (and thus optimize"
	      "away the whole test below exhibiting the bug."))
    {
      i = 1;
      while (--i >= 10000) { /* large value to skip this loop */
	x = x-1;
	sign = ~sign;
      }
    }
  /*
   * end of bogosity section, just needed to establish the threat
   * of modifying x, so that the compiler won't optimize it away.
   */

  /* This next part exhibits the error.  It should NOT goto the overflow label,
     because the (sign > 0) || (x << 1) != 0 part should resolve to FALSE.
     However, it can be shown that pgcc 1.1.3, when the optimization is on (even -O1),
     produces an "lea" instruction to do the left shift and then does a "je" without
     a "test".  This "test" IS produced when optimization is NOT enabled, and the
     program behaves as it should. */

  if ((long)x < 0 && (sign > 0 || (x << 1) != 0))
    goto overflow;
  return (long)x * sign;

 overflow:
  printf("If this prints, there is a bug\n");
}

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