[Bug c/40907] New: -O2 generates incorrect code on i386

[Bug c/40907] New: -O2 generates incorrect code on i386

[sperber at deinprogramm dot de]

When compiling Scheme 48 with -O2, an overflow check in the internal arithmetic
gets elided. This bug showed up sometime after gcc 4.0.1.  It's in 4.4.1, and
in the 4.5-20090723 snapshot.  It's been seen on 32-bit Linux and 32-bit
FreeBSD.

The code snippet in question in c/scheme48vm-32.c is this:

 L12553: {
  b_1970X = arg0K0;
  lo_a_1971X = 65535 & a_1687X;
  lo_b_1972X = 65535 & b_1970X;
  hi_a_1973X = 65535 & (((a_1687X)>>16));
  hi_b_1974X = 65535 & (((b_1970X)>>16));
  lo_c_1975X = SMALL_MULTIPLY(lo_a_1971X, lo_b_1972X);
  v_1976X = SMALL_MULTIPLY(lo_a_1971X, hi_b_1974X);
  v_1977X = SMALL_MULTIPLY(lo_b_1972X, hi_a_1973X);
  mid_c_1978X = v_1976X + v_1977X;
  c_1979X = lo_c_1975X + (((mid_c_1978X)<<16));
  if ((0 < hi_a_1973X)) {
    if ((0 < hi_b_1974X)) {
      val_1980X = integer_multiply(arg2_1113X, y_1114X);
      SvalS = val_1980X;
      Scode_pointerS = ((Scode_pointerS) + 1);
      arg3K0 = (Scode_pointerS);
      goto L32913;}
    else {
      goto L12595;}}
  else {
    goto L12595;}}
 ...
 L12595: {
  if ((536870911 < lo_c_1975X)) {
    val_2153X = integer_multiply(arg2_1113X, y_1114X);
    SvalS = val_2153X;
    Scode_pointerS = ((Scode_pointerS) + 1);
    arg3K0 = (Scode_pointerS);
    goto L32913;}
  else {
    if ((lo_c_1975X < 0)) {

That last test never gets run, and the consequent code does not get run, even
when lo_c_1975X is negative.

I've not been able to boil the code down to a smaller snippet: Smaller programs
containing this code compile OK.


-- 
           Summary: -O2 generates incorrect code on i386
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: sperber at deinprogramm dot de
 GCC build triplet: i386-unknown-freebsd7.2
  GCC host triplet: i386-unknown-freebsd7.2
GCC target triplet: i386-unknown-freebsd7.2


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40907

[Bug c/40907] -O2 generates incorrect code on i386

[sperber at deinprogramm dot de]

------- Comment #1 from sperber at deinprogramm dot de  2009-07-29 17:37 -------
Created an attachment (id=18271)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=18271&action=view)
preprocessed source file

gcc -v output:

kramer[420] gcc -v -save-temps -c  -DHAVE_CONFIG_H
-D__COMPILING_SCHEME48_ITSELF__
-DS48_HOST_ARCHITECTURE=\"i386-unknown-freebsd7.2\" -I ./c -I../s48/c
-I../s48/c/net -I../s48/c/bibop -g -O2 -pthread -o c/scheme48vm-32.o
../s48/c/scheme48vm-32.c
Using built-in specs.
Target: i386-unknown-freebsd7.2
Configured with: ./configure
--prefix=/afs/informatik.uni-tuebingen.de/i386_fbsd72/gcc-4.5-20090723
--with-gmp=/afs/informatik.uni-tuebingen.de/i386_fbsd70
--with-mpfr=/afs/informatik.uni-tuebingen.de/i386_fbsd72
Thread model: posix
gcc version 4.5.0 20090723 (experimental) (GCC) 
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-c' '-DHAVE_CONFIG_H'
'-D__COMPILING_SCHEME48_ITSELF__'
'-DS48_HOST_ARCHITECTURE="i386-unknown-freebsd7.2"' '-I' './c' '-I../s48/c'
'-I../s48/c/net' '-I../s48/c/bibop' '-g' '-O2' '-pthread' '-o'
'c/scheme48vm-32.o' '-mtune=i386'

/afs/informatik.uni-tuebingen.de/i386_fbsd72/gcc-4.5-20090723/libexec/gcc/i386-unknown-freebsd7.2/4.5.0/cc1
-E -quiet -v -I ./c -I../s48/c -I../s48/c/net -I../s48/c/bibop -DHAVE_CONFIG_H
-D__COMPILING_SCHEME48_ITSELF__
-DS48_HOST_ARCHITECTURE="i386-unknown-freebsd7.2" ../s48/c/scheme48vm-32.c
-mtune=i386 -g -fworking-directory -O2 -fpch-preprocess -o scheme48vm-32.i
ignoring nonexistent directory
"/afs/informatik.uni-tuebingen.de/i386_fbsd72/gcc-4.5-20090723/lib/gcc/i386-unknown-freebsd7.2/4.5.0/../../../../i386-unknown-freebsd7.2/include"
#include "..." search starts here:
#include <...> search starts here:
 ./c
 ../s48/c
 ../s48/c/net
 ../s48/c/bibop
 /usr/local/include
 /afs/informatik.uni-tuebingen.de/i386_fbsd72/gcc-4.5-20090723/include

/afs/informatik.uni-tuebingen.de/i386_fbsd72/gcc-4.5-20090723/lib/gcc/i386-unknown-freebsd7.2/4.5.0/include

/afs/informatik.uni-tuebingen.de/i386_fbsd72/gcc-4.5-20090723/lib/gcc/i386-unknown-freebsd7.2/4.5.0/include-fixed
 /usr/include
End of search list.
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-c' '-DHAVE_CONFIG_H'
'-D__COMPILING_SCHEME48_ITSELF__'
'-DS48_HOST_ARCHITECTURE="i386-unknown-freebsd7.2"' '-I' './c' '-I../s48/c'
'-I../s48/c/net' '-I../s48/c/bibop' '-g' '-O2' '-pthread' '-o'
'c/scheme48vm-32.o' '-mtune=i386'

/afs/informatik.uni-tuebingen.de/i386_fbsd72/gcc-4.5-20090723/libexec/gcc/i386-unknown-freebsd7.2/4.5.0/cc1
-fpreprocessed scheme48vm-32.i -quiet -dumpbase scheme48vm-32.c -mtune=i386
-auxbase-strip c/scheme48vm-32.o -g -O2 -version -o scheme48vm-32.s
GNU C (GCC) version 4.5.0 20090723 (experimental) (i386-unknown-freebsd7.2)
        compiled by GNU C version 4.5.0 20090723 (experimental), GMP version
4.2.4, MPFR version 2.4.1
GGC heuristics: --param ggc-min-expand=30 --param ggc-min-heapsize=4096
GNU C (GCC) version 4.5.0 20090723 (experimental) (i386-unknown-freebsd7.2)
        compiled by GNU C version 4.5.0 20090723 (experimental), GMP version
4.2.4, MPFR version 2.4.1
GGC heuristics: --param ggc-min-expand=30 --param ggc-min-heapsize=4096
Compiler executable checksum: 2ffc104588e5268bc0421635a8aaa323
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-c' '-DHAVE_CONFIG_H'
'-D__COMPILING_SCHEME48_ITSELF__'
'-DS48_HOST_ARCHITECTURE="i386-unknown-freebsd7.2"' '-I' './c' '-I../s48/c'
'-I../s48/c/net' '-I../s48/c/bibop' '-g' '-O2' '-pthread' '-o'
'c/scheme48vm-32.o' '-mtune=i386'
 as -o c/scheme48vm-32.o scheme48vm-32.s
COMPILER_PATH=/afs/informatik.uni-tuebingen.de/i386_fbsd72/gcc-4.5-20090723/libexec/gcc/i386-unknown-freebsd7.2/4.5.0/:/afs/informatik.uni-tuebingen.de/i386_fbsd72/gcc-4.5-20090723/libexec/gcc/i386-unknown-freebsd7.2/4.5.0/:/afs/informatik.uni-tuebingen.de/i386_fbsd72/gcc-4.5-20090723/libexec/gcc/i386-unknown-freebsd7.2/:/afs/informatik.uni-tuebingen.de/i386_fbsd72/gcc-4.5-20090723/lib/gcc/i386-unknown-freebsd7.2/4.5.0/:/afs/informatik.uni-tuebingen.de/i386_fbsd72/gcc-4.5-20090723/lib/gcc/i386-unknown-freebsd7.2/
LIBRARY_PATH=/afs/informatik.uni-tuebingen.de/i386_fbsd72/gcc-4.5-20090723/lib/gcc/i386-unknown-freebsd7.2/4.5.0/:/afs/informatik.uni-tuebingen.de/i386_fbsd72/gcc-4.5-20090723/lib/gcc/i386-unknown-freebsd7.2/4.5.0/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-c' '-DHAVE_CONFIG_H'
'-D__COMPILING_SCHEME48_ITSELF__'
'-DS48_HOST_ARCHITECTURE="i386-unknown-freebsd7.2"' '-I' './c' '-I../s48/c'
'-I../s48/c/net' '-I../s48/c/bibop' '-g' '-O2' '-pthread' '-o'
'c/scheme48vm-32.o' '-mtune=i386'


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40907

[Bug c/40907] -O2 generates incorrect code on i386

[pinskia at gcc dot gnu dot org]

------- Comment #2 from pinskia at gcc dot gnu dot org  2009-07-29 17:40 -------
../s48/c/scheme48vm-32.c: In function 'current_code_vector':
../s48/c/scheme48vm-32.c:1426:8: warning: 'v_231X' may be used uninitialized in
this function
../s48/c/scheme48vm-32.c: In function 'HtopD12136':
../s48/c/scheme48vm-32.c:3018:8: warning: 'i_572X' may be used uninitialized in
this function
../s48/c/scheme48vm-32.c:3023:8: warning: 'table_567X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c: In function 's48_restart':
../s48/c/scheme48vm-32.c:6130:8: warning: 'get_current_port0_return_value' may
be used uninitialized in this function
../s48/c/scheme48vm-32.c:6145:7: warning: 'push_list_return_tag' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:6147:7: warning: 'rest_list_setupAgc_return_tag' may
be used uninitialized in this function
../s48/c/scheme48vm-32.c:6171:8: warning: 'count_1120X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:6172:8: warning: 'wants_stack_args_1121X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:6173:8: warning: 'stack_arg_count_1122X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:6175:8: warning: 'list_arg_count_1124X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:6250:8: warning: 'count_2692X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:6267:8: warning: 'cont_2675X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:6275:10: warning: 'cont_2667X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:6563:8: warning: 'minutesP_2379X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:6617:8: warning: 'y_2325X' may be used uninitialized
in this function
../s48/c/scheme48vm-32.c:6619:8: warning: 'x_2323X' may be used uninitialized
in this function
../s48/c/scheme48vm-32.c:6680:10: warning: 'arg_top_2262X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:6681:8: warning: 'list_arg_count_2261X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:6682:8: warning: 'list_args_2260X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:6683:8: warning: 'stack_nargs_2259X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:6684:8: warning: 'bytes_used_2258X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:6928:8: warning: 'y_2014X' may be used uninitialized
in this function
../s48/c/scheme48vm-32.c:6998:10: warning: 'arg_top_1944X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:7000:8: warning: 'count_1942X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:7012:8: warning: 'list_arg_count_1930X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:7013:8: warning: 'list_args_1929X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:7014:8: warning: 'stack_nargs_1928X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:7027:8: warning: 'proc_1915X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:7041:8: warning: 'stack_nargs_1901X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:7042:8: warning: 'list_args_1900X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:7051:8: warning: 'skip_1891X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:7057:8: warning: 'skip_1885X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:7103:8: warning: 'length_1839X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:7105:8: warning: 'list_args_1837X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:7106:8: warning: 'stack_nargs_1836X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:7107:8: warning: 'maybe_cont_1835X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:7118:8: warning: 'return_address_1824X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:7119:8: warning: 'template_1823X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:7290:8: warning: 'arg2_1652X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:7635:8: warning: 'nargs_1307X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:7636:8: warning: 'code_1306X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:7637:8: warning: 'template_1305X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:7645:8: warning: 'stack_nargs_1297X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:7651:8: warning: 'length_1291X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:7653:8: warning: 'stack_nargs_1289X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:7654:8: warning: 'list_args_1288X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:7662:8: warning: 'stack_arg_count_1280X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:7663:8: warning: 'stack_arg_count_1279X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:7725:8: warning: 'n_1217X' may be used uninitialized
in this function
../s48/c/scheme48vm-32.c:7750:8: warning: 'final_stack_arg_count_1192X' may be
used uninitialized in this function
../s48/c/scheme48vm-32.c:7756:8: warning: 'skip_1186X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:7757:8: warning: 'skip_1185X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:7792:8: warning: 'skip_1150X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:7802:8: warning: 'stack_arg_count_1140X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:7803:8: warning: 'exception_1139X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:7791:8: warning: 'v_1151X' may be used uninitialized
in this function
../s48/c/scheme48vm-32.c:7790:8: warning: 'v_1152X' may be used uninitialized
in this function
../s48/c/scheme48vm-32.c:7789:8: warning: 'v_1153X' may be used uninitialized
in this function
../s48/c/scheme48vm-32.c:7788:8: warning: 'v_1154X' may be used uninitialized
in this function
../s48/c/scheme48vm-32.c:7786:8: warning: 'v_1156X' may be used uninitialized
in this function
../s48/c/scheme48vm-32.c:7785:8: warning: 'v_1157X' may be used uninitialized
in this function
../s48/c/scheme48vm-32.c:7784:8: warning: 'v_1158X' may be used uninitialized
in this function
../s48/c/scheme48vm-32.c:7783:8: warning: 'v_1159X' may be used uninitialized
in this function
../s48/c/scheme48vm-32.c:7782:8: warning: 'v_1160X' may be used uninitialized
in this function
../s48/c/scheme48vm-32.c:7781:8: warning: 'v_1161X' may be used uninitialized
in this function
../s48/c/scheme48vm-32.c:7778:8: warning: 'v_1164X' may be used uninitialized
in this function
../s48/c/scheme48vm-32.c:7777:8: warning: 'v_1165X' may be used uninitialized
in this function
../s48/c/scheme48vm-32.c:7776:8: warning: 'v_1166X' may be used uninitialized
in this function
../s48/c/scheme48vm-32.c:7775:8: warning: 'v_1167X' may be used uninitialized
in this function
../s48/c/scheme48vm-32.c:7108:8: warning: 'v_1834X' may be used uninitialized
in this function
../s48/c/scheme48vm-32.c:7099:8: warning: 'v_1843X' may be used uninitialized
in this function
../s48/c/scheme48vm-32.c:7793:8: warning: 'nativeP_1149X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:7794:8: warning: 'stack_space_1148X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:7804:8: warning: 'total_arg_count_1138X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:7805:8: warning: 'code_1137X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:6198:8: warning: 'v_2744X' may be used uninitialized
in this function
../s48/c/scheme48vm-32.c:7796:8: warning: 'v_1146X' may be used uninitialized
in this function
../s48/c/scheme48vm-32.c: In function 's48_call_startup_procedure':
../s48/c/scheme48vm-32.c:19827:8: warning: 'output_encoding_2807X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:19828:8: warning: 'input_encoding_2806X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:19829:8: warning: 'vm_channel_2805X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:19832:8: warning: 'vm_channel_2802X' may be used
uninitialized in this function
../s48/c/scheme48vm-32.c:19835:8: warning: 'vm_channel_2799X' may be used
uninitialized in this function


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40907

[Bug c/40907] -O2 generates incorrect code on i386

[pinskia at gcc dot gnu dot org]

------- Comment #3 from pinskia at gcc dot gnu dot org  2009-07-29 17:41 -------
>an overflow check in the internal arithmetic gets elided.

Does -Wno-strict-overflow fix the issue?  Does -fwrapv fix the issue?

If so you are depending on overflow being defined as wrapping and the code is
undefined as defined by the standard.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |WAITING


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40907

[Bug c/40907] -O2 generates incorrect code on i386

[sperber at deinprogramm dot de]

------- Comment #4 from sperber at deinprogramm dot de  2009-07-29 17:42 -------
(From update of attachment 18271)
In the preprocessed file, the test that gets elided is in line 20307.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40907

[Bug c/40907] -O2 generates incorrect code on i386

[rguenth at gcc dot gnu dot org]

------- Comment #5 from rguenth at gcc dot gnu dot org  2009-07-29 18:12 -------
  long lo_b_2268X;
  long lo_a_2267X;
  long lo_c_2271X;
...
  lo_a_2267X = 65535 & a_1962X;
  lo_b_2268X = 65535 & b_2266X;
...
  lo_c_2271X = ((lo_a_2267X) * (lo_b_2268X));
...
  if ((536870911 < lo_c_2271X)) {
...
  else {
    if ((lo_c_2271X < 0)) {

as these are all signed quantities the multiplication in
lo_c_2271X = ((lo_a_2267X) * (lo_b_2268X)) if overflowing invokes
undefined behavior according to the C language standard.  Thus
the compiler assumes that multiplying two positive signed values
is always positive.

Use -fwrapv if you want twos-complement semantics for signed integers.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|WAITING                     |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40907