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Old 11-13-2011, 11:31 AM
Jurij Smakov
 
Default Bug#648016: Some analysis

reassign 648016 gcc-4.6
found 648016 4.6.2-4
thanks

This bug really is in gcc-4.6, because it is currently the default sid
gcc and it is used to (mis)compile src/build/genrecog.c during
gcc-avr build, which later crashes. I'm fairly certain that this is
gcc problem, because if the binary is compiled with -O0, the problem
goes away. All debugging output below was obtained on a sparc machine
running up-to-date sid, invoking build/genrecog under gdb with a
single argument of '../../src/gcc/config/avr/avr.md'.

Tracing the execution is somewhat tricky, since failure happens within
write_tree(), and most of the functions write_tree() calls
(write_tree_1, write_switch, write_node, write_action, etc) are
optimized out. The output generated by

build/genrecog ../../src/gcc/config/avr/avr.md

is the same as the one produced on an amd64 system until we hit the
following code in genrecog.c/write_switch():

else if (type == DT_mode
|| type == DT_veclen
|| type == DT_elt_zero_int
|| type == DT_elt_one_int
|| type == DT_elt_zero_wide_safe)
{
const char *indent = "";

/* We cast switch parameter to integer, so we must ensure that the value
fits. */
if (type == DT_elt_zero_wide_safe)
{
indent = " ";
printf(" if ((int) XWINT (x%d, 0) == XWINT (x%d, 0))
", depth, depth);
}
printf ("%s switch (", indent);
switch (type)
{
case DT_mode:
printf ("GET_MODE (x%d)", depth);
break;
case DT_veclen:
printf ("XVECLEN (x%d, 0)", depth);
break;
case DT_elt_zero_int:
printf ("XINT (x%d, 0)", depth);
break;
case DT_elt_one_int:
printf ("XINT (x%d, 1)", depth);
break;
case DT_elt_zero_wide_safe:
/* Convert result of XWINT to int for portability since some C
compilers won't do it and some will. */
printf ("(int) XWINT (x%d, 0)", depth);
break;
default:
gcc_unreachable ();
}

The problem appears after executing the

printf ("%s switch (", indent);

statetement. It looks like compiler generates a number of small stubs
within write_tree() for calling printf with all possible format
statements. Here's how the generated assembler code looks for this
particular one, starting at 0x00013e60:

Dump of assembler code from 0x13e40 to 0x13ea0:
0x00013e40 <write_tree+2144>: ld [ %i5 + 0x1c ], %o2
0x00013e44 <write_tree+2148>: sethi %hi(0x1e800), %o0
0x00013e48 <write_tree+2152>: or %l1, 0x110, %o1
0x00013e4c <write_tree+2156>: call 0x3510c <printf@plt>
0x00013e50 <write_tree+2160>: or %o0, 0x258, %o0
0x00013e54 <write_tree+2164>: b %xcc, 0x13bf4 <write_tree+1556>
0x00013e58 <write_tree+2168>: ld [ %i0 ], %i5
0x00013e5c <write_tree+2172>: be,pn %icc, 0x13850 <write_tree+624>
=> 0x00013e60 <write_tree+2176>: sethi %hi(0x1f400), %i3
0x00013e64 <write_tree+2180>: sethi %hi(0x1e800), %o0
0x00013e68 <write_tree+2184>: or %o0, 0x2b0, %o0 ! 0x1eab0
0x00013e6c <write_tree+2188>: call 0x3510c <printf@plt>
0x00013e70 <write_tree+2192>: or %i3, 0xe8, %o1
0x00013e74 <write_tree+2196>: cmp %l7, 7
0x00013e78 <write_tree+2200>: sll %l7, 2, %g1
0x00013e7c <write_tree+2204>: sethi %hi(0x1e800), %o0
0x00013e80 <write_tree+2208>: mov %l6, %o1
0x00013e84 <write_tree+2212>: call 0x3510c <printf@plt>
0x00013e88 <write_tree+2216>: or %o0, 0x3e8, %o0
0x00013e8c <write_tree+2220>: b %xcc, 0x13bac <write_tree+1484>
0x00013e90 <write_tree+2224>: ld [ %fp + -192 ], %g3
0x00013e94 <write_tree+2228>: b %xcc, 0x13ad0 <write_tree+1264>
0x00013e98 <write_tree+2232>: st %g2, [ %fp + -188 ]
0x00013e9c <write_tree+2236>: cmp %g0, %i3
End of assembler dump.

Confirmation that 0x1eab0 contains the correct format statement
(passed to printf in %o0):

(gdb) printf "%s
", (char *) 0x1eab0
%s switch (
(gdb)

A remarkable feature of this stub is that it does not have a return
branch statement, like others do (see 0x00013e54, for example). So,
instead of returning to the correct location where the stub was
invoked in write_switch(), we fall through to 0x00013e74, and start
executing the next stub, which invokes printf with a format
statement at 0x1ebe8 (== 0x1e800 | 0x3e8):

(gdb) printf "%s
", (char *) 0x1ebe8
%sreturn gen_split_%d (insn, operands);

(gdb)

This is completely unrelated code, normally invoked by
write_action(), line 2182. Once it's done, we jump back to completely
wrong location at 0x00013e8c, eventually causing a crash.

Best regards,
--
Jurij Smakov jurij@wooyd.org
Key: http://www.wooyd.org/pgpkey/ KeyID: C99E03CC



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Old 11-14-2011, 05:24 PM
Matthias Klose
 
Default Bug#648016: Some analysis

On 11/13/2011 01:31 PM, Jurij Smakov wrote:
> reassign 648016 gcc-4.6
> found 648016 4.6.2-4
> thanks
>
> This bug really is in gcc-4.6, because it is currently the default sid
> gcc and it is used to (mis)compile src/build/genrecog.c during
> gcc-avr build, which later crashes. I'm fairly certain that this is
> gcc problem, because if the binary is compiled with -O0, the problem
> goes away. All debugging output below was obtained on a sparc machine
> running up-to-date sid, invoking build/genrecog under gdb with a
> single argument of '../../src/gcc/config/avr/avr.md'.

I don't plan to work on this.

for what reason is the cross compiler built for anything besides x86?



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