Hello,
I think I came across a possible bug in gcc when using -O2 optimisation on using
the following program .
int global_var;
int switch_test(void) {
int sw = 4;
int dmy = (sw == 77) ? 11 : 0x101;
global_var = dmy * ((sw >= 5) +1);
switch (sw) {
case 4: return(0); // okay
default: return(99); //error
}
}
int main(void) {
printf("result=%d\n", switch_test()); return(0);
}
When executed, the function returns "99", i.e. at the time the switch(sw) is
calculated, the wrong case is taken.
The following assembly code is generated :
* 13 .global switch_test
* 14 .type switch_test,function
* 15 switch_test:
* 16 @ args = 0, pretend = 0, frame = 0
* 17 @ frame_needed = 1, current_function_anonymous_args = 0
* 18 0000 0DC0A0E1 mov ip, sp
* 19 0004 00D82DE9 stmfd sp!, {fp, ip, lr, pc}
* 20 0008 04B04CE2 sub fp, ip, #4
* 21 000c 0B30A0E3 mov r3, #11 ; dmy = constant 11
* 22 0010 08209FE5 ldr r2, .L40 ; load adrs of global
* 23 0014 6300A0E3 mov r0, #99 ; load result = 99 (wrong !!!)
* 24 0018 003082E5 str r3, [r2, #0] ; store dmy = 257 into global
* 25 001c 00A81BE9 ldmea fp, {fp, sp, pc}
* 27 .L41:
* 28 .align 2
* 29 .L40:
* 30 0024 00000000 .word global
Obviously, gcc optimizes away all the constant expressions and produces optimum
code, but then decides to use the wrong case.
I have tried three different compilers, and all showed the problem :
> gcc version 2.95.2 20000220 (Debian GNU/Linux) (native onSA1110)
> gcc version 2.95.3 20010125 (prerelease) (native on
>skiffclusters2.handhelds.org)
> gcc version egcs-2.91.60 19990113/philb (egcs-1.1.1 release) (cross-compiler
>hosted on x86)
Does anybody have any hint what I might be doing wrong (missing compiler
options, loose C-Syntax ?), or is this possibly a known issue that i could fix
with a newer compiler version ?
I think that this problem is somehow specific to the arm-gcc, because the error
depends on whether some constants are valid arm-immediatates (<8-bit> << n*2)
or not, even though they may eventually not be used in the code after
optimization !?
Thank you very much,
Regards klaus
In case that might help, here is a more verbose version of the program that
might provide some additional hints:
#include <stdio.h>
#include <stdlib.h>
int global = 3;
int switch_test(void)
{
#define INITIAL_SW 0x04 /* use any positive immediate arm-constant for INITIAL_SW
* (tested: 0, 1, 2, 3, 4, 10, 99, 0x0039c000) and the
error occurs
* larger constants (0x101, 1000000 and negative numbers
do not produce errors */
#define MUST_BE_SW_PLUS_1 (INITIAL_SW + 1) /* this must be INITIAL_SW + 1 */
#define ONLY_1_CAUSES_ERR 1
#define ANY_INTEGER 77
#define ANY_INTEGER1 11
#define NO_ARM_IMMEDIATE 0x101 /* 257, a constant that can not be representated as an
arm immediate value ( b << n*2, where b is 8 bit)*/
#define ANY_OPERATOR_EXCEPT_LOG_OR & /* also fails for <, ==, >=, &, |, &&,
but not || */
#define ANY_OF_dmy_sw_global_result sw /* use either dmy, sw, global, local */
int dmy;
int result;
int sw=INITIAL_SW; /* assign any integer to sw and never overwrite it later */
// I could not reproduce the error without using any of the following three
statements A, B, C
// In statement A, a comparison using any binary operator (except "||" ) seems to be
required.
// This might mean that the problem does not occur if the second operand is not
evaluated ?
/* A */ dmy = (ANY_OF_dmy_sw_global_result ANY_OPERATOR_EXCEPT_LOG_OR
ANY_INTEGER)
? ANY_INTEGER1 /* either ANY_INTEGER1 or NO_ARM_IMMEDIATE must
not be a valid */
: NO_ARM_IMMEDIATE; /* ARM constant (8 bit << n*2), otherwise no
fault occurs */
/* In statement B, to produce the fault, I need
* - ">=" Operator
* - compare <n> with <n+1> and then add one
* - multiply ("dmy * ..", "dmy + .." will not do)
* - the result must be assigned to a global variable to avoid these statements
* from being optimized away
*/
/* B */ global = dmy * ((sw >= MUST_BE_SW_PLUS_1) + ONLY_1_CAUSES_ERR); /* assigning
to global ensures result of mult is used */
/* In statement C, sw is falsely evaluated only by the "switch"-operator.
* Just assigning result = sw works correctly.
* The error is not affected by extra case-statements or changing what the
case-statements do.
*/
/* C */ switch( sw ) { /* only a switch causes errors, merely assigning b gives
expected result */
case INITIAL_SW: result=0; break; /* 0 = expected result, sw should still be
= INITIAL_SW */
default: result=99; /* 99 = error */
}
return (result); /* 0 = no error, 99 = error (values do not matter) */
}
int main(void)
{
int res = switch_test();
if (res == 0) {
printf ("okay, no Error\n");
} else {
printf ("####### Error: result=%d #########\n", res);
}
return(0);
}
--
Mobotix AG
Klaus Borchers
Luxemburgerstr. 6
D-67657 Kaiserslautern
Germany
Tel: +49 (631) 3033141
Fax: +49 (631) 3033190
E-Mail: [EMAIL PROTECTED]
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