[Bug sanitizer/64996] New: UBsan check optimized out by -O

[Bug sanitizer/64996] New: UBsan check optimized out by -O

[sami.liedes at iki dot fi]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64996

            Bug ID: 64996
           Summary: UBsan check optimized out by -O
           Product: gcc
           Version: 4.9.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: sanitizer
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sami.liedes at iki dot fi
                CC: dodji at gcc dot gnu.org, dvyukov at gcc dot gnu.org,
                    jakub at gcc dot gnu.org, kcc at gcc dot gnu.org

Consider this program:

#include "limits.h"
int main() {
        int i=INT_MIN;
        i--;
}

When compiled with -fsanitize=undefined -O0, ub is correctly detected:

$ gcc -fsanitize=undefined ub.c -o ub && ./ub
ub.c:4:10: runtime error: signed integer overflow: -2147483648 + -1 cannot be
represented in type 'int'

When compiled with -O, the entire check is optimized out, even if the program
still clearly exhibits ub:

$ gcc -fsanitize=undefined ub.c -o ub -O && ./ub
$ gcc -fsanitize=undefined ub.c -o ub.s -O -S
$ cat ub.s
        .file   "ub.c"
        .text
        .globl  main
        .type   main, @function
main:
.LFB0:
        .cfi_startproc
        rep ret
        .cfi_endproc
.LFE0:
        .size   main, .-main
        .ident  "GCC: (Debian 4.9.2-10) 4.9.2"
        .section        .note.GNU-stack,"",@progbits
$

[Bug sanitizer/64996] UBsan check optimized out by -O

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64996

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
                 CC|                            |mpolacek at gcc dot gnu.org
         Resolution|---                         |INVALID

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Since the variable is unused, we get rid of it, and then DCE removes the ubsan
check.  I don't think it is a bug.  Adding asm ("" : "+g" (i)); after the
decrement makes the runtime error appear again.

[Bug sanitizer/64996] UBsan check optimized out by -O

[y.gribov at samsung dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64996

Yury Gribov <y.gribov at samsung dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |y.gribov at samsung dot com

--- Comment #2 from Yury Gribov <y.gribov at samsung dot com> ---
Also take a look at second FAQ question at
https://code.google.com/p/address-sanitizer/wiki/AddressSanitizer#FAQ (it's for
ASan but same logic applies).