Re: Lemon crashing on Solaris? (was Re: [Wireshark-commits] buildbot failure in Wireshark (development) on Solaris-10-SPARC)

[Guy Harris <guy () alum mit edu>]

(CCing the list this time.)

On Nov 24, 2013, at 5:37 PM, Bálint Réczey <balint () balintreczey hu> wrote:

> I tried to trigger a crash using GCC and -ftrapv without success while
> the clang-compiled binary crashed as expected:


        $ gcc -S -O2 test.c
        $ mv test.s test.s.noftrapv
        $ gcc -ftrapv -S -O2 test.c
        $ diff test.s.noftrapv test.s
        $ 

whereas:

        $ clang -S -O2 test.c
        $ mv test.s test.s.noftrapv
        $ clang -ftrapv -S -O2 test.c
        $ diff test.s.noftrapv test.s
        15,21c15
        <       leaq    L_.str(%rip), %rdi
        <       movl    $-2147483550, %esi      ## imm = 0xFFFFFFFF80000062
        <       xorb    %al, %al
        <       callq   _printf
        <       xorl    %eax, %eax
        <       popq    %rbp
        <       ret
        ---
        >       ud2
        24,27d17
        <       .section        __TEXT,__cstring,cstring_literals
        < L_.str:                                 ## @.str
        <       .asciz   "res:%d\n"

and, if we make it so that the compiler can't do all the work at compile time:

        $ cat test.c
        #include <limits.h>
        #include <stdio.h>

        int
        foo(int i)
        {
         int b = 100;
        #ifdef TEST
         if ((i + b) < i) {
           printf("overflow!\n");
           return 1;
         }
        #endif
         i += b;
         printf("res:%d\n",i);
         return i;
        }
        $ gcc -S -O2 test.c
        $ mv test.s test.s.noftrapv
        $ gcc -ftrapv -S -O2 test.c
        $ diff test.s.noftrapv test.s
        $ clang -S -O2 test.c
        $ mv test.s test.s.noftrapv
        $ clang -ftrapv -S -O2 test.c
        $ diff test.s.noftrapv test.s
        20a21,22
        >       jo      LBB0_1
        > ## BB#2:
        29a32,33
        > LBB0_1:
        >       ud2

so GCC is apparently not actually doing overflow checks, while clang is - and "gcc" is, on my machine, actually 
llvm-gcc:

        $ gcc --version
        i686-apple-darwin11-llvm-gcc-4.2 (GCC) 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2336.11.00)
        Copyright (C) 2007 Free Software Foundation, Inc.
        This is free software; see the source for copying conditions.  There is NO
        warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

(you might not be surprised to hear that there's no button under my machine's trackpad - the trackpad is the 
one-and-only mouse button :-))

On SPARC, there's a TRAPV instruction that traps if the integer overflow condition code bit is set; on 32-bit x86, 
there's INTO, which traps if the overflow condition code bit is set, but it's illegal in 64-bit x86.  JO jumps if the 
overflow bit is set in both 32-bit and 64-bit mode, and UD2 is an instruction that's defined to trap (in 32-bit and 
64-bit mode), so jumping to a UD2 if the overflow flag is set is a valid "trap if overflow" instruction sequence even 
on x86-64.

I don't know why GCC isn't actually implementing -ftrapv on x86 but is implementing it on SPARC, however; it's a little 
more work on x86 than just sticking in TRAPVs on SPARC, but not *that* much more.
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