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Subject: [Bug sanitizer/55309] gcc's address-sanitizer 66% slower than clang's
Date: Fri, 08 Feb 2013 09:02:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55309

--- Comment #27 from Jakub Jelinek <jakub at gcc dot gnu.org> 2013-02-08 09:02:23 UTC ---
Zero based offset has the big disadvantage of imposing big requirements on the
executable.
Could we on x86_64 think about mem_to_shadow(x) (x >> 3) + 0x7fff8000 (note,
not |, but +)?
Then instead of something like:
        movq    %rdi, %rdx
        movabsq $17592186044416, %rax
        shrq    $3, %rdx
        cmpb    $0, (%rdx,%rax)
        jne     .L5
        movq    (%rdi), %rax
        ret
.L5:
        pushq   %rax
        call    __asan_report_load8
we could emit:
        movq    %rdi, %rdx
        shrq    $3, %rdx
        cmpb    $0, 0x7fff8000(%rdx)
        jne     .L5
        movq    (%rdi), %rax
        ret
.L5:
        pushq   %rax
        call    __asan_report_load8
which is 7 bytes shorter sequence, without the need of an extra register and
the not so cheap movabs insn.  By forcing PIE for everything, you are forcing
the PIC overhead of unnecessary extra indirections in many places (and, on
non-x86_64 usually it is even much more expensive).