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* [Bug c/30267]  New: missed optimization due to bad range propagation without -fwrapv
@ 2006-12-20 14:35 bruno at clisp dot org
  2006-12-27 16:48 ` [Bug middle-end/30267] folding (~ -x) >= (-2147483647-1) to x != -2147483648 rguenth at gcc dot gnu dot org
  2006-12-27 17:01 ` pinskia at gcc dot gnu dot org
  0 siblings, 2 replies; 4+ messages in thread
From: bruno at clisp dot org @ 2006-12-20 14:35 UTC (permalink / raw)
  To: gcc-bugs

This program shows that some range propagation became worse between
gcc 4.0.2 and gcc 4.1.1.

=========================== foo.c ========================
int notneg (int x)
{
  return (~ -x) >= (-2147483647-1);
}
int negnot (int x)
{
  return (- ~x) <= 2147483647;
}
==========================================================


# With gcc 4.0.2 on i686-pc-linux-gnu the code is fully optimized:

$ gcc -O2 -fomit-frame-pointer -S foo.c && cat foo.s
        .file   "foo.c"
        .text
        .p2align 4,,15
.globl notneg
        .type   notneg, @function
notneg:
        movl    $1, %eax
        ret
        .size   notneg, .-notneg
        .p2align 4,,15
.globl negnot
        .type   negnot, @function
negnot:
        movl    $1, %eax
        ret
        .size   negnot, .-negnot
        .ident  "GCC: (GNU) 4.0.2"
        .section        .note.GNU-stack,"",@progbits


# With gcc 4.1.1 on i686-pc-linux-gnu the code is fully optimized with -fwrapv
# but not without -fwrapv:

$ gcc -O2 -fomit-frame-pointer -S foo.c && cat foo.s
        .file   "foo.c"
        .text
        .p2align 4,,15
.globl notneg
        .type   notneg, @function
notneg:
        xorl    %eax, %eax
        cmpl    $-2147483648, 4(%esp)
        setne   %al
        ret
        .size   notneg, .-notneg
        .p2align 4,,15
.globl negnot
        .type   negnot, @function
negnot:
        xorl    %eax, %eax
        cmpl    $2147483647, 4(%esp)
        setne   %al
        ret
        .size   negnot, .-negnot
        .ident  "GCC: (GNU) 4.1.1"
        .section        .note.GNU-stack,"",@progbits

$ gcc -O2 -fomit-frame-pointer -fwrapv -S foo.c && cat foo.s
        .file   "foo.c"
        .text
        .p2align 4,,15
.globl notneg
        .type   notneg, @function
notneg:
        movl    $1, %eax
        ret
        .size   notneg, .-notneg
        .p2align 4,,15
.globl negnot
        .type   negnot, @function
negnot:
        movl    $1, %eax
        ret
        .size   negnot, .-negnot
        .ident  "GCC: (GNU) 4.1.1"
        .section        .note.GNU-stack,"",@progbits

So somehow this seems to be linked to flag_wrapv. But regardless which
value is the result after signed overflow, any int >= INT_MIN and
any int <= INT_MAX should evaluate to 1 unconditionally.


-- 
           Summary: missed optimization due to bad range propagation without
                    -fwrapv
           Product: gcc
           Version: 4.1.1
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: bruno at clisp dot org
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30267


^ permalink raw reply	[flat|nested] 4+ messages in thread
end of thread, other threads:[~2021-06-08 11:09 UTC | newest]

Thread overview: 4+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
     [not found] <bug-30267-4@http.gcc.gnu.org/bugzilla/>
2021-06-08  8:47 ` [Bug middle-end/30267] folding (~ -x) >= (-2147483647-1) to x != -2147483648 pinskia at gcc dot gnu.org
2021-06-08 11:09 ` bruno at clisp dot org
2006-12-20 14:35 [Bug c/30267] New: missed optimization due to bad range propagation without -fwrapv bruno at clisp dot org
2006-12-27 16:48 ` [Bug middle-end/30267] folding (~ -x) >= (-2147483647-1) to x != -2147483648 rguenth at gcc dot gnu dot org
2006-12-27 17:01 ` pinskia at gcc dot gnu dot org

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