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
