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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; 3+ 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] 3+ messages in thread
* [Bug middle-end/30267] folding (~ -x) >= (-2147483647-1) to x != -2147483648
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 ` rguenth at gcc dot gnu dot org
2006-12-27 17:01 ` pinskia at gcc dot gnu dot org
1 sibling, 0 replies; 3+ messages in thread
From: rguenth at gcc dot gnu dot org @ 2006-12-27 16:48 UTC (permalink / raw)
To: gcc-bugs
------- Comment #1 from rguenth at gcc dot gnu dot org 2006-12-27 16:48 -------
This is folded to
;; Function notneg (notneg)
;; enabled by -tree-original
{
return x != -2147483648;
}
;; Function negnot (negnot)
;; enabled by -tree-original
{
return x != 2147483647;
}
via
/* Convert - (~A) to A + 1. */
if (INTEGRAL_TYPE_P (type) && TREE_CODE (arg0) == BIT_NOT_EXPR)
return fold_build2 (PLUS_EXPR, type, TREE_OPERAND (arg0, 0),
build_int_cst (type, 1));
and
/* Convert ~ (-A) to A - 1. */
else if (INTEGRAL_TYPE_P (type) && TREE_CODE (arg0) == NEGATE_EXPR)
return fold_build2 (MINUS_EXPR, type, TREE_OPERAND (arg0, 0),
build_int_cst (type, 1));
and
/* Transform comparisons of the form X +- C1 CMP C2 to X CMP C2 +- C1.
*/ if ((TREE_CODE (arg0) == PLUS_EXPR || TREE_CODE (arg0) == MINUS_EXPR)
&& (TREE_CODE (TREE_OPERAND (arg0, 1)) == INTEGER_CST
&& !TREE_OVERFLOW (TREE_OPERAND (arg0, 1))
&& !TYPE_UNSIGNED (TREE_TYPE (arg1))
&& !(flag_wrapv || flag_trapv))
&& (TREE_CODE (arg1) == INTEGER_CST
&& !TREE_OVERFLOW (arg1)))
{
tree const1 = TREE_OPERAND (arg0, 1);
tree const2 = arg1;
tree variable = TREE_OPERAND (arg0, 0);
tree lhs;
int lhs_add;
lhs_add = TREE_CODE (arg0) != PLUS_EXPR;
lhs = fold_build2 (lhs_add ? PLUS_EXPR : MINUS_EXPR,
TREE_TYPE (arg1), const2, const1);
if (TREE_CODE (lhs) == TREE_CODE (arg1)
&& (TREE_CODE (lhs) != INTEGER_CST
|| !TREE_OVERFLOW (lhs)))
return fold_build2 (code, type, variable, lhs);
}
(citing from the 4.1 branch)
Note this is actually wrong-code.
--
rguenth at gcc dot gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |rguenth at gcc dot gnu dot
| |org
Status|UNCONFIRMED |NEW
Ever Confirmed|0 |1
Keywords| |wrong-code
Known to fail| |4.1.2 4.3.0
Known to work| |4.0.3
Last reconfirmed|0000-00-00 00:00:00 |2006-12-27 16:48:37
date| |
Summary|missed optimization due to |folding (~ -x) >= (-
|bad range propagation |2147483647-1) to x != -
|without -fwrapv |2147483648
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30267
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug middle-end/30267] folding (~ -x) >= (-2147483647-1) to x != -2147483648
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
1 sibling, 0 replies; 3+ messages in thread
From: pinskia at gcc dot gnu dot org @ 2006-12-27 17:01 UTC (permalink / raw)
To: gcc-bugs
------- Comment #2 from pinskia at gcc dot gnu dot org 2006-12-27 17:01 -------
(In reply to comment #1)
> Note this is actually wrong-code.
No, it is not.
In notneg, if x is -2147483647-1, it is obviously, we have an overflow as
-(-2147483647-1) is overflowed.
In negnot, if x is 2147483647, then ~ 2147483647 == -2147483647-1, and then we
have an overflow.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30267
^ permalink raw reply [flat|nested] 3+ messages in thread
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-- links below jump to the message on this page --
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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