This is an error-report. However I will provide some background
for my little piece of code. (The code itself is very simple)
I will try to post on comp.std.c++ in order to make this a part of C++
(maybe c) otherwise I might come back too beck you to implement it just in your
compiler.
There are many reasons. (One and the best is to switch a double based on
intervals) Therefore I would like a VERY FAST FUNCTION to return the sign of a
double (and float and long double) ...
(And compare on a double d<=-0.0 (without branch) wont do the trick. And I can't
blame you because you will have to respect Nan.
Since c/c++ does not have this fast function (skipping Nan)
(which I hope will come) I have no other no other options that to
write it myself (and cheat!)
That means that my trick will only work on doubles in IEEE 754 - with a size of
2.
The sizepart of double (in my case 8 bytes) and int (in my case 4 bytes) could
probably be fixed with the right macros.
However my code will only work on x86 and other machines accepting the IEEE 754
standard. I think Motorola does not follow this - but nevermind.
The code with the bug is : (Read signbit a push it to be one or zero)
int is_not_positive(double v)
{
return ((reinterpret_cast<unsigned int*>(&v)[1]) >> 31);
}
This works with option O1 (and below)
but fails with O2 (and above)
The O1 correct (but not fast fast code) looks like this:
.file "bug.cpp"
.text
.align 2
.globl _Z15is_not_positived
.type _Z15is_not_positived, @function
_Z15is_not_positived:
.LFB3:
pushl %ebp
.LCFI0:
movl %esp, %ebp
.LCFI1:
subl $8, %esp
.LCFI2:
fldl 8(%ebp)
fstpl -8(%ebp)
movl -4(%ebp), %eax
shrl $31, %eax
movl %ebp, %esp
popl %ebp
ret
.LFE3:
.size _Z15is_not_positived, .-_Z15is_not_positived
.section .note.GNU-stack,"",@progbits
.ident "GCC: (GNU) 3.3.5-20050130 (Gentoo 3.3.5.20050130-r1,
ssp-3.3.5.20050130-1, pie-8.7.7.1)"
The wrong optimization simply removes:
fldl 8(%ebp)
fstpl -8(%ebp)
// I guess that it removes the store.
---------------------------------------
The "wished optimized code" is (notice this is partly manually written so
I might be wrong. I am not an assembler expert)
.LFB4:
pushl %ebp
.LCFI0:
movl %esp, %ebp
.LCFI1:
movl 12(%ebp), %eax
popl %ebp
shrl $31, %eax
ret
.LFE4:
I am sorry that I have not testet it with a newer version.
(However I am not to bright and last time I did emerge gcc (with accept newest
version I got problems with compiling my kernel))
I hope the answer is "just upgrade you stupid man..."
Regards
Thorbjørn Martsum
PS:
BTW...
I have found a workaround using unsigned long long.
This works with O3 and has only one extra instruction compared to "my best"
movl 12(%ebp), %eax
popl %ebp
expands to
movl 12(%ebp), %ecx
popl %ebp
movl %ecx, %eax
(a missing peephole-pattern (?) )
But sill WAY WAY Better than what the MS VS6 gives =)
--
Summary: Unhealthy optimization. Accessing double with
reinterpret_cast.
Product: gcc
Version: 3.3.5
Status: UNCONFIRMED
Severity: normal
Priority: P2
Component: c++
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: martsummsw at hotmail dot com
CC: gcc-bugs at gcc dot gnu dot org
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23793
