optimization/7145: Optimize + structure init + return value optimization fails
tlb@tlb.org
tlb@tlb.org
Thu Jun 27 14:46:00 GMT 2002
>Number: 7145
>Category: optimization
>Synopsis: g++ -O with structure initializer & return value optimization generates bad code
>Confidential: no
>Severity: serious
>Priority: medium
>Responsible: unassigned
>State: open
>Class: wrong-code
>Submitter-Id: net
>Arrival-Date: Thu Jun 27 13:46:03 PDT 2002
>Closed-Date:
>Last-Modified:
>Originator: Trevor Blackwell
>Release: 3.1.1 20020617 (prerelease) [FreeBSD]
>Organization:
Umbrella Research
>Environment:
System: FreeBSD tlb.blackwells.net 4.5-RELEASE-p2 FreeBSD 4.5-RELEASE-p2 #3: Sat Mar 9 12:32:15 PST 2002 root@tlb.umbrellaresearch.com:/d1/usr-obj/usr/src/sys/TLB i386
host: i386-portbld-freebsd4.5
build: i386-portbld-freebsd4.5
target: i386-portbld-freebsd4.5
configured with: ./..//gcc-20020617/configure --disable-nls --with-gnu-as --with-gnu-ld --with-gxx-include-dir=/usr/local/lib/gcc-lib/i386-portbld-freebsd4.5/3.1.1/include/g++ --disable-libgcj --disable-shared --prefix=/usr/local i386-portbld-freebsd4.5
>Description:
An inline function subject to return value optimization, where the returned argument is initialized
with an initializer-list generates bad code with -O. It seems to only happen when it's an odd
number of halfwords.
>How-To-Repeat:
Compile the following C++ code with g++ -O. On my system, it prints junk values:
mkcolor: pixel=-1077938364 r=-2244 g=-16449 b=0
It depends on the structure; if I remove the 'blue' member it works. I suspect it has
to do with whether it's a multiple of the word size.
It seems to complete optimize away the initialization as you can see in the assembly output.
-------------------------------------------------------------------------------------------------------
extern "C" int printf(const char *, ...);
struct GdkColor {
long pixel;
short red;
short green;
short blue;
};
inline GdkColor mkcolor() {
GdkColor ret={0,1,2,3};
return ret;
}
int
main()
{
GdkColor col=mkcolor();
printf("mkcolor: pixel=%d r=%d g=%d b=%d\n", col.pixel, col.red, col.green, col.blue);
}
-----------------------------------------------------------------------------------------------------
main:
.LFB1:
pushl %ebp
.LCFI0:
movl %esp, %ebp
.LCFI1:
subl $24, %esp
.LCFI2:
andl $-16, %esp
subl $12, %esp
movswl -16(%ebp),%eax
pushl %eax
movswl -18(%ebp),%eax
pushl %eax
movswl -20(%ebp),%eax
pushl %eax
pushl -24(%ebp)
pushl $.LC0
.LCFI3:
call printf
movl $0, %eax
leave
ret
-----------------------------------------------------------------------------------------------------
>Fix:
>Release-Note:
>Audit-Trail:
>Unformatted:
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