Consider this simple class:
class TV
{
private:
float truth;
float confidence;
public:
TV(float, float);
float getT(void);
};
extern TV my_tv_maker(float tr);
float my_subr(float tr)
{
TV tv = my_tv_maker(434.23);
return tv.getT();
}
On powerPC, when compiled with gcc -O2 -S -c generates the annotated assembly,
below. Noticable is a copy of the entire structure, which is 8 bytes long, to
and from the stack, which is completely un-needed. When the structure is
*larger*, then no copy is performed, *even when no optimization is done (i.e.
even when -O2 is not specified.)*. So basically, large structures work great;
but small structures cause some extra, un-neccesary cycles to be wasted.
Problem is still there for -O3.
.L._Z7my_subrf:
.LFB3:
mflr 0 r0 = link reg value
.LCFI4:
lfs 1,....@toc(2) fpr1 = 434.23
std 0,16(1) save link reg value
.LCFI5:
stdu 1,-144(1) incr stack ptr
.LCFI6:
addi 3,1,128 alloc TV on stack. Note sizeof(TV) is 8 bytes.
bl _Z11my_tv_makerf call
ld 0,128(1) r0 = 8 bytes = copy of entire instance of class TV
addi 3,1,112 r3 = ptr to new location
std 0,112(1) store 8-byte copy of the instance to new location
on stack ... this is really not needed.
The old copy, at offset 128, could have been
used .. there was no point to making this copy.
bl _ZN2TV4getTEv call
nop
addi 1,1,144 decr stack
ld 0,16(1) get link reg val
mtlr 0 move to link reg
blr branch to link reg.
gcc -v
Using built-in specs.
Target: powerpc64-unknown-linux-gnu
Configured with: /var/tmp/portage/sys-devel/gcc-4.1.2/work/gcc-4.1.2/configure
--prefix=/usr --bindir=/usr/powerpc64-unknown-linux-gnu/gcc-bin/4.1.2
--includedir=/usr/lib/gcc/powerpc64-unknown-linux-gnu/4.1.2/include
--datadir=/usr/share/gcc-data/powerpc64-unknown-linux-gnu/4.1.2
--mandir=/usr/share/gcc-data/powerpc64-unknown-linux-gnu/4.1.2/man
--infodir=/usr/share/gcc-data/powerpc64-unknown-linux-gnu/4.1.2/info
--with-gxx-include-dir=/usr/lib/gcc/powerpc64-unknown-linux-gnu/4.1.2/include/g++-v4
--host=powerpc64-unknown-linux-gnu --build=powerpc64-unknown-linux-gnu
--enable-altivec --enable-nls --without-included-gettext --with-system-zlib
--disable-checking --disable-werror --enable-secureplt
--disable-libunwind-exceptions --disable-multilib --enable-libmudflap
--disable-libssp --enable-objc-gc
--enable-languages=c,c++,java,objc,obj-c++,fortran --enable-shared
--enable-threads=posix --enable-__cxa_atexit --enable-clocale=gnu
Thread model: posix
gcc version 4.1.2 (Gentoo 4.1.2 p1.0.2)
Bug is reproducible on:
Welcome to hazelnut.osuosl.org
Server Address : 140.211.167.137
Hardware : 8 x POWER5+ (gs),
CHRP IBM,9133-55A, 7794 MB RAM
Operating System : Gentoo Linux
(default-linux/ppc/ppc64/2007.0/64bit-userland/power5)
Support : supp...@osuosl.org
Website : http://powerdev.osuosl.org
Mailing List : http://lists.osuosl.org/mailman/listinfo/powerdev
--
Summary: [PPC64] un-needed copy generated for small structs kept
on stack
Product: gcc
Version: 4.1.2
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: linasvepstas at gmail dot com
GCC build triplet: powerpc64-unknown-linux-gnu
GCC host triplet: powerpc64-unknown-linux-gnu
GCC target triplet: powerpc64-unknown-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39067
