[Bug middle-end/40893] New: ARM and PPC truncate intermediate operations unnecessarily

["lessen42+gcc at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

Consider the following C code:

#include <inttypes.h>

void dct2x2dc_dconly( int16_t d[2][2] )
{
    int d0 = d[0][0] + d[0][1];
    int d1 = d[1][0] + d[1][1];
    d[0][0] = d0 + d1;
    d[0][1] = d0 - d1;
}

The following is generated with arm-none-linux-gnueabi-gcc-4.4.0 -O3
-mcpu=cortex-a8 -S
dct2x2dc_dconly:
        ldrsh   ip, [r0, #2]
        ldrsh   r3, [r0, #0]
        ldrsh   r1, [r0, #6]
        ldrsh   r2, [r0, #4]
        add     r3, ip, r3
        add     r2, r1, r2
        uxth    r3, r3
        uxth    r2, r2
        rsb     r1, r2, r3
        add     r3, r2, r3
        strh    r1, [r0, #2]    @ movhi
        strh    r3, [r0, #0]    @ movhi
        bx      lr
(with pre-armv6 targets the two uxth are replaced by asl #16, lsr #16 pairs.)

The following is generated with powerpc-unknown-linux-gnu-gcc-4.4.0 -O3
-mcpu=G4 -S
dct2x2dc_dconly:
        lha 10,2(3)
        lha 0,0(3)
        lha 11,6(3)
        lha 9,4(3)
        add 0,10,0
        rlwinm 0,0,0,0xffff
        add 9,11,9
        rlwinm 9,9,0,0xffff
        subf 11,9,0
        add 0,9,0
        sth 11,2(3)
        sth 0,0(3)
        blr

The two uxth in the ARM version, and the two rlwinm in the PPC version are
completely unnecessary, as letting strh/sth truncate will give equivalent
results. x86 does not exhibit this behaviour, and removing either d0 + d1 or d0
- d1 will not cause d0 and d1 be truncated to to 16 bits on both ARM and PPC.

powerpc-unknown-linux-gnu-gcc-4.4.0 -v
Using built-in specs.
Target: powerpc-unknown-linux-gnu
Configured with: /var/tmp/portage/sys-devel/gcc-4.4.0/work/gcc-4.4.0/configure
--prefix=/usr --bindir=/usr/powerpc-unknown-linux-gnu/gcc-bin/4.4.0
--includedir=/usr/lib/gcc/powerpc-unknown-linux-gnu/4.4.0/include
--datadir=/usr/share/gcc-data/powerpc-unknown-linux-gnu/4.4.0
--mandir=/usr/share/gcc-data/powerpc-unknown-linux-gnu/4.4.0/man
--infodir=/usr/share/gcc-data/powerpc-unknown-linux-gnu/4.4.0/info
--with-gxx-include-dir=/usr/lib/gcc/powerpc-unknown-linux-gnu/4.4.0/include/g++-v4
--host=powerpc-unknown-linux-gnu --build=powerpc-unknown-linux-gnu
--enable-altivec --disable-fixed-point --without-ppl --without-cloog
--disable-nls --with-system-zlib --disable-checking --disable-werror
--enable-secureplt --disable-multilib --disable-libmudflap --disable-libssp
--enable-libgomp --enable-cld --disable-libgcj --enable-languages=c,c++,fortran
--enable-shared --enable-threads=posix --enable-__cxa_atexit
--enable-clocale=gnu --with-bugurl=http://bugs.gentoo.org/
--with-pkgversion='Gentoo 4.4.0 p1.1'
Thread model: posix
gcc version 4.4.0 (Gentoo 4.4.0 p1.1) 

arm-none-linux-gnueabi-gcc-4.4.0 -v
Using built-in specs.
Target: arm-none-linux-gnueabi
Configured with: ../gcc-4.4.0/configure --target=arm-none-linux-gnueabi
--prefix=/usr/local/arm --enable-threads
--with-sysroot=/usr/local/arm/arm-none-linux-gnueabi/libc
Thread model: posix
gcc version 4.4.0 (GCC)


-- 
           Summary: ARM and PPC truncate intermediate operations
                    unnecessarily
           Product: gcc
           Version: 4.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: lessen42+gcc at gmail dot com
  GCC host triplet: i386-apple-darwin
GCC target triplet: arm-none-linux-gnueabi


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40893