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

[paul at pwsan dot com]

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

paul walmsley  changed:

   What|Removed |Added

 CC||paul at pwsan dot com

--- Comment #2 from paul walmsley  2010-10-05 18:14:35 
UTC ---
Here's a minimal test case:

void foo(unsigned int v)
{
  *(volatile unsigned short *)0xabcdefab = (v);
}

arm-linux-gcc  -O2 -march=armv7-a -c test.c; arm-linux-objdump -DS test.o 
| less


 :
   0:   e30e3fffmovwr3, #61439  ; 0xefff
   4:   e34a3bcdmovtr3, #43981  ; 0xabcd
   8:   e6ff0070uxthr0, r0
   c:   e14305b4strhr0, [r3, #-84]
  10:   e12fff1ebx  lr


As David notes, the expected behavior is that the uxth should not be generated
for >= armv6 targets, and the two shifts should not be generated on < armv6
targets, as they should be superfluous.

http://marc.info/?l=linux-omap&m=128630215909798&w=2

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

[paul at pwsan dot com]

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

--- Comment #4 from paul walmsley  2010-10-08 17:17:24 
UTC ---
The bug also appears without volatile:



/* generates an unnecessary uxth */
void foo(unsigned short a, unsigned short b, unsigned short c,
 unsigned short *e, unsigned short *f)
{
*e = (a + b) + c;
*f = (a + b) - c;
}

/* works as expected */
void bar(unsigned short a, unsigned short b, unsigned short c,
 unsigned short *e)
{
*e = (a + b) + c;
}

/* works as expected */
void baz(unsigned short a, unsigned short b, unsigned short c,
 unsigned short *e)
{
*e = (a + b) - c;
}

-

compiled and dumped with:

arm-linux-gnueabi-gcc -O2 -c test.c ; objdump -DS test.o

produces:

-

Disassembly of section .text:

 :
   0:   e0811000add r1, r1, r0
   4:   e1a01801lsl r1, r1, #16
   8:   e1a01821lsr r1, r1, #16
   c:   e0620001rsb r0, r2, r1
  10:   e0821001add r1, r2, r1
  14:   e1c310b0strhr1, [r3]
  18:   e59d3000ldr r3, [sp]
  1c:   e1c300b0strhr0, [r3]
  20:   e12fff1ebx  lr

0024 :
  24:   e0811000add r1, r1, r0
  28:   e0822001add r2, r2, r1
  2c:   e1c320b0strhr2, [r3]
  30:   e12fff1ebx  lr

0034 :
  34:   e0811000add r1, r1, r0
  38:   e0622001rsb r2, r2, r1
  3c:   e1c320b0strhr2, [r3]
  40:   e12fff1ebx  lr

-

gcc -v:

Using built-in specs.
Target: arm-linux-gnueabi
Configured with: ../src/configure -v --with-pkgversion='Debian 4.4.5-2'
--with-bugurl=file:///usr/share/doc/gcc-4.4/README.Bugs
--enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-4.4 --enable-shared --enable-multiarch
--enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib
--without-included-gettext --enable-threads=posix
--with-gxx-include-dir=/usr/arm-linux-gnueabi/include/c++/4.4.5
--libdir=/usr/lib --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug
--enable-objc-gc --disable-sjlj-exceptions --enable-checking=release
--program-prefix=arm-linux-gnueabi- --includedir=/usr/arm-linux-gnueabi/include
--build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=arm-linux-gnueabi
--with-headers=/usr/arm-linux-gnueabi/include
--with-libs=/usr/arm-linux-gnueabi/lib
Thread model: posix
gcc version 4.4.5 (Debian 4.4.5-2)