[Bug target/32107] New: bad codegen for vector initialization in Altivec

[Bug target/32107] New: bad codegen for vector initialization in Altivec

[dorit at il dot ibm dot com]

Compiling the folloxing testcase:

#define vector __attribute__((__vector_size__(16) ))
float fa[100] __attribute__ ((__aligned__(16)));
vector float foo ()
{
  float f = fa[0];
  vector float vf = {f, f, f, f};
  return vf;
}

...with gcc -O2 -maltivec, we get:

ld      r9,0(r2)
lfs     f0,0(r9)
addi    r9,r1,-16
stfs    f0,-16(r1)
lvewx   v2,r0,r9
vspltw  v2,v2,0
blr

My problem is with the {lfs,stfs,lvewx} sequence: we load a value into f0, and
then store it (with stfs) into an aligned memory location, so that it could be
loaded from there into a vector (with lvewx). However, since the address from
which f0 was loaded is known to be aligned, we could directly do an lvewx from
there, and avoid the extra {lfs,stfs}, so the following should be enough:

ld      r9,0(r2)
lvewx   v2,r0,r9
vspltw  v2,v2,0
blr

The problem is that rs6000_expand_vector_init doesn't know that f0 is
originated from an aligned address. It gets the following as vals:

(parallel:V4SF [
        (reg/v:SF 119 [ f ])
        (reg/v:SF 119 [ f ])
        (reg/v:SF 119 [ f ])
        (reg/v:SF 119 [ f ])
    ])

We somehow want to expand 'f = fa[0]' and '{f,f,f,f}' together... if
expand_vector_init could get this as vals: '{fa[0],fa[0],fa[0],fa[0]}', it
could see that the original address is aligned. 
Alternatively, the prospects of getting rid of the redundant load and store
later on during some kind of a peephole optimization don't seem so high to
me... Thoughts?

This may be related to PR31334 (though there the issue is about initialization
with constants, so I'm not sure if the idea for a solution proposed there would
help us here).


-- 
           Summary: bad codegen for vector initialization in Altivec
           Product: gcc
           Version: 4.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: dorit at il dot ibm dot com
 GCC build triplet: powerpc-linux
  GCC host triplet: powerpc-linux
GCC target triplet: powerpc-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32107

[Bug target/32107] bad codegen for vector initialization in Altivec

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2007-05-27 21:31 -------
This is unrelated to that one.  Even if we did not have an aligned address, we
can do better than the extra load/store (but I forget how to do this and I did
not write anything on this for vec_splats in the C/C++ language extension for
the Cell).



Confirmed.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2007-05-27 21:31:45
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32107

[Bug target/32107] bad codegen for vector initialization in Altivec

[pinskia at gcc dot gnu dot org]

------- Comment #2 from pinskia at gcc dot gnu dot org  2008-03-11 21:34 -------
Mine.  The patches which I have from the PS3 toolchain fixes this one, it is
related to PR 32110 also.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32107

[Bug target/32107] bad codegen for vector initialization in Altivec

[pinskia at gcc dot gnu dot org]

------- Comment #3 from pinskia at gcc dot gnu dot org  2008-03-11 21:35 -------
Mine, I said.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|unassigned at gcc dot gnu   |pinskia at gcc dot gnu dot
                   |dot org                     |org
             Status|NEW                         |ASSIGNED


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32107

[Bug target/32107] bad codegen for vector initialization in Altivec

[pinskia at gcc dot gnu dot org]

------- Comment #4 from pinskia at gcc dot gnu dot org  2008-07-14 12:47 -------
Note with the Cell, we can just use lvlx with a splat and that works without an
alignment attribute on fa :).


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
  GCC build triplet|powerpc-linux               |
   GCC host triplet|powerpc-linux               |
 GCC target triplet|powerpc-linux               |powerpc*-*-*


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32107

[Bug target/32107] bad codegen for vector initialization in Altivec

[eres at il dot ibm dot com]

------- Comment #5 from eres at il dot ibm dot com  2009-04-26 07:29 -------
(In reply to comment #2)
> Mine.  The patches which I have from the PS3 toolchain fixes this one, it is
> related to PR 32110 also.

I see this problem still exits on trunk -r146794.
If you still have the patch I will be happy to test it.


-- 

eres at il dot ibm dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |eres at il dot ibm dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32107

[Bug target/32107] bad codegen for vector initialization in Altivec

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32107

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |NEW
         AssignedTo|pinskia at gcc dot gnu.org  |unassigned at gcc dot
                   |                            |gnu.org

--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> 2011-11-17 21:59:57 UTC ---
No longer working on this.