[Bug target/66144] New: vector element operator produces very bad code

[Bug target/66144] New: vector element operator produces very bad code

[mkg at us dot ibm.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66144

            Bug ID: 66144
           Summary: vector element operator produces very bad code
           Product: gcc
           Version: tree-ssa
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mkg at us dot ibm.com
  Target Milestone: ---

Compiling the following input
#include <altivec.h>

vector char
test( vector char a, vector char b)
{
        return a == b;
}

generates the following output with gcc from from trunk/head with -O3 


test:
        vcmpequb 2,2,3
        xxlxor 32,32,32
        vspltisw 1,-1
        xxsel 34,32,33,34
        blr

The original first instruction already has the right answer, then it's
recomputed!

The inverted condition
#include <altivec.h>

vector char
test( vector char a, vector char b)
{
        return a != b;
}


produces
test:
        vcmpequb 2,2,3
        xxlxor 33,33,33
        vspltisw 0,-1
        xxsel 34,32,33,34
        blr

[Bug target/66144] vector element operator produces very bad code

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66144

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Version|tree-ssa                    |4.9.0

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I suspect a == b is all equals and not element by element equals.

[Bug target/66144] vector element operator produces very bad code

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66144

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #1)
> I suspect a == b is all equals and not element by element equals.

I am wrong.

>From the manual:
Vector comparison is supported with standard comparison operators: ==, !=, <,
<=, >, >=. Comparison operands can be vector expressions of integer-type or
real-type. Comparison between integer-type vectors and real-type vectors are
not supported. The result of the comparison is a vector of the same width and
number of elements as the comparison operands with a signed integral element
type.

Vectors are compared element-wise producing 0 when comparison is false and -1
(constant of the appropriate type where all bits are set) otherwise. Consider
the following example.

     typedef int v4si __attribute__ ((vector_size (16)));

     v4si a = {1,2,3,4};
     v4si b = {3,2,1,4};
     v4si c;

     c = a >  b;     /* The result would be {0, 0,-1, 0}  */
     c = a == b;     /* The result would be {0,-1, 0,-1}  */

[Bug target/66144] vector element operator produces very bad code

[segher at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66144

Segher Boessenkool <segher at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2015-05-14
                 CC|                            |segher at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #3 from Segher Boessenkool <segher at gcc dot gnu.org> ---
Confirmed.  This code is generated by rs6000_emit_vector_cond_expr
and combine doesn't know how to simplify it (it tries though).