[Bug target/62191] New: extra shift generated for vector integer division by constant 2

[spatel at rotateright dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62191

            Bug ID: 62191
           Summary: extra shift generated for vector integer division by
                    constant 2
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: spatel at rotateright dot com

Using gcc 4.9:

$ cat sdiv.c
typedef int vecint __attribute__((vector_size(16))); 
vecint f(vecint x) { 
    return x/2;
} 

$ gcc -O2 sdiv.c -S -o  -
...
    movdqa    %xmm0, %xmm1
    psrad    $31, %xmm1    <--- splat the sign bit
    psrld    $31, %xmm1    <--- then shift sign bit down to LSB
    paddd    %xmm1, %xmm0  <--- add sign bit to quotient
    psrad    $1, %xmm0     <--- div via alg shift right
    ret

--------------------------------------------------------------

I don't think the first shift right algebraic is necessary. We splat the sign
bit and then shift that right logically, so the upper bits are all zero'd
anyway. 

This is a special case for signed integer division by 2. You need that first
'psrad' for any other power of 2 because the subsequent logical shift would not
also be a shift of 31.