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From: "ramana at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/66675] New: Could improve vector bit_field_ref style optimizations.
Date: Thu, 25 Jun 2015 21:03:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66675

            Bug ID: 66675
           Summary: Could improve vector bit_field_ref style
                    optimizations.
           Product: gcc
           Version: 5.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ramana at gcc dot gnu.org
  Target Milestone: ---

This example 


#include <arm_neon.h>

int main(int argc, char *argv[])
{
    int8x8_t a = {argc, 1, 2, 3, 4, 5, 6, 7};
    int8x8_t b = {0, 1, 2, 3, 4, 5, 6, 7};
    int8x8_t c = vadd_s8(a, b);
    return c[0];
}


or it's variant written in gcc vector speak generate pretty terrible code for
AArch64 

main:
        adr     x1, .LC0
        ld1     {v0.8b}, [x1]
        ins     v0.b[0], w0
        adr     x0, .LC2
        ld1     {v1.8b}, [x0]
        add     v0.8b, v0.8b, v1.8b
        umov    w0, v0.b[0]
        sxtb    w0, w0
        ret
        .size   main, .-main


This could well be folded down to a simple function that returns just argc.
While this is a bit silly to expect in real life, it does show an interesting
example....


regards
Ramana