Re: [Patch] catch builtin_bswap16 construct

[Christophe Lyon]

On 21 September 2012 12:56, Eric Botcazou <ebotca...@adacore.com> wrote:
>> Here is a new patch, modifying tree-ssa-math-opts.c as you suggested.
>> It's indeed simpler :-)
>>
>> Validated with qemu-arm on target arm-none-linux-gnueabi.
>
> I cannot formally approve, but this looks good to me.
>
> However, could you check that this is also an improvement for PowerPC, which
> is the only other mainstream architecture with a bswaphi pattern AFAIK?
>
> --
> Eric Botcazou

I have not yet been able to build an environment to run the testsuite
with qemu-ppc (not sure about the best target & dejagnu board
selection).

However, when compiling a sample test
short myswaps16(short x) {
   return (x << 8) | (x >> 8);
}
unsigned short myswapu16(unsigned short x) {
  return (x << 8) | (x >> 8);
}

The generated code is now:
myswaps16:
        rlwinm 10,3,8,16,23
        rlwinm 9,3,24,24,31
        or 9,9,10
        extsh 3,9
        blr

myswapu16:
        rlwinm 10,3,8,16,23
        rlwinm 9,3,24,24,31
        or 9,9,10
        rlwinm 3,9,0,0xffff
        blr

While it was (without my patch):

myswaps16:
        slwi 9,3,8
        srawi 3,3,8
        or 3,9,3
        extsh 3,3
        blr

myswapu16:
        srwi 9,3,8
        rlwinm 3,3,8,16,23
        or 3,3,9
        blr

I don't know PowerPC, but I am not sure it's an improvement. Is it?

Christophe.