Add bswapsi2/bswapdi2 support to libcompiler-rt for mips platforms.

[dim]

LGTM, we should upstream the contrib part, though. Unfortunately llvm.org is down at the moment...

[emaste]

I'm fine with this so that we can unblock MIPS w/ xGCC, but would hope we can replace it with a better asm version in the future. Depending on the MIPS ISA we target I suspect there's no huge gain to be had , but there some clever tricks might save a couple of instructions.

[dim]

Using -target mips-freebsd, the assembly produced for e.g. bswapsi2.c is:

__bswapsi2:                             # @__bswapsi2
        .frame  $fp,8,$ra
        .mask   0x40000000,-4
        .fmask  0x00000000,0
        .set    noreorder
        .set    nomacro
        .set    noat
# BB#0:                                 # %entry
        addiu   $sp, $sp, -8
        sw      $fp, 4($sp)             # 4-byte Folded Spill
        move     $fp, $sp
        wsbh    $1, $4
        rotr    $2, $1, 16
        move     $sp, $fp
        lw      $fp, 4($sp)             # 4-byte Folded Reload
        jr      $ra
        addiu   $sp, $sp, 8
        .set    at
        .set    macro
        .set    reorder
        .end    __bswapsi2
$func_end0:
        .size   __bswapsi2, ($func_end0)-__bswapsi2

I'm not a mips expert, but it looks pretty OK to me... :)

[emaste]

Indeed, that looks fine.