Re: LLVM calling convention for AVX2 and AVX512 registers

15 Mar 2017


      Ugh. I apparently had a misunderstanding about how that was compiled.

-Edward

On Wed, Mar 15, 2017 at 5:14 PM, Ben Gamari  wrote:
...
Edward Kmett  writes:
...
Currently if you try to use a DoubleX4# and don't have AVX2 turned on, it
deliberately crashes out during code generation, no?
I very well be missing something, but I don't believe this is true. This
program compiles just fine with merely -fllvm -msse,
{-# LANGUAGE MagicHash #-}
    {-# LANGUAGE UnboxedTuples #-}
    module Hi where
    import GHC.Prim
    import GHC.Float
addIt :: DoubleX4# -> DoubleX4# -> DoubleX4#
    addIt x y = plusDoubleX4# x y
    {-# NOINLINE addIt #-}
It produces the following assembler,,
movupd 0x10(%rbp),%xmm0
    movupd 0x0(%rbp),%xmm1
    movupd 0x30(%rbp),%xmm2
    movupd 0x20(%rbp),%xmm3
    addpd  %xmm1,%xmm3
    addpd  %xmm0,%xmm2
    movupd %xmm2,0x30(%rbp)
    movupd %xmm3,0x20(%rbp)
    mov    0x40(%rbp),%rax
    lea    0x20(%rbp),%rbp
    jmpq   *%rax
The reason for this is that the LLVM code generator just blindly
translates DoubleX4# to LLVM's <4 x double> type. The LLVM code
generator then does whatever it can to produce the code we ask of it,
even if the target doesn't have support for this vector variety.
Cheers,
- Ben