[Mesa-dev] [PATCH] gallivm: Use vector selects on LLVM 3.4+.

[Roland Scheidegger]

Am 01.04.2016 um 00:42 schrieb Jose Fonseca:
> This is an old patch I had around.
> 
> Vector selects seem to work well from LLVM 3.4.  Using them might
> improve code quality, as it might make constant propagation pass more
> effective.
> 
> Tested lp_test_*
> ---
>  src/gallium/auxiliary/gallivm/lp_bld_logic.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/src/gallium/auxiliary/gallivm/lp_bld_logic.c b/src/gallium/auxiliary/gallivm/lp_bld_logic.c
> index 19d30d0..b3694ea 100644
> --- a/src/gallium/auxiliary/gallivm/lp_bld_logic.c
> +++ b/src/gallium/auxiliary/gallivm/lp_bld_logic.c
> @@ -314,11 +314,13 @@ lp_build_select(struct lp_build_context *bld,
>        mask = LLVMBuildTrunc(builder, mask, LLVMInt1TypeInContext(lc), "");
>        res = LLVMBuildSelect(builder, mask, a, b, "");
>     }
> -   else if (0) {
> +   else if (HAVE_LLVM >= 0x0304) {
>        /* Generate a vector select.
>         *
> -       * XXX: Using vector selects would avoid emitting intrinsics, but they aren't
> -       * properly supported yet.
> +       * Using vector selects would avoid emitting intrinsics, but they weren't
> +       * properly supported yet for a long time.
> +       *
> +       * LLVM 3.4 appears to reliably support it.
>         *
>         * LLVM 3.1 supports it, but it yields buggy code (e.g. lp_blend_test).
>         *
> 

Wouldn't they already work with llvm 3.3 (the minimum required
nowadays)? I think we just didn't really test after 3.1. I'm fine though
with requiring 3.4.
(If anything I'd be more worried about the lowering code doing awful
things, that is when sse41 isn't available - earlier llvm versions had
tendencies to lower things to scalars when things weren't directly
doable with vector operations...).
Hopefully it works on non-x86 backends too but we can't test this, so
I'd say we just wait for the bug reports ;-).

Reviewed-by: Roland Scheidegger <sroland at vmware.com>