[Mesa-dev] [PATCH] st/glsl_to_tgsi: Translate float ir_unop_neg into float MOV with modifier.

[Ilia Mirkin]

Hmmm... I wonder if this should only be done for the native_integers
case. I'm concerned that this will cause perf regressions on weaker hw
like nv30 and r300, as the neg will no longer be inserted as a
modifier into the next instruction. Any opinion on this?

On Sat, Mar 4, 2017 at 2:16 PM, Francisco Jerez <currojerez at riseup.net> wrote:
> Otherwise result_src may be provided to an integer instruction whose
> negate modifier has different semantics.  Example is UCMP as in the
> bug linked below, where an unrelated change in the GLSL built-in
> lowering code for atan2 (e9ffd12827ac11a2d2002a42fa8eb1df847153ba)
> caused the generation of floating-point ir_unop_neg instructions
> followed by ir_triop_csel, which is lowered into UCMP on back-ends
> with native integer support.
>
> Fixes a regression of piglit glsl-fs-tan-1 on softpipe introduced by
> the above-mentioned glsl front-end commit.  Even though the commit
> that triggered the regression doesn't seem to have made it to any
> stable branches yet, this seems worth back-porting since I don't see
> any reason why the bug couldn't have been reproduced before that
> point.
>
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99817
> Tested-by: Vinson Lee <vlee at freedesktop.org>
> Cc: mesa-stable at lists.freedesktop.org
> ---
>  src/mesa/state_tracker/st_glsl_to_tgsi.cpp | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/src/mesa/state_tracker/st_glsl_to_tgsi.cpp b/src/mesa/state_tracker/st_glsl_to_tgsi.cpp
> index af41bdb..6bf3c89 100644
> --- a/src/mesa/state_tracker/st_glsl_to_tgsi.cpp
> +++ b/src/mesa/state_tracker/st_glsl_to_tgsi.cpp
> @@ -1633,7 +1633,7 @@ glsl_to_tgsi_visitor::visit_expression(ir_expression* ir, st_src_reg *op)
>           emit_asm(ir, TGSI_OPCODE_DNEG, result_dst, op[0]);
>        else {
>           op[0].negate = ~op[0].negate;
> -         result_src = op[0];
> +         emit_asm(ir, TGSI_OPCODE_MOV, result_dst, op[0]);
>        }
>        break;
>     case ir_unop_subroutine_to_int:
> --
> 2.10.2
>
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