[Mesa-dev] [PATCH 05/22] glsl: Add sqrt, rsq, exp, exp2 to get_range

Connor Abbott cwabbott0 at gmail.com
Thu Jan 8 19:15:45 PST 2015


On Sat, Jan 3, 2015 at 2:18 PM, Thomas Helland
<thomashelland90 at gmail.com> wrote:
> Also handle undefined behaviour for sqrt(x) where x < 0
> and rsq(x) where x <= 0.
>
> This gives us some reduction in instruction count on three
> Dungeon Defenders shaders as they are doing: max(exp(x), 0)

So initially when you said that Dungeon Defenders was doing
max(exp(x), 0), my thought was "wat?" but after thinking about it some
more, I can see why it would do this. The GLSL spec doesn't guarantee
that implementations of +, *, exp(), etc. will return NaN when one of
the arguments is NaN, but it also doesn't guarantee that they *won't*;
in other words, if for some strange reason you need the old-style
never-return-NaN functionality, you need to do something like what
this game is doing. For implementations that don't return NaN, this
optimization is just fine, but if you remove it when the HW does
return NaN, then whatever's using the result might get a NaN when it's
not expecting it, leading to Bad Things happening. Maybe it isn't an
issue with this particular game, but in order to be correct here it
seems like we do have to take NaN's into account after all.

There was a related thread (and other discussions) about the behavior
of min/max wrt NaN's:

http://lists.freedesktop.org/archives/mesa-dev/2014-December/073182.html

My conclusion is that basically everyone that actually produces NaN's
follows the IEEE/D3D behavior here, which I'm assuming the Dungeon
Defenders developers were probably depending on.

>
> v2: Change to use new IS_CONSTANT() macro
>     Fix high unintenionally not being returned
>     Add some air for readability
>     Comment on the exploit of undefined behavior
>     Constify mem_ctx
> ---
>  src/glsl/opt_minmax.cpp | 31 +++++++++++++++++++++++++++++++
>  1 file changed, 31 insertions(+)
>
> diff --git a/src/glsl/opt_minmax.cpp b/src/glsl/opt_minmax.cpp
> index 56805c0..2faa3c3 100644
> --- a/src/glsl/opt_minmax.cpp
> +++ b/src/glsl/opt_minmax.cpp
> @@ -274,9 +274,40 @@ get_range(ir_rvalue *rval)
>     minmax_range r0;
>     minmax_range r1;
>
> +   void *const mem_ctx = ralloc_parent(rval);
> +
> +   ir_constant *low = NULL;
> +   ir_constant *high = NULL;
> +
>     if (expr) {
>        switch (expr->operation) {
>
> +      case ir_unop_exp:
> +      case ir_unop_exp2:
> +      case ir_unop_sqrt:
> +      case ir_unop_rsq:
> +         r0 = get_range(expr->operands[0]);
> +
> +         /* The spec says sqrt is undefined if x < 0
> +          * We can use this to set the range to whatever we want
> +          */
> +         if (expr->operation == ir_unop_sqrt &&
> +             IS_CONSTANT(r0.high, <, 0.0f))
> +            high = new(mem_ctx) ir_constant(0.0f);
> +
> +         /* The spec says rsq is undefined if x <= 0
> +          * We can use this to set the range to whatever we want
> +          */
> +         if (expr->operation == ir_unop_rsq &&
> +             IS_CONSTANT(r0.high, <=, 0.0f))
> +            high = new(mem_ctx) ir_constant(0.0f);
> +
> +         /* TODO: If we know, i.e, the lower range of the operand
> +          * we can calculate the lower range
> +          */
> +         low = new(mem_ctx) ir_constant(0.0f);
> +         return minmax_range(low, high);
> +
>        case ir_binop_min:
>        case ir_binop_max:
>           r0 = get_range(expr->operands[0]);
> --
> 2.2.1
>
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