[Mesa-dev] [PATCH] avoid crashing in mod by 0 with llvmpipe

[Roland Scheidegger]

Am 15.01.2016 um 20:13 schrieb Jeff Muizelaar:
> This adds code that is basically the same as the code in smod, div and idiv.
That would be umod, not smod, above (and udiv).

> However, unlike idiv we return -1.
> 
> diff --git a/src/gallium/auxiliary/gallivm/lp_bld_tgsi_action.c
> b/src/gallium/auxiliary/gallivm/lp_bld_tgsi_action.c
> index 0ad78b0..d17ef31 100644
> --- a/src/gallium/auxiliary/gallivm/lp_bld_tgsi_action.c
> +++ b/src/gallium/auxiliary/gallivm/lp_bld_tgsi_action.c
> @@ -1524,18 +1524,32 @@ min_emit_cpu(
> 
>  /* TGSI_OPCODE_MOD (CPU Only) */
>  static void
>  mod_emit_cpu(
>     const struct lp_build_tgsi_action * action,
>     struct lp_build_tgsi_context * bld_base,
>     struct lp_build_emit_data * emit_data)
>  {
> -   emit_data->output[emit_data->chan] = lp_build_mod(&bld_base->int_bld,
> -                                   emit_data->args[0], emit_data->args[1]);
> +   LLVMBuilderRef builder = bld_base->base.gallivm->builder;
> +   LLVMValueRef div_mask = lp_build_cmp(&bld_base->uint_bld,
> +                    PIPE_FUNC_EQUAL, emit_data->args[1],
> +                    bld_base->uint_bld.zero);
> +   /* We want to make sure that we never divide/mod by zero to not
> +    * generate sigfpe. We don't want to crash just because the
> +    * shader is doing something weird. */
> +   LLVMValueRef divisor = LLVMBuildOr(builder,
> +                      div_mask,
> +                      emit_data->args[1], "");
> +   LLVMValueRef result = lp_build_mod(&bld_base->int_bld,
> +                      emit_data->args[0], divisor);
> +   /* umod by zero is guaranteed to return 0xffffffff */
I think this should use language along the lines of what idiv does, i.e.
no guaranteed value, pick -1. Because we don't say anything about that
in the gallium tgsi docs section about that (unlike udiv/umod).
(AFAIK we only really care for udiv/umod due to d3d10, which doesn't
have the signed versions - glsl doesn't care either way.)

> +   emit_data->output[emit_data->chan] = LLVMBuildOr(builder,
> +                            div_mask,
> +                            result, "");
>  }
> 
>  /* TGSI_OPCODE_NOT */
>  static void
>  not_emit_cpu(
>     const struct lp_build_tgsi_action * action,
>     struct lp_build_tgsi_context * bld_base,
>     struct lp_build_emit_data * emit_data)

Subject should probably say: gallivm: ...

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