On 30/04/18 23:12, Jason Ekstrand wrote:
> On Mon, Apr 30, 2018 at 7:18 AM, Iago Toral Quiroga <ito...@igalia.com
> <mailto:ito...@igalia.com>> wrote:
>
> From: Jose Maria Casanova Crespo <jmcasan...@igalia.com
> <mailto:jmcasan...@igalia.com>>
>
> 16-bit immediates need to replicate the 16-bit immediate value
> in both words of the 32-bit value. This needs to be careful
> to avoid sign-extension, which the previous implementation was
> not handling properly.
>
> For example, with the previous implementation, storing the value
> -3 would generate imm.d = 0xfffffffd due to signed integer sign
> extension, which is not correct. Instead, we should cast to
> unsigned, which gives us the correct result: imm.ud = 0xfffdfffd.
>
> We only had a couple of cases hitting this path in the driver
> until now, one with value -1, which would work since all bits are
> one in this case, and another with value -2 in brw_clip_tri(),
> which would hit the aforementioned issue (this case only affects
> gen4 although we are not aware of whether this was causing an
> actual bug somewhere).
> ---
> src/intel/compiler/brw_reg.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/src/intel/compiler/brw_reg.h b/src/intel/compiler/brw_reg.h
> index dff9b970b2..0084a78af6 100644
> --- a/src/intel/compiler/brw_reg.h
> +++ b/src/intel/compiler/brw_reg.h
> @@ -705,7 +705,7 @@ static inline struct brw_reg
> brw_imm_w(int16_t w)
> {
> struct brw_reg imm = brw_imm_reg(BRW_REGISTER_TYPE_W);
> - imm.d = w | (w << 16);
> + imm.ud = (uint16_t)w | ((uint16_t)w << 16);
> Uh... Is this cast right? Doing a << 16 on a 16-bit data type should
> yield undefined results. I think you want a (uint32_t) cast.
In my test code it was working at least with GCC, I think it is because
at the end we have an integer promotion for any type with lower rank
than int.
"Formally, the rule says (C11 6.3.1.1):
If an int can represent all values of the original type (as
restricted by the width, for a bit-field), the value is converted to an
int; otherwise, it is converted to an unsigned int. These are called the
integer promotions."
But I agree that is clearer if we just use (uint32_t).
I can change also the brw_imm_uw case that has the same issue.
> return imm;
> }
>
> --
> 2.14.1
>
>
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