[Mesa-dev] [PATCH v2 06/18] intel/compiler: fix brw_imm_w for negative 16-bit integers

[Jason Ekstrand]

On Mon, Apr 30, 2018 at 3:53 PM, Chema Casanova <jmcasanova at igalia.com>
wrote:

>
>
> On 30/04/18 23:12, Jason Ekstrand wrote:
> > On Mon, Apr 30, 2018 at 7:18 AM, Iago Toral Quiroga <itoral at igalia.com
> > <mailto:itoral at igalia.com>> wrote:
> >
> >     From: Jose Maria Casanova Crespo <jmcasanova at igalia.com
> >     <mailto:jmcasanova at 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.
>

Yeah, best to make it clear. :-)
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