[Mesa-dev] [PATCH 23/24] i965/vec4: Replace src_reg(imm) constructors with brw_imm_*().

[Matt Turner]

On Thu, Nov 26, 2015 at 9:22 AM, Emil Velikov <emil.l.velikov at gmail.com> wrote:
> On 25 November 2015 at 22:48, Matt Turner <mattst88 at gmail.com> wrote:
>
>> I can't see it, but that might be because I can't stop thinking like I
>> was when I wrote the code.
>>
> Understandable, we've all been there.
>
>> Yeah, we skip any instruction that has a full writemask -- because if
>> it has a full writemask, we can just load the immediate as-is and we
>> don't need to turn it into 4x restricted floats. But that doesn't seem
>> to be related to the warning.
>>
>> The code seems fine to me. Here's what I see:
>>
>> We start with remaining_channels = WRITEMASK_XYZW.
> Actually we start with WRITEMASK_NONE (0). Based on your knowledge you
> can establish that on the first iteration(?) remaining_channels will
> be set to WRITEMASK_XYZW, although the compiler cannot determine that.
>
>> We initialize an
>> element of imm[] for each enabled bit of inst->dst.writemask, and then
>> we disable those bits from remaining_channels.
> If all channels are set we bail out just before that. Thus as we hit
> the imm[x] assignments of at least one channel is not set. Be that
> left uninitialized or storing data from a previous instruction. I take
> that this is by design ?
>
>> When remaining_channels
>> is zero (should be if and only if all elements of imm[] are
>> initialized), we read imm[].
>>
> Barring your existing knowledge, remaining_channels can be zero on the
> first, second,  N'th iteration, albeit extremely unlikely. Thus
> regardless of what data we have in imm[], we'll proceed to use it.
>
>> Do you see anything wrong or anything I've missed?
>>
>> FWIW, Clang does not warn about this and doesn't warn if I remove the
>> useless initializations of remaining_channels = 0 and inst_count = 0.
>> I think this is gcc and Coverity just being stupid.
> I think the whole things is that you know who it will run, whereas the
> code makes is a bit ambiguous.
>
> To fix (?) and make things a lot more obvious I'd suggest initializing
> (upon declaration) remaining_channels with WRITEMASK_XYZW. If
> gcc/coverity/other complains with that in place then it's definitely a
> defect on their end.

The initialization of remaining_writemask is *dead* -- the first block
inside the for loop initializes it upon seeing the first instruction.
Initializing it to WRITEMASK_XYZW doesn't fix the warning in any case.

> Out of curiosity: what is the input from gcc devs, do you have the bug
> number handy ?

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68548

They think it's a deficiency in gcc.