[Mesa-dev] GLSL compiler bug

[Dragomir Ivanov]

Probably, the optimizer sees that there is second assignment, and skips the
first, f=1.0. It never gets to second though.


On Wed, Apr 10, 2013 at 11:08 PM, Ian Romanick <idr at freedesktop.org> wrote:

> On 04/10/2013 12:20 PM, Vadim Girlin wrote:
>
>> Hi,
>>
>> It seems there is a bug in the compiler. The problem may be reproduced
>> with the following shader (complete shader_test file attached):
>>
>>      void main()
>>      {
>>          float f = 0.0;
>>          while (true) {
>>              f = 1.0;
>>              break;
>>              f = 0.5;
>>          }
>>          gl_FragColor = vec4(1.0 - f, f, 0.0, 1.0);
>>      }
>>
>> The result of compilation is equal to:
>>
>>      while (true) {
>>          break;
>>      }
>>      gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);
>>
>> In other words, GLSL compiler eliminates both assignments to "f" in the
>> loop body and the resulting value of the "f" variable is 0.
>>
>
> Blarg.  I can believe it.  The loop analysis code is a disaster.  Part of
> the reason I'm (slowly) making changes to the IR is so that we can do more
> credible loop handling.
>
> I probably won't be able to get to this problem for quite some time. :(
>
>  Vadim
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>
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