[Mesa-dev] Mesa (master): glsl: Fix constant-folding for reciprocal expressions

[Chad Versace]

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On 02/02/2011 01:36 PM, Ian Romanick wrote:
> This behavior is actually correct.

That's unexpected...


> The 4.10 spec contains that language, but it also requires IEEE 754
> behavior.  Specifically, page 54 (page 59 of the PDF) says, "The
> following rules apply to both single and double-precision operations:
> Dividing by 0 results in the appropriately signed IEEE Inf."
> 
> For now I believe 1/0 for both integer and real values should result in
> 0.  

I think that 1/0, for real values, should result in Inf now.
1. The compiler will have to do it eventually, anyway; the 4.10 spec
   requires it.
2. This does not differ from the compiler's previous
   behavior (previous, as in before my patches). The compiler constant-
   folded floating point ir_binop_div, ir_binop_mod, and ir_unop_rsq to
   inf when the denominator was 0.
3. Doing so now will not diverge from specified behavior, since the
   previous specs are lenient here [1].

> This also applies to e7c1f058.

I'll revert b3cf92 and fix e7c1f058 so that integer valued 1/0 results
in 0. Once we decide how to resolve real valued 1/0, I'll fix e7c1f058
in those cases too.

- ----
[1] OpenGL 3.3 spec, page 20 (page 26 of PDF): "Similarly, treatment of
conditions such as divide by 0 [for floating point numbers] may lead to
an unspecified result, but in no case should such a condition lead to
the interruption or termination of processing."

- -- 
Chad Versace
chad at chad-versace.us
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