[Bug 34266] New: Division by zero in shader results in NaNs

[wine-bugs at winehq.org]

http://bugs.winehq.org/show_bug.cgi?id=34266

             Bug #: 34266
           Summary: Division by zero in shader results in NaNs
           Product: Wine
           Version: 1.7.0
          Platform: x86
        OS/Version: Linux
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: directx-d3d
        AssignedTo: wine-bugs at winehq.org
        ReportedBy: achurch+wine at achurch.org
    Classification: Unclassified


Created attachment 45619
  --> http://bugs.winehq.org/attachment.cgi?id=45619
Proof-of-concept patch

As reported in the AppDB, if the HDR rendering option is enabled in Final
Fantasy XIV, some scenes have large black squares covering the screen.  This
appears to stem from NaNs getting stored in floating-point framebuffers as the
result of division by zero in a fragment shader.

Granted there's a whole bunch of wild guessing involved, but changing the
translation of the HLSL rcp instruction to explicitly check for zero and return
a huge value instead of performing the division seems to fix the problem
without causing any side effects (see attached patch).

I do note that HLSL defines the rcp instruction to return infinity for a zero
operand (*1), while the GLSL spec says that division by zero "result[s] in an
unspecified value" (*2).

(*1)
http://msdn.microsoft.com/en-us/library/windows/desktop/hh447205%28v=vs.85%29.aspx
(*2) http://www.opengl.org/registry/doc/GLSLangSpec.Full.1.20.8.pdf (section
5.9 "Expressions") -- bizarrely, the GLSL 4.4 spec keeps that language but adds
section 4.7.1 "Range and Precision" which says that "dividing a non-zero by 0
results in the appropriately signed IEEE Inf", so the spec is internally
inconsistent and who knows what you'll get.

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