[Bug 93339] glLinkProgram() should fail when a varying is never written to in a previous stage

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=93339

            Bug ID: 93339
           Summary: glLinkProgram() should fail when a varying is never
                    written to in a previous stage
           Product: Mesa
           Version: git
          Hardware: x86-64 (AMD64)
                OS: Linux (All)
            Status: NEW
          Severity: normal
          Priority: medium
         Component: glsl-compiler
          Assignee: idr at freedesktop.org
          Reporter: bugs at linkmauve.fr
        QA Contact: intel-3d-bugs at lists.freedesktop.org

Created attachment 120455
  --> https://bugs.freedesktop.org/attachment.cgi?id=120455&action=edit
Testcase exhibiting this bug, red patterns randomly happen when line 55 is
commented.

If a varying is declared in one stage and read from in the next stage, without
having been written to, the link step don’t fail and will generate a working
program object, but that varying will read from seemingly uninitialized video
memory (at least on i965, llvmpipe seems to read from 0-initialized memory
instead).

I haven’t looked at the specification, but I can’t see any usecase for this
specific behaviour, which should imo be reported to the user before they can
use the program object.

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