Keith Whitwell wrote:
A while ago, Mesa started using the full width of variables like
Stencil.WriteMask, etc. to store values, while previously it had
restricted itself to only the low 8 bits.
Unfortunately there are many cases in the dri drivers that assumed the
old behaviour, and did things like comparisons:
if (ctx->Stencil.WriteMask[0] == 0xff)
and also building hardware commands:
OUT_BATCH( SOME_COMMAND_TAG |
ctx->Stencil.WriteMask[0] |
SOME_OTHER_STUFF )
This was the cause of the recent i915 performance regression. The
change seems to have been noticed and local fixes for the change are
sprinkled through the drivers, but the attached patch is the first
attempt to look at all cases throughout the drivers where this may have
had an effect.
I'm travelling at the moment, and can't test the changes even on the
hardware I have. Hopefully people can do a little testing in the
meantime, I expect I'll commit this on Monday.
Some background on this: The OpenGL spec isn't totally clear on what
the initial value for the write mask should be. It just says: 1's
We (and other OpenGL implementors) had assumed the value should be
0xff for 8-bit stencil buffers.
That was OK prior to GL_EXT_framebuffer_object. With that extension
it's possible for a rendering context to use a number of stencil
renderbuffers of different depths. So 0xff was an incorrect assumption.
After some discussion among the ARB members we determined the default
value should be ~0 (typically 32 bits). When we use this value now we
just grab the lowest N bits of the write mask if we have an N-bit
stencil buffer.
Since the DRI drivers use 8-bit stencil, the tests for
if (ctx->Stencil.WriteMask[0] == 0xff)
should probably be:
if ((ctx->Stencil.WriteMask[0] & 0xff) == 0xff)
-Brian
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