[Mesa-dev] [PATCH 2/2] prog_optimize: Add reads without writes optimization pass

Eric Anholt eric at anholt.net
Tue Mar 29 14:16:39 PDT 2011

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On Tue, 29 Mar 2011 11:35:33 -0700, Tom Stellard <tstellar at gmail.com> wrote:
> On Tue, Mar 29, 2011 at 10:19:13AM -0700, Eric Anholt wrote:
> > On Mon, 28 Mar 2011 22:59:31 -0700, Tom Stellard <tstellar at gmail.com> wrote:
> > > This pass scans programs for instructions that read registers that have
> > > not been written and replaces these reads with a read from a constant
> > > register with the value of zero.  This pass prevents phantom
> > > dependencies from being introduced by the register allocator and
> > > improves the efficiency of subsequent optimization passes.
> > 
> > I'm not sure why optimizing a program with undefined behavior (using
> > undefined register values) is interesting.  It just seems like an
> > opportunity to make a mistake and break programs that should have
> > defined behavior.
> The main problem I am trying to fix with this is that these reads
> from undefined register values are causing the Mesa IR register allocator
> to alter the structure of the program by adding dependencies between
> instructions that should not be there.  This is limiting the number of
> optimization opportunities that are available to drivers, especially
> on architectures like r300 that need IFs lowered and thus use a lot of
> CMP instructions.

Why do you have programs reading undefined values?  Why are these
programs interesting?
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