i965: Fix INTEL_DEBUG=shader_time for fragment shaders with discards.

i965: Fix INTEL_DEBUG=shader_time for fragment shaders with discards.

"discard" instructions generate HALT instructions which jump to a final
HALT near the end of the shader.  Previously, fs_generator created this
final jump target when it saw the first FS_OPCODE_FB_WRITE, causing it
to jump right before the FB write epilogue.  This is normally good.

However, INTEL_DEBUG=shader_time also has an epilogue section which
records the final timestamp.  The frontend emits IR for this just before
FS_OPCODE_FB_WRITE.  Unfortunately, this led to the following ordering:

1. Shader Time Epilogue
2. Final HALT (where discards jump)
3. Framebuffer Write Epilogue

This meant that discarded pixels completely skipped the shader time
epilogue, causing no ending timestamp to be written.  This obviously
led to inaccurate results.

This patch adds a new FS_OPCODE_PLACEHOLDER_HALT in the IR stream just
before any epilogue sections.  This is where the final HALT should be
generated, and makes it easy to ensure the correct ordering:

1. Final HALT
2. Shader Time Epilogue
3. Framebuffer Write Epilogue

For shaders that don't discard, this opcode compiles away to nothing.
The scheduler adds barrier dependencies to make sure that it doesn't
get moved above any FS_OPCODE_DISCARD_JUMP instructions.

One 8-wide shader in GLBenchmark 2.7 dropped from 2291.67 Gcycles to
a mere 5.13 Gcycles.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>