[Mesa-dev] [PATCH] i965/vec4: Avoid reswizzling MACH instructions in opt_register_coalesce().

[Kenneth Graunke]

opt_register_coalesce() was optimizing sequences such as:

   mul(8) acc0:D, attr18.xyyy:D, attr19.xyyy:D
   mach(8) vgrf5.xy:D, attr18.xyyy:D, attr19.xyyy:D
   mov(8) m4.zw:F, vgrf5.xxxy:F

into:

   mul(8) acc0:D, attr18.xyyy:D, attr19.xyyy:D
   mach(8) m4.zw:D, attr18.xxxy:D, attr19.xxxy:D

This doesn't work - if we're going to reswizzle MACH, we'd need to
reswizzle the MUL as well.  Here, the MUL fills the accumulator's .zw
components with attr18.yy * attr19.yy.  But the MACH instruction expects
.z to contain attr18.x * attr19.x.  Bogus results ensue.

No change in shader-db on Haswell.  Prevents regressions in Timothy's
patches to use enhanced layouts for varying packing (which rearrange
code just enough to trigger this pre-existing bug, but were fine
themselves).

Cc: Timothy Arceri <tarceri at itsqueeze.com>
---
 src/intel/compiler/brw_vec4.cpp | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/src/intel/compiler/brw_vec4.cpp b/src/intel/compiler/brw_vec4.cpp
index 0b92ba704e5..4bb774bf10e 100644
--- a/src/intel/compiler/brw_vec4.cpp
+++ b/src/intel/compiler/brw_vec4.cpp
@@ -1071,6 +1071,13 @@ vec4_instruction::can_reswizzle(const struct gen_device_info *devinfo,
    if (devinfo->gen == 6 && is_math() && swizzle != BRW_SWIZZLE_XYZW)
       return false;
 
+   /* Don't touch MACH - it uses the accumulator results from an earlier
+    * MUL - so we'd need to reswizzle both.  We don't do that, so just
+    * avoid it entirely.
+    */
+   if (opcode == BRW_OPCODE_MACH)
+      return false;
+
    if (!can_do_writemask(devinfo) && dst_writemask != WRITEMASK_XYZW)
       return false;
 
-- 
2.12.2