i965/vec4: Avoid reswizzling MACH instructions in opt_register_coalesce().

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).

Acked-by: Timothy Arceri <[email protected]>
Reviewed-by: Jason Ekstrand <[email protected]>
(cherry picked from commit 2faf227ec2e22c7a37e0a54783a3f0a0062ac852)

Squashed with commit:

i965/vec4: Use reads_accumulator_implicitly(), not MACH checks.

Curro pointed out that I should not just check for MACH, but use
the reads_accumulator_implicitly() helper, which would also prevent
the same bug with MAC and SADA2 (if we ever decide to use them).

Cc: [email protected]
Reviewed-by: Francisco Jerez <[email protected]>
(cherry picked from commit 6b10c37b9c3a73add73f444fe1aee73c9ec82c94)

1 files changed, 7 insertions, 0 deletions

diff --git a/src/mesa/drivers/dri/i965/brw_vec4.cpp b/src/mesa/drivers/dri/i965/brw_vec4.cpp
index 5e60eb657a7..a7694089e93 100644
--- a/src/mesa/drivers/dri/i965/brw_vec4.cpp
+++ b/src/mesa/drivers/dri/i965/brw_vec4.cpp
@@ -1066,6 +1066,13 @@ vec4_instruction::can_reswizzle(const struct gen_device_info *devinfo,
    if (devinfo->gen == 6 && is_math() && swizzle != BRW_SWIZZLE_XYZW)
       return false;
 
+   /* We can't swizzle implicit accumulator access.  We'd have to
+    * reswizzle the producer of the accumulator value in addition
+    * to the consumer (i.e. both MUL and MACH).  Just skip this.
+    */
+   if (reads_accumulator_implicitly())
+      return false;
+
    if (!can_do_writemask(devinfo) && dst_writemask != WRITEMASK_XYZW)
       return false;