[Mesa-dev] [PATCH 2/2] i965/fs: Take # of components into account in try_rewrite_rhs_to_dst.

[Kenneth Graunke]

Commit dc7f449d1ac53a66e6efb56ccf2a5953418a26ca introduced a new method
for avoiding MOVs: try to rewrite the destination of the instruction
that produced the RHS so it writes into the LHS.

Unfortunately, this is not safe for swizzled texturing operations, as
they return a set of four contiguous registers.  Consider the following:

(assign (x)
        (var_ref vec_ctor_x)
        (swiz x (tex vec4 (var_ref m_sampY) (var_ref m_cordY) 0 1 ())))

In this case, the source and destination registers are equal, since
reg_offset is 0 for both.  Yet, this is only a partial move: the texture
operation generates four registers, and the LHS only covers one.

Fixes color distortion in XBMC when using GLSL shaders.

NOTE: This is a candidate for the 8.0 branch (with the previous commit).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=44333
Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
---
 src/mesa/drivers/dri/i965/brw_fs_visitor.cpp |    6 ++++++
 1 files changed, 6 insertions(+), 0 deletions(-)

diff --git a/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp b/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
index e670ba7..bf1da9d 100644
--- a/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
+++ b/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
@@ -605,6 +605,12 @@ fs_visitor::try_rewrite_rhs_to_dst(ir_assignment *ir,
        !src.equals(&last_rhs_inst->dst))
       return false;
 
+   /* If last_rhs_inst wrote a different number of components than our LHS,
+    * we can't safely rewrite it.
+    */
+   if (ir->lhs->type->vector_elements != last_rhs_inst->regs_written())
+      return false;
+
    /* Success!  Rewrite the instruction. */
    last_rhs_inst->dst = dst;
 
-- 
1.7.9