i965/vs: When asked to make a dst_reg for a src.xxxx, just write to src.x.

We have several places in our pull constant handling where we make a
temporary src_reg for an int, and then turn it into a dst.  In doing so,
we were writing to the dst.xyzw, so we never register coalesced it with a
later mov from dst.x to real_dst.x.

These extra channels written would be removed if we had channel-wise DCE
in the backend, but we don't.  Fix it for now by just not writing these
extra channels that won't get used.

Reviewed-by: Matt Turner <mattst88@gmail.com>

diff --git a/src/mesa/drivers/dri/i965/brw_vec4.cpp b/src/mesa/drivers/dri/i965/brw_vec4.cpp
index c58fb44..e470ac8 100644 (file)
--- a/src/mesa/drivers/dri/i965/brw_vec4.cpp
+++ b/src/mesa/drivers/dri/i965/brw_vec4.cpp
@@ -200,7 +200,14 @@ dst_reg::dst_reg(src_reg reg)
    this->reg = reg.reg;
    this->reg_offset = reg.reg_offset;
    this->type = reg.type;
-   this->writemask = WRITEMASK_XYZW;
+   /* How should we do writemasking when converting from a src_reg?  It seems
+    * pretty obvious that for src.xxxx the caller wants to write to src.x, but
+    * what about for src.wx?  Just special-case src.xxxx for now.
+    */
+   if (reg.swizzle == BRW_SWIZZLE_XXXX)
+      this->writemask = WRITEMASK_X;
+   else
+      this->writemask = WRITEMASK_XYZW;
    this->reladdr = reg.reladdr;
    this->fixed_hw_reg = reg.fixed_hw_reg;
 }