glsl: Rework assignments with write_masks to have LHS chan count match RHS.

It turns out that most people new to this IR are surprised when an
assignment to (say) 3 components on the LHS takes 4 components on the
RHS.  It also makes for quite strange IR output:

(assign (constant bool (1)) (x) (var_ref color) (swiz x (var_ref v) ))
(assign (constant bool (1)) (y) (var_ref color) (swiz yy (var_ref v) ))
(assign (constant bool (1)) (z) (var_ref color) (swiz zzz (var_ref v) ))

But even worse, even we get it wrong, as shown by this line of our
current step(float, vec4):

(assign (constant bool (1)) (w)
	(var_ref t)
	(expression float b2f (expression bool >=
		    (swiz w (var_ref x))(var_ref edge))))

where we try to assign a float to the writemasked-out x channel and
don't supply anything for the actual w channel we're writing.  Drivers
right now just get lucky since ir_to_mesa spams the float value across
all the source channels of a vec4.

Instead, the RHS will now have a number of components equal to the
number of components actually being written.  Hopefully this confuses
everyone less, and it also makes codegen for a scalar target simpler.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

1 files changed, 12 insertions, 4 deletions

diff --git a/src/glsl/ir_constant_propagation.cpp b/src/glsl/ir_constant_propagation.cpp
index f7a0599f42..390f295d6e 100644
--- a/src/glsl/ir_constant_propagation.cpp
+++ b/src/glsl/ir_constant_propagation.cpp
@@ -168,18 +168,26 @@ ir_constant_propagation_visitor::handle_rvalue(ir_rvalue **rvalue)
       if (!found)
 	 return;
 
+      int rhs_channel = 0;
+      for (int j = 0; j < 4; j++) {
+	 if (j == channel)
+	    break;
+	 if (found->write_mask & (1 << j))
+	    rhs_channel++;
+      }
+
       switch (type->base_type) {
       case GLSL_TYPE_FLOAT:
-	 data.f[i] = found->constant->value.f[channel];
+	 data.f[i] = found->constant->value.f[rhs_channel];
 	 break;
       case GLSL_TYPE_INT:
-	 data.i[i] = found->constant->value.i[channel];
+	 data.i[i] = found->constant->value.i[rhs_channel];
 	 break;
       case GLSL_TYPE_UINT:
-	 data.u[i] = found->constant->value.u[channel];
+	 data.u[i] = found->constant->value.u[rhs_channel];
 	 break;
       case GLSL_TYPE_BOOL:
-	 data.b[i] = found->constant->value.b[channel];
+	 data.b[i] = found->constant->value.b[rhs_channel];
 	 break;
       default:
 	 assert(!"not reached");