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, 28 insertions, 1 deletions

diff --git a/src/glsl/ir.cpp b/src/glsl/ir.cpp
index 7cc55d40b7..5e2109ecc6 100644
--- a/src/glsl/ir.cpp
+++ b/src/glsl/ir.cpp
@@ -55,6 +55,9 @@ update_rhs_swizzle(ir_swizzle_mask &m, unsigned from, unsigned to)
 void
 ir_assignment::set_lhs(ir_rvalue *lhs)
 {
+   void *mem_ctx = this;
+   bool swizzled = false;
+
    while (lhs != NULL) {
       ir_swizzle *swiz = lhs->as_swizzle();
 
@@ -82,7 +85,21 @@ ir_assignment::set_lhs(ir_rvalue *lhs)
       this->write_mask = write_mask;
       lhs = swiz->val;
 
-      this->rhs = new(this) ir_swizzle(this->rhs, rhs_swiz);
+      this->rhs = new(mem_ctx) ir_swizzle(this->rhs, rhs_swiz);
+      swizzled = true;
+   }
+
+   if (swizzled) {
+      /* Now, RHS channels line up with the LHS writemask.  Collapse it
+       * to just the channels that will be written.
+       */
+      ir_swizzle_mask rhs_swiz = { 0, 0, 0, 0, 0, 0 };
+      int rhs_chan = 0;
+      for (int i = 0; i < 4; i++) {
+	 if (write_mask & (1 << i))
+	    update_rhs_swizzle(rhs_swiz, i, rhs_chan++);
+      }
+      this->rhs = new(mem_ctx) ir_swizzle(this->rhs, rhs_swiz);
    }
 
    assert((lhs == NULL) || lhs->as_dereference());
@@ -122,6 +139,16 @@ ir_assignment::ir_assignment(ir_dereference *lhs, ir_rvalue *rhs,
    this->rhs = rhs;
    this->lhs = lhs;
    this->write_mask = write_mask;
+
+   if (lhs->type->is_scalar() || lhs->type->is_vector()) {
+      int lhs_components = 0;
+      for (int i = 0; i < 4; i++) {
+	 if (write_mask & (1 << i))
+	    lhs_components++;
+      }
+
+      assert(lhs_components == this->rhs->type->vector_elements);
+   }
 }
 
 ir_assignment::ir_assignment(ir_rvalue *lhs, ir_rvalue *rhs,