[Mesa-dev] [PATCH] glsl: Annotate as_foo functions that the this pointer cannot be NULL

[Ian Romanick]

From: Ian Romanick <ian.d.romanick at intel.com>

We use the idoim

   ir_foo *x = y->as_foo();
   if (x == NULL)
      return;

all over the place.  GCC generates some quite lovely code for this.
One such example:

  340a5b:       83 7d 18 04             cmpl   $0x4,0x18(%rbp)
  340a5f:       0f 85 06 04 00 00       jne    340e6b
  340a65:       48 85 ed                test   %rbp,%rbp
  340a68:       0f 84 fd 03 00 00       je     340e6b

This case used as_expression() (ir_type_expression is 4).  Note that it
checkes the ir_type, then checks that the pointer isn't NULL.  There is
some disconnect in GCC around the condition in the as_foo functions.

      return ir_type == ir_type_##TYPE ? (ir_##TYPE *) this : NULL; \

It believes "this" could be NULL, so it emits check outside the function
just for fun.

This patch uses unreachable() to tell GCC that it need not bother with
extra NULL checking of pointer returned by the as_foo functions.

   text	   data	    bss	    dec	    hex	filename
4836430	 158688	  26248	5021366	 4c9eb6	i965_dri-before.so
4836173	 158688	  26248	5021109	 4c9db5	i965_dri-after.so

Signed-off-by: Ian Romanick <ian.d.romanick at intel.com>
---
 src/glsl/ir.h | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/src/glsl/ir.h b/src/glsl/ir.h
index 25f2eca..89887ba 100644
--- a/src/glsl/ir.h
+++ b/src/glsl/ir.h
@@ -119,6 +119,9 @@ public:
    /*@{*/
    class ir_rvalue *as_rvalue()
    {
+      if (this == NULL)
+         unreachable("this cannot be NULL");
+
       if (ir_type == ir_type_dereference_array ||
           ir_type == ir_type_dereference_record ||
           ir_type == ir_type_dereference_variable ||
@@ -132,6 +135,9 @@ public:
 
    class ir_dereference *as_dereference()
    {
+      if (this == NULL)
+         unreachable("this cannot be NULL");
+
       if (ir_type == ir_type_dereference_array ||
           ir_type == ir_type_dereference_record ||
           ir_type == ir_type_dereference_variable)
@@ -141,6 +147,9 @@ public:
 
    class ir_jump *as_jump()
    {
+      if (this == NULL)
+         unreachable("this cannot be NULL");
+
       if (ir_type == ir_type_loop_jump ||
           ir_type == ir_type_return ||
           ir_type == ir_type_discard)
@@ -151,6 +160,7 @@ public:
    #define AS_CHILD(TYPE) \
    class ir_##TYPE * as_##TYPE() \
    { \
+      if (this == NULL) unreachable("this cannot be NULL");         \
       return ir_type == ir_type_##TYPE ? (ir_##TYPE *) this : NULL; \
    }
    AS_CHILD(variable)
-- 
2.1.0