glsl: replace 'x + (-x)' with constant 0

This fixes a hang in shadertoy for radeonsi where a buffer was initialized with:

   value -= value

with value being undefined.
In this case LLVM replace the operation with an assignment to NaN.

Cc: 19.1 19.2 <mesa-stable@lists.freedesktop.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111241
Reviewed-by: Marek Olšák <marek.olsak@amd.com>

diff --git a/src/compiler/glsl/opt_algebraic.cpp b/src/compiler/glsl/opt_algebraic.cpp
index ff4be269578c1f95152e36138594514131977b19..3147d25aea86a2a18bf566a630a2b9213ad86f42 100644 (file)
--- a/src/compiler/glsl/opt_algebraic.cpp
+++ b/src/compiler/glsl/opt_algebraic.cpp
@@ -507,6 +507,18 @@ ir_algebraic_visitor::handle_expression(ir_expression *ir)
       if (is_vec_zero(op_const[1]))
         return ir->operands[0];
 
+      /* Replace (x + (-x)) with constant 0 */
+      for (int i = 0; i < 2; i++) {
+         if (op_expr[i]) {
+            if (op_expr[i]->operation == ir_unop_neg) {
+               ir_rvalue *other = ir->operands[(i + 1) % 2];
+               if (other && op_expr[i]->operands[0]->equals(other)) {
+                  return ir_constant::zero(ir, ir->type);
+               }
+            }
+         }
+      }
+
       /* Reassociate addition of constants so that we can do constant
        * folding.
        */