Support evaluating simple driver expressions without Python interpreter.

Recently @sergey found that hard-coding evaluation of certain very
common driver expressions without calling the Python interpreter
produces a 30-40% performance improvement. Since hard-coding is
obviously not suitable for production, I implemented a proper
parser and interpreter for simple arithmetic expressions in C.

The evaluator supports +, -, *, /, (), ==, !=, <, <=, >, >=,
and, or, not, ternary if; driver variables, frame, pi, True, False,
and a subset of standard math functions that seem most useful.

Booleans are represented as numbers, since within the supported
operation set it seems to be impossible to distinguish True/False
from 1.0/0.0. Boolean operations properly implement lazy evaluation
with jumps, and comparisons support chaining like 'a < b < c...'.

Expressions are parsed into a very simple stack machine program
that can then be safely evaluated in multiple threads.

Reviewers: sergey, campbellbarton

Differential Revision: https://developer.blender.org/D3698

1 files changed, 10 insertions, 5 deletions

diff --git a/source/blender/editors/space_graph/graph_buttons.c b/source/blender/editors/space_graph/graph_buttons.c
index 642e8e074fa..1dc57a0da7d 100644
--- a/source/blender/editors/space_graph/graph_buttons.c
+++ b/source/blender/editors/space_graph/graph_buttons.c
@@ -817,13 +817,18 @@ static void graph_draw_driver_settings_panel(uiLayout *layout, ID *id, FCurve *f
 		col = uiLayoutColumn(layout, true);
 		block = uiLayoutGetBlock(col);
 
-		if ((G.f & G_SCRIPT_AUTOEXEC) == 0) {
-			/* TODO: Add button to enable? */
-			uiItemL(col, IFACE_("WARNING: Python expressions limited for security"), ICON_ERROR);
-		}
-		else if (driver->flag & DRIVER_FLAG_INVALID) {
+		if (driver->flag & DRIVER_FLAG_INVALID) {
 			uiItemL(col, IFACE_("ERROR: Invalid Python expression"), ICON_CANCEL);
 		}
+		else if (!BKE_driver_has_simple_expression(driver)) {
+			if ((G.f & G_SCRIPT_AUTOEXEC) == 0) {
+				/* TODO: Add button to enable? */
+				uiItemL(col, IFACE_("WARNING: Python expressions limited for security"), ICON_ERROR);
+			}
+			else {
+				uiItemL(col, IFACE_("Slow Python expression"), ICON_INFO);
+			}
+		}
 
 		/* Explicit bpy-references are evil. Warn about these to prevent errors */
 		/* TODO: put these in a box? */