Simplify window management for the transformation engine

Generic instructions have a min_window field. Its purpose is to
avoid calling transform_engine() when there are too few instructions
in the current "transformation window" for a transformation to
succeed.

Currently it does not do much good since the window size will be
decremented by one before being used. The reason for the subtraction
is probably that in some circumstances in the past, the loader could
read past the end of the BEAM module while attempting to fetch
instructions to increase the window size. Therefore, it would not
be safe to just remove the subtraction by one.

The simplest and safest solution seems to always ensure that there
are always at least TWO instructions when calling transform_engine().
That will be safe, as long as a BEAM module is always finished with
an int_code_end/0 that is not involved in any transformation.

1 files changed, 8 insertions, 8 deletions

diff --git a/erts/emulator/beam/beam_load.c b/erts/emulator/beam/beam_load.c
index c6c35e74c9..5d03c98657 100644
--- a/erts/emulator/beam/beam_load.c
+++ b/erts/emulator/beam/beam_load.c
@@ -2030,14 +2030,14 @@ load_code(LoaderState* stp)
     do_transform:
 	ASSERT(stp->genop != NULL);
 	if (gen_opc[stp->genop->op].transform != -1) {
-	    int need;
-	    tmp_op = stp->genop;
-
-	    for (need = gen_opc[stp->genop->op].min_window-1; need > 0; need--) {
-		if (tmp_op == NULL) {
-		    goto get_next_instr;
-		}
-		tmp_op = tmp_op->next;
+	    if (stp->genop->next == NULL) {
+		/*
+		 * Simple heuristic: Most transformations requires
+		 * at least two instructions, so make sure that
+		 * there are. That will reduce the number of
+		 * TE_SHORT_WINDOWs.
+		 */
+		goto get_next_instr;
 	    }
 	    switch (transform_engine(stp)) {
 	    case TE_FAIL: