This patch adds a multiply-and-add instruction expression to
simple_rhs_p function. When debugging my sms patches, the following
situation was found. Imagine a loop, with constant step > 1, where "n"
and "fin" is unknown at compile time.
for (c = n * step + fin; c != fin; c -= step) ...;
c is a pseudo-register and is initialized as one multiply-and-add rtl
instruction before the loop. In this case, get_simple_loop_desc
function gives a non-null infinite condition which looks like:
(c - fin)%step == 0
It's obviously always true when you try to substitute the initialization
expression. But no substitution is done, because the exression is not
simple_rhs_p. So, this patch allows get_simple_loop_desc analysis to be
more precise. How can this influence other optimizations?
2011-07-20 Roman Zhuykov <zhr...@ispras.ru>
* loop-iv.c (simple_rhs_p): Support multiply-and-add operations.
---
gcc/loop-iv.c | 10 ++++++++--
1 files changed, 8 insertions(+), 2 deletions(-)
diff --git a/gcc/loop-iv.c b/gcc/loop-iv.c
index 83d2501..dbb7728 100644
--- a/gcc/loop-iv.c
+++ b/gcc/loop-iv.c
@@ -1340,9 +1340,15 @@ simple_rhs_p (rtx rhs)
case AND:
op0 = XEXP (rhs, 0);
op1 = XEXP (rhs, 1);
- /* Allow reg OP const and reg OP reg. */
+ /* Allow op0 to be reg or expression like "reg mult const".
+ * Allow op1 to be reg or const. */
if (!(REG_P (op0) && !HARD_REGISTER_P (op0))
- && !function_invariant_p (op0))
+ && !function_invariant_p (op0)
+ && !(((GET_CODE (op0) == ASHIFT)
+ || (GET_CODE (op0) == ASHIFTRT)
+ || (GET_CODE (op0) == LSHIFTRT)
+ || (GET_CODE (op0) == MULT)
+ ) && simple_rhs_p (op0)))
return false;
if (!(REG_P (op1) && !HARD_REGISTER_P (op1))
&& !function_invariant_p (op1))
--
Roman Zhuykov
zhr...@ispras.ru
