On Tue, 11 Oct 2016, Bin Cheng wrote:
We missed folding (convert)(X op const) -> (convert)X op (convert)const for
unsigned narrowing because of reason reported at
https://gcc.gnu.org/ml/gcc/2016-07/msg00126.html
This patch fixes the issue by adding new match&simplify pattern, it also adds a
test case. This is the prerequisite patch for next patch adding new vectorization
pattern.
Some technical comments below. I am sure Jeff and/or Richi will have more
to say on the approach. I am a bit surprised to see it as adding a new
transformation, instead of moving an old one.
+/* (convert (X op C)) -> ((convert)X op (convert)C) if it is narrowing
+ conversion and both types wrap when overflow. */
+(for op (plus minus)
+ (simplify
We used to indent by a single space in this file, but I see that other
transforms have made it in with double spacing, so I guess it doesn't
matter.
+ (convert (op @0 @1))
+ (if (INTEGRAL_TYPE_P (type)
+ && TYPE_OVERFLOW_WRAPS (type)
+ && TREE_CODE (@1) == INTEGER_CST
You can write (convert (op @0 INTEGER_CST@1)) and skip this line.
+ && INTEGRAL_TYPE_P (TREE_TYPE (@0))
+ && TYPE_OVERFLOW_WRAPS (TREE_TYPE (@0))
This seems quite restrictive, TYPE_OVERFLOW_UNDEFINED should also be fine
for this type. I guess you are trying to avoid saturating / trapping
types?
+ && TYPE_PRECISION (type) < TYPE_PRECISION (TREE_TYPE (@0)))
+ (op (convert @0) (convert @1)))))
+
/* If we have a narrowing conversion to an integral type that is fed by a
BIT_AND_EXPR, we might be able to remove the BIT_AND_EXPR if it merely
masks off bits outside the final type (and nothing else). */
diff --git a/gcc/testsuite/gcc.dg/fold-narrow-unsgn-opcst.c
b/gcc/testsuite/gcc.dg/fold-narrow-unsgn-opcst.c
new file mode 100644
index 0000000..aff96a9
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/fold-narrow-unsgn-opcst.c
@@ -0,0 +1,8 @@
+/* { dg-do compile } */
+/* { dg-options "-O1 -fdump-tree-gimple" } */
+
+unsigned char foo (unsigned short s)
+{
+ return (unsigned char)(s + 65530);
+}
+/* { dg-final { scan-tree-dump-not " 65530" "gimple" } } */
As I understand it, C says that s is promoted to int and added to 65530,
but int is not TYPE_OVERFLOW_WRAPS so your transformation doesn't apply
(the test already passes without your patch). It is better to write tests
for the gimple version of transformations, i.e. don't write everything as
a single expression, use intermediate variables.
--
Marc Glisse
