This is the mail archive of the
gcc-patches@gcc.gnu.org
mailing list for the GCC project.
Re: [PATCH] Fold integer divisions to arithmetic shifts
On 10/14/2010 08:32 PM, Jakub Jelinek wrote:
> Consider
> long long x = foo ();
> x = (x & -0x200000000LL) / 0x200000000LL;
> The above code would optimize this as
> x = x >> -1;
> on 32-bit HWI targets. Also, I think checking that arg1 is actually
> INTEGER_CST would be desirable too (integer_pow2p can return true
> even for COMPLEX_CST and then tree_int_cst_sgn accesses something the
> COMPLEX_CST doesn't have).
I've modified the patch similarly to
http://gcc.gnu.org/ml/gcc-patches/2010-10/msg01282.html, added
check for INTEGER_CST, and modified test to check for shift operation in
'optimized' dump.
Bootstrapped and regtested on x86_64 and i686.
Ok now?
--
Best regards,
Dmitry
2010-10-13 Dmitry Melnik <dm@ispras.ru>
gcc/
* fold-const.c (fold_binary_loc): New transformation.
gcc/testsuite/
* gcc.dg/20101013-1.c: New test.
diff --git a/gcc/fold-const.c b/gcc/fold-const.c
index 8146920..80fb248 100644
--- a/gcc/fold-const.c
+++ b/gcc/fold-const.c
@@ -11599,6 +11599,31 @@ fold_binary_loc (location_t loc,
return NULL_TREE;
case TRUNC_DIV_EXPR:
+ /* Optimize (X & (-A)) / A where A is a power of 2,
+ to X >> log2(A) */
+ if (TREE_CODE (arg0) == BIT_AND_EXPR
+ && !TYPE_UNSIGNED (type) && TREE_CODE (arg1) == INTEGER_CST
+ && integer_pow2p (arg1) && tree_int_cst_sgn (arg1) > 0)
+ {
+ tree sum = fold_binary_loc (loc, PLUS_EXPR, TREE_TYPE (arg1),
+ arg1, TREE_OPERAND (arg0, 1));
+ if (sum && integer_zerop (sum)) {
+ unsigned long pow2;
+
+ if (TREE_INT_CST_LOW (arg1))
+ pow2 = exact_log2 (TREE_INT_CST_LOW (arg1));
+ else
+ pow2 = exact_log2 (TREE_INT_CST_HIGH (arg1))
+ + HOST_BITS_PER_WIDE_INT;
+
+ return fold_build2_loc (loc, RSHIFT_EXPR, type,
+ TREE_OPERAND (arg0, 0),
+ build_int_cst (NULL_TREE, pow2));
+ }
+ }
+
+ /* Fall thru */
+
case FLOOR_DIV_EXPR:
/* Simplify A / (B << N) where A and B are positive and B is
a power of 2, to A >> (N + log2(B)). */
diff --git a/gcc/testsuite/gcc.dg/20101013-1.c b/gcc/testsuite/gcc.dg/20101013-1.c
new file mode 100644
index 0000000..b5bd3a7
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/20101013-1.c
@@ -0,0 +1,11 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-optimized" } */
+
+int foo(int a)
+{
+ int x = (a & (~15)) / 16;
+ return x;
+}
+
+/* { dg-final { scan-tree-dump ">>" "optimized" } } */
+/* { dg-final { cleanup-tree-dump "optimized" } } */