Hi,
As subject, for the testcase in the patch:
unsigned long
f2 (unsigned long a, int b)
{
unsigned long x = 1UL << b;
return a / x;
}
We currently generate:
f2:
mov x2, 1
lsl x1, x2, x1
udiv x0, x0, x1
ret
Which could instead be transformed to:
f2:
lsr x0, x0, x1
ret
OK?
Thanks,
James
---
gcc/
2017-06-12 James Greenhalgh <[email protected]>
* match.pd (A / (1 << B) -> A >> B): New.
gcc/testsuite/
2017-06-12 James Greenhalgh <[email protected]>
* gcc.dg/tree-ssa/forwprop-37.c: New.
diff --git a/gcc/match.pd b/gcc/match.pd
index 54a8e04..656ede2 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -147,6 +147,14 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
(op @0 integer_onep)
(non_lvalue @0)))
+/* For unsigned A: (A / (1 << B)), (A >> B).
+ We can't do the same for signed A, as it might be negative, which would
+ introduce undefined behaviour. */
+(simplify
+ (trunc_div @0 (lshift integer_onep@1 @2))
+ (if (TYPE_UNSIGNED (type))
+ (rshift @0 @2)))
+
/* Preserve explicit divisions by 0: the C++ front-end wants to detect
undefined behavior in constexpr evaluation, and assuming that the division
traps enables better optimizations than these anyway. */
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/forwprop-37.c b/gcc/testsuite/gcc.dg/tree-ssa/forwprop-37.c
new file mode 100644
index 0000000..dec826c
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/forwprop-37.c
@@ -0,0 +1,25 @@
+/* { dg-do compile } */
+/* { dg-options "-O -fdump-tree-forwprop1-raw" } */
+
+unsigned int
+f1 (unsigned int a, unsigned int b)
+{
+ unsigned int x = 1U << b;
+ return a / x;
+}
+
+unsigned long
+f2 (unsigned long a, int b)
+{
+ unsigned long x = 1UL << b;
+ return a / x;
+}
+
+unsigned long long
+f3 (unsigned long long a, int b)
+{
+ unsigned long long x = 1ULL << b;
+ return a / x;
+}
+
+/* { dg-final { scan-tree-dump-not "trunc_div_expr" "forwprop1" } } */
