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[COMMITTED,range-op] Restrict division by power of 2 optimization to positive numbers.

Message ID 20221107114238.663927-1-aldyh@redhat.com
State New
Headers show
Series [COMMITTED,range-op] Restrict division by power of 2 optimization to positive numbers. | expand

Commit Message

Aldy Hernandez Nov. 7, 2022, 11:42 a.m. UTC 
The problem here is that we are transforming a division by a power of
2 into a right shift, and using this to shift the maybe nonzero bits.
This gives the wrong result when the number being divided is negative.

In the testcase we are dividing this by 8:

	[irange] int [-256, -255] NONZERO 0xffffff01

and coming up with:

	[irange] int [-32, -31] NONZERO 0xffffffe0

The maybe nonzero bits are wrong as -31 has the LSB set (0xffffffe1)
whereas the bitmask says the lower 4 bits are off.

	PR tree-optimization/107541

gcc/ChangeLog:

	* range-op.cc (operator_div::fold_range): Restrict power of 2
	optimization to positive numbers.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/pr107541.c: New test.
---
 gcc/range-op.cc                          |  4 +++-
 gcc/testsuite/gcc.dg/tree-ssa/pr107541.c | 16 ++++++++++++++++
 2 files changed, 19 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/pr107541.c
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Patch

diff --git a/gcc/range-op.cc b/gcc/range-op.cc
index 5e94c3d2282..2b5db0cac85 100644
--- a/gcc/range-op.cc
+++ b/gcc/range-op.cc
@@ -1953,7 +1953,9 @@  operator_div::fold_range (irange &r, tree type,
     return true;
 
   tree t;
-  if (rh.singleton_p (&t))
+  if (code == TRUNC_DIV_EXPR
+      && rh.singleton_p (&t)
+      && !wi::neg_p (lh.lower_bound ()))
     {
       wide_int wi = wi::to_wide (t);
       int shift = wi::exact_log2 (wi);
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/pr107541.c b/gcc/testsuite/gcc.dg/tree-ssa/pr107541.c
new file mode 100644
index 00000000000..475142186b5
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/pr107541.c
@@ -0,0 +1,16 @@ 
+// { dg-do run }
+// { dg-options "-O1" }
+
+unsigned char a = 1;
+char b, e;
+long c;
+short d;
+int main() {
+  a = ~(1 && a);
+  c = ~((~a / 8 | -2) & 11007578330939886389LLU);
+  e = -c;
+  d = ~c / e;
+  if (d < 2000)
+    __builtin_abort();
+  return 0;
+}