[PATCH] tree-optimization/110838 - vectorization of widened shifts

Richard Biener rguenther@suse.de
Mon Jul 31 14:01:49 GMT 2023


The following makes sure to limit the shift operand when vectorizing
(short)((int)x >> 31) via (short)x >> 31 as the out of bounds shift
operand otherwise invokes undefined behavior.  When we determine
whether we can demote the operand we know we at most shift in the
sign bit so we can adjust the shift amount.

Note this has the possibility of un-CSEing common shift operands
as there's no good way to share pattern stmts between patterns.
We'd have to separately pattern recognize the definition.

Bootstrapped on x86_64-unknown-linux-gnu, testing in progress.

Not sure about LSHIFT_EXPR, it probably has the same issue but
the fallback optimistic zero for out-of-range shifts is at least
"corrrect".  Not sure we ever try to demote rotates (probably not).

OK?

Thanks,
Richard.

	PR tree-optimization/110838
	* tree-vect-patterns.cc (vect_recog_over_widening_pattern):
	Adjust the shift operand of RSHIFT_EXPRs.

	* gcc.dg/torture/pr110838.c: New testcase.
---
 gcc/testsuite/gcc.dg/torture/pr110838.c | 43 +++++++++++++++++++++++++
 gcc/tree-vect-patterns.cc               | 24 ++++++++++++++
 2 files changed, 67 insertions(+)
 create mode 100644 gcc/testsuite/gcc.dg/torture/pr110838.c

diff --git a/gcc/testsuite/gcc.dg/torture/pr110838.c b/gcc/testsuite/gcc.dg/torture/pr110838.c
new file mode 100644
index 00000000000..f039bd6c8ea
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/pr110838.c
@@ -0,0 +1,43 @@
+/* { dg-do run } */
+
+typedef __UINT32_TYPE__ uint32_t;
+typedef __UINT8_TYPE__ uint8_t;
+typedef __INT8_TYPE__ int8_t;
+typedef uint8_t pixel;
+
+/* get the sign of input variable (TODO: this is a dup, make common) */
+static inline int8_t signOf(int x)
+{
+  return (x >> 31) | ((int)((((uint32_t)-x)) >> 31));
+}
+
+__attribute__((noipa))
+static void calSign_bug(int8_t *dst, const pixel *src1, const pixel *src2, const int endX)
+{
+  for (int x = 0; x < endX; x++)
+    dst[x] = signOf(src1[x] - src2[x]);
+}
+
+__attribute__((noipa, optimize(0)))
+static void calSign_ok(int8_t *dst, const pixel *src1, const pixel *src2, const int endX)
+{
+  for (int x = 0; x < endX; x++)
+    dst[x] = signOf(src1[x] - src2[x]);
+}
+
+__attribute__((noipa, optimize(0)))
+int main()
+{
+  const pixel s1[9] = { 0xcd, 0x33, 0xd4, 0x3e, 0xb0, 0xfb, 0x95, 0x64, 0x70, };
+  const pixel s2[9] = { 0xba, 0x9f, 0xab, 0xa1, 0x3b, 0x29, 0xb1, 0xbd, 0x64, };
+  int endX = 9;
+  int8_t dst[9];
+  int8_t dst_ok[9];
+
+  calSign_bug(dst, s1, s2, endX);
+  calSign_ok(dst_ok, s1, s2, endX);
+
+  if (__builtin_memcmp(dst, dst_ok, endX) != 0)
+    __builtin_abort ();
+  return 0;
+}
diff --git a/gcc/tree-vect-patterns.cc b/gcc/tree-vect-patterns.cc
index ef806e2346e..e4ab8c2d65b 100644
--- a/gcc/tree-vect-patterns.cc
+++ b/gcc/tree-vect-patterns.cc
@@ -3099,9 +3099,33 @@ vect_recog_over_widening_pattern (vec_info *vinfo,
   tree ops[3] = {};
   for (unsigned int i = 1; i < first_op; ++i)
     ops[i - 1] = gimple_op (last_stmt, i);
+  /* For right shifts limit the shift operand.  */
   vect_convert_inputs (vinfo, last_stmt_info, nops, &ops[first_op - 1],
 		       op_type, &unprom[0], op_vectype);
 
+  /* Limit shift operands.  */
+  if (code == RSHIFT_EXPR)
+    {
+      wide_int min_value, max_value;
+      if (TREE_CODE (ops[1]) == INTEGER_CST)
+	ops[1] = wide_int_to_tree (op_type,
+				   wi::bit_and (wi::to_wide (ops[1]),
+						new_precision - 1));
+      else if (!vect_get_range_info (ops[1], &min_value, &max_value)
+	       || wi::ge_p (max_value, new_precision, TYPE_SIGN (op_type)))
+	{
+	  /* ???  Note the following bad for SLP as that only supports
+	     same argument widened shifts and it un-CSEs same arguments.  */
+	  tree new_var = vect_recog_temp_ssa_var (op_type, NULL);
+	  gimple *pattern_stmt
+	    = gimple_build_assign (new_var, BIT_AND_EXPR, ops[1],
+				   build_int_cst (op_type, new_precision - 1));
+	  ops[1] = new_var;
+	  gimple_set_location (pattern_stmt, gimple_location (last_stmt));
+	  append_pattern_def_seq (vinfo, last_stmt_info, pattern_stmt);
+	}
+    }
+
   /* Use the operation to produce a result of type OP_TYPE.  */
   tree new_var = vect_recog_temp_ssa_var (op_type, NULL);
   gimple *pattern_stmt = gimple_build_assign (new_var, code,
-- 
2.35.3


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