[PATCH] ipa: fix bit CPP when combined with IPA bit CP

Martin Liška mliska@suse.cz
Wed Aug 12 11:35:55 GMT 2020


Hello.

First, I must confess I'm not familiar with bit constant propagation.
However, I learnt something during debugging session related to this PR.
  
As mentioned in the PR, let's consider the following example:

int
__attribute__((noinline))
foo(int arg)
{
   if (arg == 3)
     return 1;
   if (arg == 4)
     return 123;

   __builtin_unreachable ();
}

during WPA we find all calls of the function
(yes the call with value 5 is UBSAN):

   Node: foo/0:
     param [0]: 5 [loc_time: 4, loc_size: 2, prop_time: 0, prop_size: 0]
                3 [loc_time: 3, loc_size: 3, prop_time: 0, prop_size: 0]
          ctxs: VARIABLE
          Bits: value = 0x5, mask = 0x6

in LTRANS we have the following VRP info:

   # RANGE [3, 3] NONZERO 3

when we AND masks in get_default_value we end up with 6 & 3 = 2 (0x010).
That means the only second (least significant bit) is unknown and
value (5 = 0x101) & ~mask gives us either 7 (0x111) or 5 (0x101).

That's why if (arg_2(D) == 3) gets optimized to false.

Patch can bootstrap on x86_64-linux-gnu and survives regression tests.

Thoughts?
Martin

gcc/ChangeLog:

	PR ipa/96482
	* tree-ssa-ccp.c (get_default_value): Known bits from IPA bits
	CP should be masked against nonzero bits get from VRP.

gcc/testsuite/ChangeLog:

	PR ipa/96482
	* gcc.dg/ipa/pr96482.c: New test.
---
  gcc/testsuite/gcc.dg/ipa/pr96482.c | 44 ++++++++++++++++++++++++++++++
  gcc/tree-ssa-ccp.c                 | 14 ++++++----
  2 files changed, 53 insertions(+), 5 deletions(-)
  create mode 100644 gcc/testsuite/gcc.dg/ipa/pr96482.c

diff --git a/gcc/testsuite/gcc.dg/ipa/pr96482.c b/gcc/testsuite/gcc.dg/ipa/pr96482.c
new file mode 100644
index 00000000000..68ead798d28
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/ipa/pr96482.c
@@ -0,0 +1,44 @@
+/* PR ipa/96482 */
+/* { dg-do run } */
+/* { dg-options "-O2 -flto"  } */
+/* { dg-require-effective-target lto } */
+
+int
+__attribute__((noinline))
+foo(int arg)
+{
+  if (arg == 3)
+    return 1;
+  if (arg == 4)
+    return 123;
+
+  __builtin_unreachable ();
+}
+
+int
+__attribute__((noinline))
+baz(int x)
+{
+  if (x != 0)
+    return foo(3); /* called */
+
+  return 1;
+}
+
+int
+__attribute__((noinline))
+bar(int x)
+{
+  if (x == 0)
+    return foo(5); /* not executed */
+
+  return 1;
+}
+
+int main(int argc, char **argv)
+{
+  if (bar(argc) != baz(argc))
+    __builtin_abort ();
+
+  return 0;
+}
diff --git a/gcc/tree-ssa-ccp.c b/gcc/tree-ssa-ccp.c
index 7e3921869b8..d8ba5379c4a 100644
--- a/gcc/tree-ssa-ccp.c
+++ b/gcc/tree-ssa-ccp.c
@@ -299,24 +299,28 @@ get_default_value (tree var)
  	      wide_int nonzero_bits = get_nonzero_bits (var);
  	      tree value;
  	      widest_int mask;
+	      tree type = TREE_TYPE (var);
  
  	      if (SSA_NAME_VAR (var)
  		  && TREE_CODE (SSA_NAME_VAR (var)) == PARM_DECL
  		  && ipcp_get_parm_bits (SSA_NAME_VAR (var), &value, &mask))
  		{
  		  val.lattice_val = CONSTANT;
-		  val.value = value;
+		  /* Known bits from IPA CP should be masked
+		     with nonzero_bits.  */
+		  wide_int ipa_value = wi::to_wide (value);
+		  ipa_value &= nonzero_bits;
+		  val.value = wide_int_to_tree (type, ipa_value);
+
  		  val.mask = mask;
  		  if (nonzero_bits != -1)
-		    val.mask &= extend_mask (nonzero_bits,
-					     TYPE_SIGN (TREE_TYPE (var)));
+		    val.mask &= extend_mask (nonzero_bits, TYPE_SIGN (type));
  		}
  	      else if (nonzero_bits != -1)
  		{
  		  val.lattice_val = CONSTANT;
  		  val.value = build_zero_cst (TREE_TYPE (var));
-		  val.mask = extend_mask (nonzero_bits,
-					  TYPE_SIGN (TREE_TYPE (var)));
+		  val.mask = extend_mask (nonzero_bits, TYPE_SIGN (type));
  		}
  	    }
  	}
-- 
2.28.0



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