Re: [PATCH] Fix warn_if_not_align ICE (PR c/83844)

[Jakub Jelinek <jakub at redhat dot com>]

On Tue, Jan 16, 2018 at 08:57:38AM +0100, Richard Biener wrote:
> > -  unsigned HOST_WIDE_INT off
> > -    = (tree_to_uhwi (DECL_FIELD_OFFSET (field))
> > -       + tree_to_uhwi (DECL_FIELD_BIT_OFFSET (field)) / BITS_PER_UNIT);
> > -  if ((off % warn_if_not_align) != 0)
> > -    warning (opt_w, "%q+D offset %wu in %qT isn't aligned to %u",
> > +  tree off = byte_position (field);
> > +  if (!multiple_of_p (TREE_TYPE (off), off, size_int (warn_if_not_align)))
> 
> multiple_of_p also returns 0 if it doesn't know (for the non-constant
> case obviously), so the warning should say "may be not aligned"?  Or
> we don't want any false positives which means multiple_of_p should get
> a worker factored out that returns a tri-state value?

tri-state sounds optimizing for the very uncommon case, I think it must be
very rare in practice when we could prove it must be not aligned and
especially we'd need to extend it a lot to handle those cases.

Here is an updated patch which says may not be aligned if off is
non-constant.  When extending the testcase, I've noticed we don't handle
IMHO quite important case in multiple_of_p, so the patch handles that too.
I've tried not to increase asymptotic complexity of multiple_of_p, so except
for the cases where both arguments are INTEGER_CSTs it shouldn't call
multiple_of_p more times than before.

Ok for trunk if this passes bootstrap/regtest?

2018-01-16  Jakub Jelinek  <jakub@redhat.com>

	PR c/83844
	* stor-layout.c (handle_warn_if_not_align): Use byte_position and
	multiple_of_p instead of unchecked tree_to_uhwi and UHWI check.
	If off is not INTEGER_CST, issue a may not be aligned warning
	rather than isn't aligned.  Use isn%'t rather than isn't.
	* fold-const.c (multiple_of_p) <case BIT_AND_EXPR>: Don't fall through
	into MULT_EXPR.
	<case MULT_EXPR>: Improve the case when bottom and one of the
	MULT_EXPR operands are INTEGER_CSTs and bottom is multiple of that
	operand, in that case check if the other operand is multiple of
	bottom divided by the INTEGER_CST operand.

	* gcc.dg/pr83844.c: New test.

--- gcc/stor-layout.c.jj	2018-01-15 22:40:14.009263280 +0100
+++ gcc/stor-layout.c	2018-01-16 10:01:48.135111031 +0100
@@ -1150,12 +1150,16 @@ handle_warn_if_not_align (tree field, un
     warning (opt_w, "alignment %u of %qT is less than %u",
 	     record_align, context, warn_if_not_align);
 
-  unsigned HOST_WIDE_INT off
-    = (tree_to_uhwi (DECL_FIELD_OFFSET (field))
-       + tree_to_uhwi (DECL_FIELD_BIT_OFFSET (field)) / BITS_PER_UNIT);
-  if ((off % warn_if_not_align) != 0)
-    warning (opt_w, "%q+D offset %wu in %qT isn't aligned to %u",
-	     field, off, context, warn_if_not_align);
+  tree off = byte_position (field);
+  if (!multiple_of_p (TREE_TYPE (off), off, size_int (warn_if_not_align)))
+    {
+      if (TREE_CODE (off) == INTEGER_CST)
+	warning (opt_w, "%q+D offset %E in %qT isn%'t aligned to %u",
+		 field, off, context, warn_if_not_align);
+      else
+	warning (opt_w, "%q+D offset %E in %qT may not be aligned to %u",
+		 field, off, context, warn_if_not_align);
+    }
 }
 
 /* Called from place_field to handle unions.  */
--- gcc/fold-const.c.jj	2018-01-15 10:02:04.119181355 +0100
+++ gcc/fold-const.c	2018-01-16 10:48:10.444360796 +0100
@@ -12595,9 +12595,34 @@ multiple_of_p (tree type, const_tree top
 	 a multiple of BOTTOM then TOP is a multiple of BOTTOM.  */
       if (!integer_pow2p (bottom))
 	return 0;
-      /* FALLTHRU */
+      return (multiple_of_p (type, TREE_OPERAND (top, 1), bottom)
+	      || multiple_of_p (type, TREE_OPERAND (top, 0), bottom));
 
     case MULT_EXPR:
+      if (TREE_CODE (bottom) == INTEGER_CST)
+	{
+	  op1 = TREE_OPERAND (top, 0);
+	  op2 = TREE_OPERAND (top, 1);
+	  if (TREE_CODE (op1) == INTEGER_CST)
+	    std::swap (op1, op2);
+	  if (TREE_CODE (op2) == INTEGER_CST)
+	    {
+	      if (multiple_of_p (type, op2, bottom))
+		return 1;
+	      /* Handle multiple_of_p ((x * 2 + 2) * 4, 8).  */
+	      if (multiple_of_p (type, bottom, op2))
+		{
+		  widest_int w = wi::sdiv_trunc (wi::to_widest (bottom),
+						 wi::to_widest (op2));
+		  if (wi::fits_to_tree_p (w, TREE_TYPE (bottom)))
+		    {
+		      op2 = wide_int_to_tree (TREE_TYPE (bottom), w);
+		      return multiple_of_p (type, op1, op2);
+		    }
+		}
+	      return multiple_of_p (type, op1, bottom);
+	    }
+	}
       return (multiple_of_p (type, TREE_OPERAND (top, 1), bottom)
 	      || multiple_of_p (type, TREE_OPERAND (top, 0), bottom));
 
--- gcc/testsuite/gcc.dg/pr83844.c.jj	2018-01-16 09:56:57.459175232 +0100
+++ gcc/testsuite/gcc.dg/pr83844.c	2018-01-16 10:02:55.494096157 +0100
@@ -0,0 +1,36 @@
+/* PR c/83844 */
+/* { dg-do compile } */
+/* { dg-options "-O0 -Wall" } */
+
+typedef unsigned long long __u64 __attribute__((aligned(4),warn_if_not_aligned(8)));
+void bar (void *, void *, void *);
+
+void
+foo (int n)
+{
+  struct A
+  {
+    int i1;
+    int i2;
+    int i3[n];
+    __u64 x;	/* { dg-warning "in 'struct A' may not be aligned to 8" } */
+  } __attribute__((aligned (8)));
+  struct B
+  {
+    int i1;
+    int i2;
+    long long i3[n];
+    __u64 x;
+  } __attribute__((aligned (8)));
+  struct C
+  {
+    int i1;
+    int i2;
+    int i3[2 * n];
+    __u64 x;
+  } __attribute__((aligned (8)));
+  struct A a;
+  struct B b;
+  struct C c;
+  bar (&a, &b, &c);
+}


	Jakub