[backport gcc-4.8/trunk r194800, also fixes parts of PR54120 <mikpe@it.uu.se>]

From: Richard Sandiford <rdsandiford at googlemail dot com>
Subject: RFA: Fix uint128_t range checking in VRP
Date: Sun, 23 Dec 2012 16:48:59 +0000
List-Archive: <http://gcc.gnu.org/ml/gcc-patches/>

The minimum uint128_t value is an all-zeros double_int and the maximum
value is an all-ones double_int.  Truncating these values to int and
then sign-extending them gives the same all-zeros and all-ones values,
so tree-vrp.c:range_fits_type_p concludes that uint128_t fits in an int.

This showed up as a miscompilation of __fractutasf on mips64-linux-gnu,
which in turn showed up in convert-float-4.c.

There was already code to handle this problem when changing the sign
and keeping the precision, but I think the rule applies regardless
of precision.

Tested on x86_64-linux-gnu and mips64-linux-gnu.  OK to install?

Richard

gcc/

2013-01-02  Richard Sandiford  <rdsandiford@googlemail.com>

	* tree-vrp.c (range_fits_type_p): Require the MSB of the double_int
	to be clear for sign changes.

gcc/testsuite/

2013-01-02  Richard Sandiford  <rdsandiford@googlemail.com>

	* gcc.dg/torture/fp-int-convert-2.c: New test.

--- gcc-4.7.2/gcc/testsuite/gcc.dg/torture/fp-int-convert-2.c.~1~	1970-01-01 01:00:00.000000000 +0100
+++ gcc-4.7.2/gcc/testsuite/gcc.dg/torture/fp-int-convert-2.c	2013-02-02 15:03:54.079100877 +0100
@@ -0,0 +1,18 @@
+/* { dg-do run } */
+/* { dg-require-effective-target int128 } */
+
+extern void abort (void);
+
+float __attribute__((noinline))
+f (__uint128_t x)
+{
+  return x + 1;
+}
+
+int
+main (void)
+{
+  if (f (0xffffffffu) == 0)
+    abort ();
+  return 0;
+}
--- gcc-4.7.2/gcc/tree-vrp.c.~1~	2012-09-07 15:04:38.000000000 +0200
+++ gcc-4.7.2/gcc/tree-vrp.c	2013-02-02 15:03:54.079100877 +0100
@@ -7378,9 +7378,11 @@ range_fits_type_p (value_range_t *vr, un
       || TREE_CODE (vr->max) != INTEGER_CST)
     return false;
 
-  /* For precision-preserving sign-changes the MSB of the double-int
-     has to be clear.  */
-  if (src_precision == precision
+  /* For sign changes, the MSB of the double_int has to be clear.
+     An unsigned value with its MSB set cannot be represented by
+     a signed double_int, while a negative value cannot be represented
+     by an unsigned double_int.  */
+  if (TYPE_UNSIGNED (src_type) != unsigned_p
       && (TREE_INT_CST_HIGH (vr->min) | TREE_INT_CST_HIGH (vr->max)) < 0)
     return false;