Date: Wed, 23 Sep 2009 22:39:45 +0200
From: Mikael Pettersson <mikpe at it dot uu dot se>
Subject: [PATCH] proposed fix for PR40987 if-conversion bug on HWI32 hosts
List-Archive: <http://gcc.gnu.org/ml/gcc-patches/>

This patch addresses PR rtl-optimization/40987, a wrong-code
error in ifcvt.c (noce_try_store_flag_constants) on 32-bit
HOST_WIDE_INT hosts.

noce_try_store_flag_constants() tries to optimize "if (test)
x = a; else x = b;" where a and b are constants. The problem
is that it performs various compile-time computations on
those CONST_INT values without first checking if they are
fully representable in HOST_WIDE_INT integers. When that is
not the case, it computes on truncated values resulting in
incorrect code being generated.

The problem is known to affect i386. It can be triggered with
gcc-4.5 by compiling with -O1. With gcc-4.4 and gcc-4.3 it also
requires a few -fno- options to disable optimizations that
appear to mask it. It also occurs with older gcc-4.x releases
with plain -O1, but gcc-3.4 appears immune to the problem.

This proposed fix compares GET_MODE_BITSIZE of the mode of the
destination variable with HOST_BITS_PER_WIDE_INT to see if
compile-time arithmetic may be inadequate, and in that case
performs an early failure return from the function.

Bootstrapped and regtested on {i686,x86_64}-linux.
4.4 and 4.3 backports (with adjusted dg-options in the test
case) bootstrapped and regtested on {i686,powerpc64}-linux.

Ok for 4.5?
(I don't have write access.)

gcc/

2009-09-23  Mikael Pettersson  <mikpe@it.uu.se>

	PR rtl-optimization/40987
	* ifcvt.c (noce_try_store_flag_constants): Skip it if
	the integer constants are too wide for host arithmetic.

gcc/testsuite/

2009-09-23  Mikael Pettersson  <mikpe@it.uu.se>

	PR rtl-optimization/40987
	* gcc.dg/torture/pr40987.c: New test.

--- gcc-4.6.1/gcc/ifcvt.c.~1~	2010-12-14 01:23:40.000000000 +0100
+++ gcc-4.6.1/gcc/ifcvt.c	2011-08-02 21:51:08.000000000 +0200
@@ -1075,6 +1075,9 @@ noce_try_store_flag_constants (struct no
       ifalse = INTVAL (if_info->a);
       itrue = INTVAL (if_info->b);
 
+      if (GET_MODE_BITSIZE (mode) > HOST_BITS_PER_WIDE_INT)
+	return FALSE;
+
       /* Make sure we can represent the difference between the two values.  */
       if ((itrue - ifalse > 0)
 	  != ((ifalse < 0) != (itrue < 0) ? ifalse < 0 : ifalse < itrue))
--- gcc-4.6.1/gcc/testsuite/gcc.dg/torture/pr40987.c.~1~	1970-01-01 01:00:00.000000000 +0100
+++ gcc-4.6.1/gcc/testsuite/gcc.dg/torture/pr40987.c	2011-08-02 21:51:08.000000000 +0200
@@ -0,0 +1,20 @@
+/* PR rtl-optimization/40987 */
+/* { do-do run } */
+/* { dg-options "-O1" } */
+
+long long __attribute__((noinline)) func(long arg)
+{
+    long long val = 0;
+    if (arg < 0)
+	val = 0xffffffff80000000ull;
+    return val;
+}
+
+extern void abort (void);
+int main (void)
+{
+    long long result = func(-1);
+    if (result != 0xffffffff80000000ull)
+	abort ();
+    return 0;
+}