>Number: 154445
>Category: gnu
>Synopsis: Attempt to "fix" gcc -ftree-vrp
>Confidential: no
>Severity: non-critical
>Priority: low
>Responsible: freebsd-bugs
>State: open
>Quarter:
>Keywords:
>Date-Required:
>Class: sw-bug
>Submitter-Id: current-users
>Arrival-Date: Tue Feb 01 19:10:06 UTC 2011
>Closed-Date:
>Last-Modified:
>Originator: Pedro Giffuni
>Release: 8.1-Release
>Organization:
>Environment:
FreeBSD mogwai.giffuni.net 8.1-RELEASE FreeBSD 8.1-RELEASE #1: Sat Jul 17
14:19:59 PDT 2010
r...@build8x64.pcbsd.org:/usr/obj/usr/pcbsd-build81/fbsd-source/8.1/sys/PCBSD
amd64
>Description:
gcc's tree-vrp optimization was turned of by default on FreeBSD (svn 172419)
because it's known to generate bad code, specially on java ports, Looking at
the gcc 4.2 branch logs I found the following change that appears to fix it.
2007-10-10 Richard Guenther
PR tree-optimization/33099
PR tree-optimization/33381
* tree-vrp.c (adjust_range_with_scev): Do not adjust ranges
from pointer typed chrecs.
Unfortunately this change is GPL3 and the owner will not reconsider changing
the license. A workaround that seems to work in my system is undoing the last
patch in that file/function.
This is not the ideal fix but since the code is disabled by default anyways it
works well enough.
>How-To-Repeat:
Check gcc PR 330099 and build the testcase with -ftree-vrp
>Fix:
REVERSE (patch -R) the attached patch in /usr/src/contrib:
2007-06-04 Ian Lance Taylor
* tree-vrp.c (adjust_range_with_scev): When loop is not expected
to overflow, reduce overflow infinity to regular infinity.
Patch attached with submission follows:
--- gcc/tree-vrp.c 2007/05/30 22:52:02 125204
+++ gcc/tree-vrp.c 2007/06/04 21:58:42 125320
@@ -2558,6 +2558,13 @@
if (compare_values (min, max) == 1)
return;
}
+
+ /* According to the loop information, the variable does not
+ overflow. If we think it does, probably because of an
+ overflow due to arithmetic on a different INF value,
+ reset now. */
+ if (is_negative_overflow_infinity (min))
+ min = tmin;
}
else
{
@@ -2570,12 +2577,61 @@
if (compare_values (min, max) == 1)
return;
}
+
+ if (is_positive_overflow_infinity (max))
+ max = tmax;
}
set_value_range (vr, VR_RANGE, min, max, vr->equiv);
}
}
+/* Return true if VAR may overflow at STMT. This checks any available
+ loop information to see if we can determine that VAR does not
+ overflow. */
+
+static bool
+vrp_var_may_overflow (tree var, tree stmt)
+{
+ struct loop *l;
+ tree chrec, init, step;
+
+ if (current_loops == NULL)
+ return true;
+
+ l = loop_containing_stmt (stmt);
+ if (l == NULL)
+ return true;
+
+ chrec = instantiate_parameters (l, analyze_scalar_evolution (l, var));
+ if (TREE_CODE (chrec) != POLYNOMIAL_CHREC)
+ return true;
+
+ init = initial_condition_in_loop_num (chrec, l->num);
+ step = evolution_part_in_loop_num (chrec, l->num);
+
+ if (step == NULL_TREE
+ || !is_gimple_min_invariant (step)
+ || !valid_value_p (init))
+ return true;
+
+ /* If we get here, we know something useful about VAR based on the
+ loop information. If it wraps, it may overflow. */
+
+ if (scev_probably_wraps_p (init, step, stmt,
+ current_loops->parray[CHREC_VARIABLE (chrec)],
+ true))
+ return true;
+
+ if (dump_file && (dump_flags & TDF_DETAILS) != 0)
+ {
+ print_generic_expr (dump_file, var, 0);
+ fprintf (dump_file, ": loop information indicates does not overflow\n");
+ }
+
+ return false;
+}
+
/* Given two numeric value ranges VR0, VR1 and a comparison code COMP:
@@ -4786,7 +4842,8 @@
if (vrp_val_is_max (vr_result.max))
goto varying;
- if (!needs_overflow_infinity (TREE_TYPE (vr_result.min)))
+ if (!needs_overflow_infinity (TREE_TYPE (vr_result.min))
+ || !vrp_var_may_overflow (lhs, phi))
vr_result.min = TYPE_MIN_VALUE (TREE_TYPE (vr_result.min));
else if (supports_overflow_infinity (TREE_TYPE (vr_result.min)))
vr_result.min =
@@ -4804,7 +4861,8 @@
if (vrp_val_is_min (vr_result.min))
goto varying;
- if (!needs_overflow_infinity (TREE_TYPE (vr_result.max)))
+ if (!needs_overflow_infinity (TREE_TYPE (vr_result.max))
+ || !vrp_var_may_overflow (lhs, phi))
vr_result.max = TYPE_MAX_VALUE (TREE_TYPE (vr_result.max));
else if (supports_overflow_infinity (TREE_TYPE (vr_result.max)))
vr_result.max =
>Release-Note:
>Audit-Trail:
>Unformatted:
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