public inbox for gcc-bugs@sourceware.org
help / color / mirror / Atom feed
* [Bug tree-optimization/50729] New: Silent code gen fault: Value range propagation seems to propagate values across narrowing/widening
@ 2011-10-14 13:40 mgretton at sourceware dot org
  2011-10-14 13:41 ` [Bug tree-optimization/50729] " mgretton at sourceware dot org
                   ` (9 more replies)
  0 siblings, 10 replies; 11+ messages in thread
From: mgretton at sourceware dot org @ 2011-10-14 13:40 UTC (permalink / raw)
  To: gcc-bugs

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50729

             Bug #: 50729
           Summary: Silent code gen fault: Value range propagation seems
                    to propagate values across narrowing/widening
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: mgretton@sourceware.org
            Target: arm*-*-*


Created attachment 25497
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=25497
Executable test case.

The attached executable test rtest.c fails when compiled and executed as
follows:

$ arm-none-eabi-gcc rtest.c -O2 -o rtest.axf
$ ~ramrad01/public/qemu-arm rtest.axf 
1616
20019
FAIL!!

The test consists of two identical functions f0a and f0b which are compiled
with different optimization levels, and produce different results at -O0 and
-O2.

Investigations lead me to believe that VRP is at fault - initially because the
issue goes away if you compile -O2 -fdisable-tree-vrp1 -fdisable-tree-vrp2.

The attached cutdown testcase (rtest-cutdown.c) when compiled as follows
produces the attached tree dumps.

$ arm-none-eabi-gcc rtest-cutdown.c -O2 -o rtest.axf -fdump-tree-all

Looking at the difference between the two attached trees indicates that before
VRP1 the tree looked something like:

  int8_t temp_1;
  uint64_t D.6414;
  short unsigned int D.6416;
  temp_1_27 = (int8_t) D.6414_26;
  D.6416_33 = (short unsigned int) temp_1_27;

[note short unsigned int is 16-bits]

VRP1's analysis finds the following:

D.6414_26: [0, 4611686018427388031]
temp_1_27: VARYING

After VRP1 this bit of tree is transformed into:

  int8_t temp_1;
  uint64_t D.6414;
  short unsigned int D.6416;
  temp_1_27 = (int8_t) D.6414_26;
  D.6416_33 = (short unsigned int) D.6414_26;

That is D.6416_33 is now assigned directly from D.6414_26 instead of going
through temp_1_27.

[Similar replacements of temp_1_27 by D.6414_26 occur throughout the example
but this particular case serves its purpose].

This is incorrect as it stops the 64-bit value D6414_26 being narrowed to an
8-bit signed value before being widened to a 16-bit, now it just gets narrowed
from 64-bits to 16-bits.


^ permalink raw reply	[flat|nested] 11+ messages in thread
end of thread, other threads:[~2011-12-06 12:05 UTC | newest]

Thread overview: 11+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2011-10-14 13:40 [Bug tree-optimization/50729] New: Silent code gen fault: Value range propagation seems to propagate values across narrowing/widening mgretton at sourceware dot org
2011-10-14 13:41 ` [Bug tree-optimization/50729] " mgretton at sourceware dot org
2011-10-14 13:49 ` mgretton at sourceware dot org
2011-10-14 13:50 ` mgretton at sourceware dot org
2011-10-14 14:04 ` [Bug tree-optimization/50729] [4.7 Regression] " rguenth at gcc dot gnu.org
2011-10-17 12:24 ` rguenth at gcc dot gnu.org
2011-10-17 12:57 ` rguenth at gcc dot gnu.org
2011-11-18 23:24 ` amylaar at gcc dot gnu.org
2011-11-19  2:05 ` amylaar at gcc dot gnu.org
2011-11-19  4:25 ` amylaar at gcc dot gnu.org
2011-12-06 12:05 ` rguenth at gcc dot gnu.org

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).