[Bug bootstrap/61583] New: stage2 and stage3 compare failure due to value range loss

[amodra at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61583

            Bug ID: 61583
           Summary: stage2 and stage3 compare failure due to value range
                    loss
           Product: gcc
           Version: 4.9.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: bootstrap
          Assignee: unassigned at gcc dot gnu.org
          Reporter: amodra at gmail dot com

Seen first on ibm-4.9 branch with powerpc64, but also occurs on 4.9 branch with
both powerpc64 and x86_64.  x86_64 configured with --enable-threads=posix
--disable-shared --disable-libmudflap --disable-libssp --disable-lto
--disable-libgomp --without-ppl --without-cloog --disable-checking
--enable-languages=c,c++

results for revision 211867 x86_64 as follows, powerpc64 doesn't hit the
builtins.o fail.  Note that --disable-checking seems necessary.

Bootstrap comparison failure!
gcc/tree-vect-stmts.o differs
gcc/builtins.o differs
gcc/tree-vect-loop.o differs

Disassembling tree-vect-stmts.o shows loop exit branches differ for
      for (j = 0; j < ncopies; ++j)
in vectorizable_mask_load_store.


ivopts details dump differences (- no debug, + debug):

@@ -16220,10 +19173,11 @@

 Analyzing # of iterations of loop 1
   exit condition [1, + , 1](no_overflow) < ncopies_19
-  bounds on difference of bases: 0 ... 2147483646
+  bounds on difference of bases: -2147483649 ... 2147483646
   result:
+    zero if ncopies_19 <= 0
     # of iterations (unsigned int) ncopies_19 + 4294967295, bounded by
2147483646
-  number of iterations (unsigned int) ncopies_19 + 4294967295
+  number of iterations (unsigned int) ncopies_19 + 4294967295; zero if
ncopies_19 <= 0

 Induction variables:

@@ -16334,7 +19288,7 @@
   0    0       0       1
   1    0       0       1
   2    0       0       1
-  3    0       0       1
+  3    4       1       1

 Use 2:
   cand cost    compl.  depends on
@@ -16428,18 +19382,17 @@
   base 0
   step 1

-Replacing exit test: if (ncopies_19 > j_153)
 bool vectorizable_assignment(gimple, gimple_stmt_iterator*,
gimple_statement_base**, slp_tree) (struct gimple_statement_base * stmt
 {
[snip]

So there's why the exit test changes, with no debug output the exit test is
if (ncopies_19 != j_153), with debug info you keep the original test.

A gdb session shows that ncopies_19 has NULL SSA_NAME_RANGE_INFO during ivopts
with -g, but does have SSA_NAME_RANGE_INFO without -g, which agrees with the
dump output above.