The testcase gcc.dg/tree-ssa/20030922-2.c FAILs on m68k (and possibly
elsewhere), not on x86_64 though.
So on x86_64 ifcombine has "removed" the if but after dom1 we still have
_14 = *_13;
if (_8 != _14)
goto <bb 3>;
else
goto <bb 5>;
<bb 3>:
target_bb.1_15 = target_bb;
_16 = _8 != target_bb.1_15;
_17 = _14 == target_bb.1_15;
_18 = _16 & _17;
if (_18 != 0)
which shows that running ifcombine before DOM might not be the best idea.
It also shows that the testcase should disable ifcombine to really test
what it was supposed to test. And then it also fails on x86_64.
VRP does optimize this case because it has a more powerful way to track
equivalences (and does a quadratic job in trying to simplify compares
using
all equivalences of the lhs vs all equivalences of the rhs).
Which is another hint at the fact that DOM maybe shouldn't treat
equivalences
derived from conditionals in the SSA_VALUE table where you can't really
record
both _14 = target_bb.1_15 and target_bb.1_15 = _14 (which would get you a
nice cycle there).
For the existing way of DOM handlign this I don't see anything in the
testcase
that could be used to build a new heuristic (in fact another heuristic
would say that what we do now is good -- _14 dies at the point where we
build the equivalency, target_bb.1_15 does not, so we don't want to
propagate
_14 because that will increase its lifetime!)
So the following "fixes" the testcase to also FAIL on x86_64 and
XFAILs it.
Tested on x86_64, applied.
Richard.
2015-04-29 Richard Biener <rguent...@suse.de>
PR tree-optimization/65917
* gcc.dg/tree-ssa/20030922-2.c: Disable ifcombine and XFAIL.
Index: gcc/testsuite/gcc.dg/tree-ssa/20030922-2.c
===================================================================
--- gcc/testsuite/gcc.dg/tree-ssa/20030922-2.c (revision 222560)
+++ gcc/testsuite/gcc.dg/tree-ssa/20030922-2.c (working copy)
@@ -1,5 +1,5 @@
/* { dg-do compile } */
-/* { dg-options "-O1 -fdump-tree-dom1" } */
+/* { dg-options "-O1 -fdump-tree-dom1 -fdisable-tree-ifcombine" } */
struct rtx_def;
typedef struct rtx_def *rtx;
@@ -20,5 +20,9 @@ rgn_rank (rtx insn1, rtx insn2)
}
/* There should be two IF conditionals. */
-/* { dg-final { scan-tree-dump-times "if " 2 "dom1" } } */
+/* This now fails as it requires a very specific decision of DOM which
+ SSA name to record as a copy of the other when DOM derives copies
+ from temporary equivalences. The heuristics there no longer do
+ the correct thing. VRP still optimizes this testcase. */
+/* { dg-final { scan-tree-dump-times "if " 2 "dom1" { xfail *-*-* } } } */
/* { dg-final { cleanup-tree-dump "dom1" } } */
