https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70020
Bug ID: 70020
Summary: Forward propagation leaves compile-time computable
conditional in IL
Product: gcc
Version: 6.0
Status: UNCONFIRMED
Keywords: wrong-code
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: law at redhat dot com
CC: law at gcc dot gnu.org, law at redhat dot com, su at cs dot
ucdavis.edu
Depends on: 70005
Target Milestone: ---
+++ This bug was initially created as a clone of Bug #70005 +++
Given this code originally from BZ70005
-------------------------------------------------------
unsigned char a = 6;
int b, c;
static void
fn1 ()
{
int i = a > 1 ? 1 : a, j = 6 & (c = a && (b = a));
int d = 0, e = a, f = ~c, g = b || a;
unsigned char h = ~a;
if (a)
f = j;
if (h && g)
d = a;
i = -~(f * d * h) + c && (e || i) ^ f;
if (i != 1)
__builtin_abort ();
}
int
main ()
{
fn1 ();
return 0;
}
---
At -O2 on x86-64 we have the following in the .dse3 dump:
;; basic block 6, loop depth 0, count 0, freq 7300, maybe hot
;; prev block 5, next block 7, flags: (NEW, REACHABLE)
;; pred: 8 [100.0%] (FALLTHRU,EXECUTABLE)
;; 5 (TRUE_VALUE,EXECUTABLE)
# iftmp.2_35 = PHI <1(8), 0(5)>
# e_58 = PHI <_5(8), 0(5)>
# iftmp.0_67 = PHI <1(8), 0(5)>
_23 = iftmp.2_35 + 1;
if (_23 != 0)
goto <bb 7>;
else
goto <bb 10>;
Which forwprop turns into:
;; basic block 6, loop depth 0, count 0, freq 7300, maybe hot
;; prev block 5, next block 7, flags: (NEW, REACHABLE)
;; pred: 8 [100.0%] (FALLTHRU,EXECUTABLE)
;; 5 (TRUE_VALUE,EXECUTABLE)
# iftmp.2_35 = PHI <1(8), 0(5)>
# e_58 = PHI <_5(8), 0(5)>
# iftmp.0_67 = PHI <1(8), 0(5)>
if (iftmp.2_35 != -1)
goto <bb 7>;
else
goto <bb 10>;
The condition is obviously the compile-time constant true.
Walking backwards to vrp2 we have:
# iftmp.2_35 = PHI <1(10), 0(12)>
[ ... ]
;; basic block 8, loop depth 0, count 0, freq 7300, maybe hot
;; Invalid sum of incoming frequencies 4999, should be 7300
;; prev block 7, next block 9, flags: (NEW, REACHABLE)
;; pred: 7 [100.0%] (FALLTHRU,EXECUTABLE)
;; 6 [50.0%] (TRUE_VALUE,EXECUTABLE)
# d_17 = PHI <0(7), e_58(6)>
# iftmp.2_56 = PHI <iftmp.2_35(7), iftmp.2_35(6)>
# e_59 = PHI <e_58(7), e_58(6)>
# f_61 = PHI <0(7), 0(6)>
# h_65 = PHI <0(7), h_52(6)>
# iftmp.0_68 = PHI <iftmp.0_67(7), iftmp.0_67(6)>
_19 = d_17 * f_61;
_20 = (int) h_65;
_21 = _19 * _20;
_22 = _21 + 1;
_23 = _22 + iftmp.2_56;
if (_23 != 0)
goto <bb 9>;
else
goto <bb 13>;
Unfortunately VRP doesn't catch that f_61 is [0, 0]. Most likely because there
was another path through bb8 which was eliminated by jump threading (which is a
subroutine that runs after VRP is complete). phicopyprop comes along and
simplifies that mess into:
# iftmp.2_35 = PHI <1(10), 0(12)>
[ ... ]
;; basic block 8, loop depth 0, count 0, freq 7300, maybe hot
;; Invalid sum of incoming frequencies 4999, should be 7300
;; prev block 7, next block 9, flags: (NEW, REACHABLE)
;; pred: 7 [100.0%] (FALLTHRU,EXECUTABLE)
;; 6 [50.0%] (TRUE_VALUE,EXECUTABLE)
# d_17 = PHI <0(7), e_58(6)>
# h_65 = PHI <0(7), h_52(6)>
_20 = (int) h_65;
_23 = iftmp.2_35 + 1;
if (_23 != 0)
goto <bb 9>;
else
goto <bb 13>;
Which is probably the first place we might have a reasonable chance at catching
this. But forwprop is probably a better place because it's working with:
;; basic block 6, loop depth 0, count 0, freq 7300, maybe hot
;; prev block 5, next block 7, flags: (NEW, REACHABLE)
;; pred: 8 [100.0%] (FALLTHRU,EXECUTABLE)
;; 5 (TRUE_VALUE,EXECUTABLE)
# iftmp.2_35 = PHI <1(8), 0(5)>
# e_58 = PHI <_5(8), 0(5)>
# iftmp.0_67 = PHI <1(8), 0(5)>
if (iftmp.2_35 != -1)
goto <bb 7>;
else
goto <bb 10>;
Anyway this probably isn't terribly important, but I didn't want it to get
lost.
Referenced Bugs:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70005
[Bug 70005] [6 Regression] wrong code at -O2 and -O3 on x86_64-linux-gnu
