https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108360
--- Comment #4 from Andrew Macleod <amacleod at redhat dot com> ---
The IL is different in VRP2 between GCC12 and GCC13. IN GCC 12 I see:
<bb 2> [local count: 1073741824]:
b.2_1 = b;
_2 = b.2_1 <= 0;
h.0_20 = (unsigned short) _2;
_21 = h.0_20 + 65535;
_22 = (short int) _21;
_3 = _22 >= 0;
_4 = (char) _3;
f = _4;
f.5_5 = (unsigned char) _3;
_6 = f.5_5 << 4;
e = _6;
h_23 = (short int) _6;
if (_6 == 0)
goto <bb 5>; [50.00%]
else
goto <bb 3>; [50.00%]
<bb 3> [local count: 805306368]:
_8 = b.2_1 == h_23;
Global ranges for bb2 are:
_4 : char [0, 1]
f.5_5 : unsigned char [0, 1]
_6 : unsigned char [0, 0][16, 16]
h.0_20 : unsigned short [0, 1]
_21 : unsigned short [0, 0][+INF, +INF]
_22 : short int [-1, 0]
Looking at what ranger calculates for the edge 2->3 based on _6 having to be 0:
2->3 (F) b.2_1 : short int [-INF, 0]
2->3 (F) _2 : _Bool [1, 1]
2->3 (F) _3 : _Bool [1, 1]
2->3 (F) _4 : char [1, 1]
2->3 (F) f.5_5 : unsigned char [1, 1]
2->3 (F) _6 : unsigned char [16, 16]
2->3 (F) h.0_20 : unsigned short [1, 1]
2->3 (F) _21 : unsigned short [0, 0]
2->3 (F) _22 : short int [0, 0]
2->3 (F) h_23 : short int [16, 16]
It can determine that h_23 is [16,16] and therefore _8 is always false. and
when you use _8 = [0, 0] the condition leading to the call can never be true
so it's eliminated.
The code sequence at -Os is quite different coming into VRP2 in GCC13. It
involves a PHI node and the condition uses _21 == -1 instead of _6 == 0.
This cahnges what we can evaluate going out, and when we get to valuation of
_8, it sees:
_8 = b.2_1 == iftmp.8_28;
and iftmp.8_28 is evaluated as [0, 0][16, 16][512, 512]
Because 0 hasnt been eliminated, we cant fold the condition.
We do still get this at -O2... just not at -Os.
