https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82666
Jeffrey A. Law <law at redhat dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |law at redhat dot com
See Also| |https://gcc.gnu.org/bugzill
| |a/show_bug.cgi?id=80520
--- Comment #2 from Jeffrey A. Law <law at redhat dot com> ---
This is another case where PRE does the right thing on a local basis, but the
result isn't well handled later in the pipeline.
Prior to PRE the key blocks look like:
<bb3>
[ ... ]
if (_4 > 127)
goto <bb 4>; [50.00%]
else
goto <bb 5>; [50.00%]
<bb 4> [local count: 531502203]:
<bb 5> [local count: 1063004407]:
# _17 = PHI <_4(4), 0(3)>
iftmp.0_8 = (long long int) _17;
sum_13 = iftmp.0_8 + sum_19;
Not surprisingly PRE realizes it can reduce the total expressions evaluated on
the path 3->5 by moving the arithmetic into bb4:
<bb 3>
[ ... ]
if (_4 > 127)
goto <bb 4>; [50.00%]
else
goto <bb 5>; [50.00%]
<bb 4> [local count: 531502203]:
_22 = (long long int) _4;
_24 = sum_19 + _22;
<bb 5> [local count: 1063004407]:
# _17 = PHI <_4(4), 0(3)>
# prephitmp_25 = PHI <_24(4), sum_19(3)>
We keep that form through the rest of the gimple optimizers and into RTL where
it turns into the problematic cmov during the first if-conversion pass.
Compiling with -fno-tree-pre gives code that I believe is equivalent to
gcc-6.x.
