[Bug middle-end/95669] -O3 generates more complicated code to return 8-byte struct of zeros, sometimes
rguenth at gcc dot gnu.org
gcc-bugzilla@gcc.gnu.org
Mon Jun 15 06:55:15 GMT 2020
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95669
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Component|c++ |middle-end
Target| |x86_64-*-*
Last reconfirmed| |2020-06-15
Ever confirmed|0 |1
Status|UNCONFIRMED |NEW
Keywords| |missed-optimization
--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
with 'dummy' and the implicit zero initialization of it we retain
<bb 2> [local count: 1073741824]:
if (a_3(D) < b_4(D))
goto <bb 3>; [50.00%]
else
goto <bb 4>; [50.00%]
<bb 3> [local count: 536870913]:
D.2350 = {};
goto <bb 5>; [100.00%]
<bb 4> [local count: 536870913]:
_1 = a_3(D) * b_4(D);
D.2350.val = _1;
MEM <unsigned int> [(void *)&D.2350 + 4B] = 1;
<bb 5> [local count: 1073741824]:
return D.2350;
wich generates straigt-forward code while with 'dummy' elided we manage
to completely scalarize things and do
<bb 2> [local count: 1073741824]:
if (a_3(D) < b_4(D))
goto <bb 4>; [50.00%]
else
goto <bb 3>; [50.00%]
<bb 3> [local count: 536870913]:
_1 = a_3(D) * b_4(D);
<bb 4> [local count: 1073741824]:
# cstore_11 = PHI <_1(3), 0(2)>
# cstore_10 = PHI <1(3), 0(2)>
D.2349.ok = cstore_10;
D.2349.val = cstore_11;
return D.2349;
which is basically two conditional moves we expand via strange bit
shufflings because D.2349 (struct res) is assigned a register.
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