https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103218
Bug ID: 103218
Summary: (a < 0) << signbit is not always optimized to a &
signbitmask at the gimple level
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Keywords: missed-optimization, TREE
Severity: enhancement
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: pinskia at gcc dot gnu.org
Target Milestone: ---
Take:
int f(signed char a)
{
signed char t = a < 0;
return (unsigned char)(t << 7);
}
At the gimple level we get:
int f(signed char a)
{
signed char t = a < 0;
return (unsigned char)(t << 7);
}
But combine is able to it:
Trying 9 -> 10:
9: {r89:QI=r91:SI#0 0>>0x7;clobber flags:CC;}
REG_DEAD r91:SI
REG_UNUSED flags:CC
10: {r90:QI=r89:QI<<0x7;clobber flags:CC;}
REG_DEAD r89:QI
REG_UNUSED flags:CC
Successfully matched this instruction:
(parallel [
(set (reg:QI 90)
(and:QI (subreg:QI (reg:SI 91) 0)
(const_int -128 [0xffffffffffffff80])))
(clobber (reg:CC 17 flags))
])
allowing combination of insns 9 and 10
original costs 4 + 4 = 8
replacement cost 4
deferring deletion of insn with uid = 9.
modifying insn i3 10: {r90:QI=r91:SI#0&0xffffffffffffff80;clobber flags:CC;}
REG_DEAD r91:SI
REG_UNUSED flags:CC
deferring rescan insn with uid = 10.
If we had wrote the testcase like:
int f(signed char a)
{
return (a < 0) << 7;
}
GCC does optimize it to:
(int) NON_LVALUE_EXPR <a> & 128
