https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86882
--- Comment #2 from Uroš Bizjak <ubizjak at gmail dot com> ---
Also suspicious combine transformation before the failure:
Trying 10, 11 -> 13:
10: r94:SI=zero_extend(r92:QI)
11: {r95:SI=r94:SI^0x101;clobber flags:CC;}
REG_DEAD r94:SI
REG_UNUSED flags:CC
13: {r97:HI=r95:SI#0^r92:QI#0;clobber flags:CC;}
REG_DEAD r95:SI
REG_DEAD r92:QI
REG_UNUSED flags:CC
Failed to match this instruction:
(parallel [
(set (reg:HI 97)
(xor:HI (and:HI (subreg:HI (reg:QI 92 [ <retval> ]) 0)
(const_int -256 [0xffffffffffffff00]))
(const_int 257 [0x101])))
(clobber (reg:CC 17 flags))
])
Failed to match this instruction:
(set (reg:HI 97)
(xor:HI (and:HI (subreg:HI (reg:QI 92 [ <retval> ]) 0)
(const_int -256 [0xffffffffffffff00]))
(const_int 257 [0x101])))
Successfully matched this instruction:
(set (reg:HI 95 [ jd ])
(and:HI (subreg:HI (reg:QI 92 [ <retval> ]) 0)
(const_int -256 [0xffffffffffffff00])))
Successfully matched this instruction:
(set (reg:HI 97)
(xor:HI (reg:HI 95 [ jd ])
(const_int 257 [0x101])))
allowing combination of insns 10, 11 and 13
original costs 4 + 4 + 4 = 12
replacement costs 6 + 4 = 10
deferring deletion of insn with uid = 10.
modifying insn i2 11: {r95:HI=r92:QI#0&0xffffffffffffff00;clobber flags:CC;}
REG_UNUSED flags:CC
REG_DEAD r92:QI
deferring rescan insn with uid = 11.
modifying insn i3 13: {r97:HI=r95:HI^0x101;clobber flags:CC;}
REG_UNUSED flags:CC
REG_DEAD r95:HI
deferring rescan insn with uid = 13.
The problematic insn:
(set (reg:HI 95 [ jd ])
(and:HI (subreg:HI (reg:QI 92 [ <retval> ]) 0)
(const_int -256 [0xffffffffffffff00])))
clears defined bits from the subreg and keeps undefined bits from the
paradoxical subreg.
