https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83393
Bug ID: 83393
Summary: [8 Regression] wrong code with -O2
-fno-forward-propagate -fno-tree-bit-ccp
Product: gcc
Version: 8.0
Status: UNCONFIRMED
Keywords: wrong-code
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: zsojka at seznam dot cz
Target Milestone: ---
Host: x86_64-pc-linux-gnu
Target: x86_64-pc-linux-gnu
Created attachment 42848
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=42848&action=edit
reduced testcase
Output:
$ x86_64-pc-linux-gnu-gcc -O2 -fno-forward-propagate -fno-tree-bit-ccp
testcase.c
$ ./a.out
Aborted
$ x86_64-pc-linux-gnu-gcc -v
Using built-in specs.
COLLECT_GCC=/repo/gcc-trunk/binary-latest-amd64/bin/x86_64-pc-linux-gnu-gcc
COLLECT_LTO_WRAPPER=/repo/gcc-trunk/binary-trunk-255576-checking-yes-rtl-df-extra-nographite-amd64/bin/../libexec/gcc/x86_64-pc-linux-gnu/8.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /repo/gcc-trunk//configure --enable-languages=c,c++
--enable-valgrind-annotations --disable-nls --enable-checking=yes,rtl,df,extra
--without-cloog --without-ppl --without-isl --build=x86_64-pc-linux-gnu
--host=x86_64-pc-linux-gnu --target=x86_64-pc-linux-gnu
--with-ld=/usr/bin/x86_64-pc-linux-gnu-ld
--with-as=/usr/bin/x86_64-pc-linux-gnu-as --disable-libstdcxx-pch
--prefix=/repo/gcc-trunk//binary-trunk-255576-checking-yes-rtl-df-extra-nographite-amd64
Thread model: posix
gcc version 8.0.0 20171212 (experimental) (GCC)
Tested revisions:
trunk r255576 - FAIL
trunk r255386 - FAIL
.combine shows:
insn_cost 4 for 8: r111:SI=[`d']
insn_cost 4 for 9: r112:HI=r111:SI#0
REG_DEAD r111:SI
insn_cost 4 for 11: {r114:HI=r112:HI<<0xf;clobber flags:CC;}
REG_DEAD r112:HI
REG_UNUSED flags:CC
insn_cost 6 for 14: {r108:HI=r114:HI-0x23;clobber flags:CC;}
REG_DEAD r114:HI
REG_UNUSED flags:CC
insn_cost 4 for 15: r115:SI=zero_extend(r108:HI)
insn_cost 4 for 16: flags:CC=cmp(r108:HI,0x23)
REG_DEAD r108:HI
insn_cost 4 for 17: r117:QI=leu(flags:CC,0)
REG_DEAD flags:CC
insn_cost 4 for 18: r116:SI=zero_extend(r117:QI)
REG_DEAD r117:QI
insn_cost 4 for 19: {r93:SI=r115:SI<<r116:SI#0;clobber flags:CC;}
REG_DEAD r116:SI
REG_DEAD r115:SI
REG_UNUSED flags:CC
insn_cost 4 for 20: r108:HI=r93:SI#0
Trying 8 -> 9:
...
deferring deletion of insn with uid = 8.
modifying insn i3 9: r112:HI=[`d']
deferring rescan insn with uid = 9.
Trying 19 -> 20:
...
deferring deletion of insn with uid = 19.
modifying insn i3 20: {r108:HI=r115:SI#0<<r116:SI#0;clobber flags:CC;}
REG_UNUSED flags:CC
REG_DEAD r115:SI
REG_DEAD r116:SI
deferring rescan insn with uid = 20.
Trying 15 -> 20:
15: r115:SI=zero_extend(r108:HI)
20: {r108:HI=r115:SI#0<<r116:SI#0;clobber flags:CC;}
REG_UNUSED flags:CC
REG_DEAD r115:SI
REG_DEAD r116:SI
Successfully matched this instruction:
(parallel [
(set (reg/v:HI 108 [ fD.1899 ])
(ashift:HI (reg/v:HI 108 [ fD.1899 ])
(subreg:QI (reg:SI 116) 0)))
(clobber (reg:CC 17 flags))
])
allowing combination of insns 15 and 20
original costs 4 + 4 = 8
replacement cost 4
deferring deletion of insn with uid = 15.
modifying insn i3 20: {r108:HI=r108:HI<<r116:SI#0;clobber flags:CC;}
REG_DEAD r116:SI
REG_UNUSED flags:CC
deferring rescan insn with uid = 20.
Trying 14, 16 -> 18:
14: {r108:HI=r114:HI-0x23;clobber flags:CC;}
REG_DEAD r114:HI
REG_UNUSED flags:CC
16: flags:CC=cmp(r108:HI,0x23)
REG_DEAD r108:HI
18: r116:SI=leu(flags:CC,0)
REG_DEAD flags:CC
Successfully matched this instruction:
(set (reg:SI 116)
(const_int 0 [0]))
allowing combination of insns 14, 16 and 18
original costs 6 + 4 + 4 = 14
replacement cost 4
deferring deletion of insn with uid = 16.
deferring deletion of insn with uid = 14.
deferring deletion of insn with uid = 9.
deferring deletion of insn with uid = 11.
modifying insn i3 18: r116:SI=0
deferring rescan insn with uid = 18.
It seems insns 9, 11, 14, 16 get deleted, even though insn 16 output (r108)
should be used insn 20; it is likely I overlooked something though. I don't
know if r108 reuse (insn 14 sets; insn 16 dead; insn 20 sets again) might be
related to this.
