[Bug c/98269] gcc 6.5.0 __builtin_add_overflow() with small uint32_t values incorrectly detects overflow
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98269 --- Comment #5 from stli at linux dot ibm.com --- Just as information, I've just committed this glibc patch: "s390x: Require GCC 7.1 or later to build glibc." https://sourceware.org/git/?p=glibc.git;a=commit;h=844b4d8b4b937fe6943d2c0c80ce7d871cdb1eb5
[Bug c/98269] gcc 6.5.0 __builtin_add_overflow() with small uint32_t values incorrectly detects overflow
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98269
Andreas Krebbel changed:
What|Removed |Added
Status|UNCONFIRMED |RESOLVED
Resolution|--- |DUPLICATE
CC||krebbel at gcc dot gnu.org
--- Comment #4 from Andreas Krebbel ---
The problem is a CC mode mismatch generated by combine. After splitting the add
insn 135 generates a CCL1mode cc while the conditional jump consumes it as
CCUmode. This leads to the wrong condition code mask being generated in the
end.
(insn 135 56 136 7 (parallel [
(set (reg:CCL1 33 %cc)
(compare:CCL1 (plus:SI (reg:SI 108)
(mem:SI (plus:DI (reg:DI 88 [ ivtmp.10 ])
(const_int 12 [0xc])) [3 MEM[base: previous_25,
offset: 12B]+0 S4 A32]))
(reg:SI 108)))
(set (reg:SI 109)
(plus:SI (reg:SI 108)
(mem:SI (plus:DI (reg:DI 88 [ ivtmp.10 ])
(const_int 12 [0xc])) [3 MEM[base: previous_25,
offset: 12B]+0 S4 A32])))
]) t.c:31 1358 {*addsi3_carry1_cc}
(expr_list:REG_DEAD (reg:SI 108)
(nil)))
(note 136 135 64 7 NOTE_INSN_DELETED)
(insn 64 136 65 7 (set (mem:SI (plus:DI (reg:DI 88 [ ivtmp.10 ])
(const_int 28 [0x1c])) [3 MEM[base: previous_25, offset: 28B]+0
S4 A32])
(reg:SI 109)) t.c:31 1077 {*movsi_zarch}
(nil))
(note 65 64 66 7 NOTE_INSN_DELETED)
(jump_insn 66 65 67 7 (set (pc)
(if_then_else (geu (reg:CCU 33 %cc)
(const_int 0 [0]))
(label_ref 78)
(pc))) t.c:31 1661 {*cjump_64}
(int_list:REG_BR_PROB 9500 (expr_list:REG_DEAD (reg:CCZ 33 %cc)
(nil)))
The failure disappears with:
commit bf7499197fbb065123257c374064f6bb715c951b
Author: Dominik Vogt
Date: Mon Jul 4 14:25:22 2016 +
S/390: Add support for z13 instructions lochi and locghi.
The attached patch adds patterns to make use of the z13 LOCHI and
LOCGHI instructions.
...
But that one only hides the problem. The mere presence of the lochi
alternatives lead to different RTL being emitted (although the alternative is
not enabled for -march=z196). The split then doesn't happen anymore.
Reverting the patch and continue bisecting. The failure finally disappears
with:
3f54004b095d1cd513e63753ee0f8f9f13698347 is the first bad commit
commit 3f54004b095d1cd513e63753ee0f8f9f13698347
Author: Bin Cheng
Date: Fri Jan 27 14:42:23 2017 +
re PR rtl-optimization/78559 (wrong code due to tree if-conversion?)
PR rtl-optimization/78559
* combine.c (try_combine): Discard REG_EQUAL and REG_EQUIV for
other_insn in combine.
This looks like the actual fix to me. The wrong CC mode survives as part of a
REG_EQUAL note:
Successfully matched this instruction:
(set (reg:SI 93 [ _27+4 ])
(if_then_else:SI (geu (reg:CCL1 33 %cc)
(const_int 0 [0]))
(reg:SI 93 [ _27+4 ])
(reg:SI 118)))
allowing combination of insns 56 and 135
original costs 4 + 4 = 16
replacement cost 8
deferring deletion of insn with uid = 56.
modifying other_insn 136: r93:SI={(geu(%cc:CCL1,0))?r93:SI:r118:SI}
REG_DEAD %cc:CCU
REG_EQUAL ltu(%cc:CCU,0)
deferring rescan insn with uid = 136.
modifying insn i3 135:
{%cc:CCL1=cmp(r108:SI+[r88:DI+0xc],r108:SI);r109:SI=r108:SI+[r88:DI+0xc];}
REG_DEAD r108:SI
deferring rescan insn with uid = 135.
So we probably should mark it as duplicate of PR78559.
*** This bug has been marked as a duplicate of bug 78559 ***
[Bug c/98269] gcc 6.5.0 __builtin_add_overflow() with small uint32_t values incorrectly detects overflow
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98269 --- Comment #3 from Jakub Jelinek --- For documentation, the bisection would be best, if you can preprocess the testcase and git bisect it between some 6.0-ish and 8.0-ish trunk revisions, we could know if the bug has been fixed or is just mere latent.
[Bug c/98269] gcc 6.5.0 __builtin_add_overflow() with small uint32_t values incorrectly detects overflow
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98269 stli at linux dot ibm.com changed: What|Removed |Added Target||s390x Known to work||10.1.0, 5.4.0, 5.5.0, ||7.1.0, 8.1.0, 9.1.0 CC||stli at linux dot ibm.com Known to fail||6.3.0, 6.4.0, 6.5.0 --- Comment #2 from stli at linux dot ibm.com --- That's okay for me. But I wanted to document it. Currently glibc is requiring gcc 6.2 as minimum. For s390x, I will post a patch which requires gcc 7.1 as minimum.
[Bug c/98269] gcc 6.5.0 __builtin_add_overflow() with small uint32_t values incorrectly detects overflow
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98269 Jakub Jelinek changed: What|Removed |Added CC||jakub at gcc dot gnu.org --- Comment #1 from Jakub Jelinek --- gcc 6.5 is unsupported since October 2018 and 7.x since November 2019, so if currently supported branches work, there is nothing to do for us. If you want, you can bisect which gcc change fixed it.
