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Re: port-sh3/56311: GCC 9 and 10 miscompile lint1 for sh3 (by mischoice of register)
The following reply was made to PR port-sh3/56311; it has been noted by GNATS.
From: Rin Okuyama <rokuyama.rk%gmail.com@localhost>
To: "gnats-bugs%NetBSD.org@localhost" <gnats-bugs%NetBSD.org@localhost>
Cc: Valery Ushakov <uwe%stderr.spb.ru@localhost>
Subject: Re: port-sh3/56311: GCC 9 and 10 miscompile lint1 for sh3 (by
mischoice of register)
Date: Fri, 16 Jul 2021 17:34:30 +0900
In my previous analysis, I carelessly forgot delay slots!
Please ignore that (I've corrected wrong title of the PR).
I've examined objdump again, more carefully this time ;)
Source code and dump for initdecl() with comments are:
/* declaration stack */
dcs = xcalloc(1, sizeof(*dcs));
dcs->d_ctx = EXTERN;
dcs->d_ldlsym = &dcs->d_dlsyms;
/* type information and classification */
407eae: mov.l r12,@-r15 ! push r12
407eb0: mova 407fb4 <initdecl+0x106>,r0 ! r0 = 0x407fb4
407eb2: mov.l 407fb4 <initdecl+0x106>,r12 ! r12 = 0x03a9b8
407eb4: sts.l pr,@-r15 ! push pr
407eb6: add r0,r12 ! r12 = .got
407eb8: mov.l 407fb8 <initdecl+0x10a>,r1 ! r1 = 0x96d4
407eba: mov #80,r5 ! r5 = 80
407ebc: bsrf r1 ! r0 = xcalloc(1, 80)
407ebe: mov #1,r4 ! r4 = 1
407ec0: mov.l 407fbc <initdecl+0x10e>,r1 ! r1 = 0x02f0
407ec2: mov r0,r2 ! r2 = r0
407ec4: mov r12,r0 ! r0 = .got
407ec6: mov.l r2,@(r0,r1) ! @(.got, 0x02f0) = dcs = r2
407ec8: mov #1,r1 ! r1 = EXTERN
407eca: mov.l r1,@(36,r2) ! @(36, r2) = dcs->d_ctx = r1
407ecc: add #64,r2 ! r2 = &dcs + 64 = &dcs->d_dlsyms
407ece: mov.l 407fc0 <initdecl+0x112>,r1 ! r1 = 0x7954
407ed0: bsrf r1 ! inittyp()
407ed2: mov.l r2,@(4,r12) ! @(4, .got) = ??? = r2
GCC seems to make a mistake at the last line; if it were
407ed2: mov.l r2,@(4,r2)
it would work as expected, i.e.,
@(4, &dcs->d_dlsyms) = dcs->d_ldlsym = r2 = &dcs->d_dlsyms
However, since it mistakes r12 (= .got) for r2, dcs->d_ldlsym is not
correctly initialized, and .got becomes broken.
I think this is what happens with the default optimization level.
Thanks uwe@ for kind advice offlist!
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