https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88962
Bug ID: 88962
Summary: Invalid/inconsistent PowerPC TLS variable access
Product: gcc
Version: 8.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: jirik.svoboda at seznam dot cz
Target Milestone: ---
If I build a GCC cross-compiler for PowerPC (ppc-linux-gnu):
binutils-2.31.1
./configure --target=ppc-linux-gnu --prefix=... --dsiable-nls --disable-werror
--enable-gold --enable-deterministic-archives
make all
make install
gcc-8.2.0
./configure --target=ppc-linux-gnu --prefix=... --with-gnu-as --with-gnu-ld
--disable-nls --disable-threads --enable-languages=c --disable-multilib
--disable-libgcj --without-headers --disable-shared --enable-lto
--disable-werror
make all-gcc
make install-gcc
and then build a shared library that accesses a static thread-local variable
and a non-static thread-local variable:
[jirka@omelette tmp]$ cat test.c
static __thread int a;
__thread int b;
int geta(void)
{
return a;
}
int getb(void)
{
return b;
}
int seta(int n)
{
a = n;
}
[jirka@omelette tmp]$ /usr/local/xcross/ppc-linux-gnu/bin/ppc-linux-gnu-gcc
-nostdlib -fPIC -O3 -c -o test.o test.c
[jirka@omelette tmp]$ /usr/local/xcross/ppc-linux-gnu/bin/ppc-linux-gnu-gcc
-nostdlib -shared -fPIC -O3 -o test.so test.o
[jirka@omelette tmp]$ /usr/local/xcross/ppc-linux-gnu/bin/ppc-linux-gnu-objdump
-D test.so
00000248 <geta>:
248: 7c 08 02 a6 mflr r0
24c: 42 9f 00 09 bcl 20,4*cr7+so,254 <geta+0xc>
250: 00 01 fd c4 .long 0x1fdc4
254: 7d 28 02 a6 mflr r9
258: 94 21 ff f0 stwu r1,-16(r1)
25c: 80 69 00 00 lwz r3,0(r9)
260: 90 01 00 14 stw r0,20(r1)
264: 7c 69 1a 14 add r3,r9,r3
268: 93 c1 00 08 stw r30,8(r1)
26c: 38 63 ff f4 addi r3,r3,-12
270: 48 01 fd f9 bl 20068 <__tls_get_addr@plt>
274: 80 01 00 14 lwz r0,20(r1)
278: 3c 63 00 00 addis r3,r3,0
27c: 83 c1 00 08 lwz r30,8(r1)
280: 38 63 80 04 addi r3,r3,-32764 <--- **** subtract bias 32K
284: 7c 08 03 a6 mtlr r0
288: 80 63 00 00 lwz r3,0(r3)
28c: 38 21 00 10 addi r1,r1,16
290: 4e 80 00 20 blr
00000294 <getb>:
294: 7c 08 02 a6 mflr r0
298: 42 9f 00 09 bcl 20,4*cr7+so,2a0 <getb+0xc>
29c: 00 01 fd 78 .long 0x1fd78
2a0: 7d 28 02 a6 mflr r9
2a4: 94 21 ff f0 stwu r1,-16(r1)
2a8: 80 69 00 00 lwz r3,0(r9)
2ac: 90 01 00 14 stw r0,20(r1)
2b0: 7c 69 1a 14 add r3,r9,r3
2b4: 93 c1 00 08 stw r30,8(r1)
2b8: 38 63 ff ec addi r3,r3,-20
2bc: 48 01 fd ad bl 20068 <__tls_get_addr@plt>
2c0: 80 01 00 14 lwz r0,20(r1)
2c4: 80 63 00 00 lwz r3,0(r3) < ---- **** no bias subtracted
2c8: 83 c1 00 08 lwz r30,8(r1)
2cc: 7c 08 03 a6 mtlr r0
2d0: 38 21 00 10 addi r1,r1,16
2d4: 4e 80 00 20 blr
As you can see, for static thread-local variable, we subtract 32K bias from
result of __tls_get_addr@plt. For non-static thread-local variable, we do not
subtract anything. This looks like a bug.
For -O0 optimization we never subtract anything.
Not sure which behavior is correct -- subtract 32 K or not) -- I ran into this
when implementing PowerPC architecture support for HelenOS dynamic linker. I am
inclined to think it should always subtract 32K bias according to the ABI. In
any case I think it needs to behave consistently, otherwise cannot make the TLS
work in all cases.
