http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58259
Bug ID: 58259
Summary: PowerPC: Atomic flag test and set generates NULL
pointer read and write
Product: gcc
Version: 4.7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: sebastian.hu...@embedded-brains.de
The GCC C compiler generates a NULL pointer read and write for the atomic flag
test and set method in GCC 4.7, 4.8 and 4.9. The GCC C++ compiler doesn't have
this problem.
C test case:
#include <stdatomic.h>
_Bool test_and_set(atomic_flag *flag)
{
return atomic_flag_test_and_set_explicit(flag, memory_order_acquire);
}
C++ test case:
#include <atomic>
bool test_and_set(std::atomic_flag &flag)
{
return flag.test_and_set(std::memory_order_acquire);
}
GCC assembler output:
powerpc-eabi-gcc-4.7.0 -std=c11 -Wall -Wextra -O2 -S test.c && cat test.s
.file "test.c"
.section ".text"
.align 2
.globl test_and_set
.type test_and_set, @function
test_and_set:
li 10,0
rlwinm 9,3,3,27,28
lbz 5,0(10) <--- HERE IS THE NULL POINTER READ
xori 9,9,24
li 7,255
li 6,1
rlwinm 3,3,0,0,29
slw 7,7,9
slw 5,5,9
slw 6,6,9
.L2:
lwarx 10,0,3
and 8,10,7
cmpw 0,8,5
andc 8,10,7
or 8,8,6
bne- 0,.L3
stwcx. 8,0,3
bne- 0,.L2
.L3:
isync
srw 10,10,9
li 9,0
mfcr 3
rlwinm 3,3,3,1
stb 10,0(9) <--- HERE IS THE NULL POINTER WRITE
blr
.size test_and_set, .-test_and_set
.ident "GCC: (GNU) 4.7.0 20120202 (experimental) [master revision
022fc2d:bffd767:fd457cef14f3bc6673e90a2de80005feea743ab7]"
powerpc-eabi-gcc -std=c11 -Wall -Wextra -O2 -S test.c && cat test.s
.file "test.c"
.section ".text"
.align 2
.globl test_and_set
.type test_and_set, @function
test_and_set:
li 9,0
li 7,255
lbz 5,0(9)
rlwinm 9,3,3,27,28
xori 9,9,24
li 6,1
rlwinm 3,3,0,0,29
slw 7,7,9
slw 5,5,9
slw 6,6,9
.L2:
lwarx 10,0,3
and 4,10,7
andc 8,10,7
cmpw 0,4,5
or 8,8,6
bne- 0,.L3
stwcx. 8,0,3
bne- 0,.L2
.L3:
isync
srw 10,10,9
li 9,0
stb 10,0(9)
mfcr 3
rlwinm 3,3,3,1
blr
.size test_and_set, .-test_and_set
.ident "GCC: (GNU) 4.9.0 20130828 (experimental) [master revision
022fc2d:bffd767:fd457cef14f3bc6673e90a2de80005feea743ab7]"
powerpc-eabi-g++ -std=c++11 -Wall -Wextra -O2 -S test.cc && cat test.s
.file "test.cc"
.section ".text"
.align 2
.globl _Z12test_and_setRSt11atomic_flag
.type _Z12test_and_setRSt11atomic_flag, @function
_Z12test_and_setRSt11atomic_flag:
.LFB326:
.cfi_startproc
rlwinm 9,3,3,27,28
li 6,255
xori 9,9,24
li 7,1
rlwinm 8,3,0,0,29
slw 6,6,9
slw 7,7,9
.L2:
lwarx 3,0,8
andc 10,3,6
or 10,10,7
stwcx. 10,0,8
bne- 0,.L2
isync
srw 3,3,9
rlwinm 3,3,0,0xff
blr
.cfi_endproc
.LFE326:
.size _Z12test_and_setRSt11atomic_flag,
.-_Z12test_and_setRSt11atomic_flag
.ident "GCC: (GNU) 4.9.0 20130828 (experimental) [master revision
022fc2d:bffd767:fd457cef14f3bc6673e90a2de80005feea743ab7]"
The ARMv7 architecture has a very similar atomic model and GCC generates valid
code:
arm-eabi-gcc -march=armv7-a -std=c11 -Wall -Wextra -O2 -S test.c && cat test.s
.arch armv7-a
.fpu softvfp
.eabi_attribute 20, 1
.eabi_attribute 21, 1
.eabi_attribute 23, 3
.eabi_attribute 24, 1
.eabi_attribute 25, 1
.eabi_attribute 26, 1
.eabi_attribute 30, 2
.eabi_attribute 34, 1
.eabi_attribute 18, 4
.file "test.c"
.text
.align 2
.global test_and_set
.type test_and_set, %function
test_and_set:
@ args = 0, pretend = 0, frame = 0
@ frame_needed = 0, uses_anonymous_args = 0
@ link register save eliminated.
mov r3, #0
mov r1, #1
ldrb r2, [r3] @ zero_extendqisi2
.L2:
ldrexb r3, [r0]
cmp r3, r2
bne .L3
strexb ip, r1, [r0]
cmp ip, #0
bne .L2
.L3:
mov r2, #0
dmb sy
movne r0, #0
moveq r0, #1
strb r3, [r2]
bx lr
.size test_and_set, .-test_and_set
.ident "GCC: (GNU) 4.8.2 20130807 (prerelease) [master revision
022fc2d:bffd767:fd457cef14f3bc6673e90a2de80005feea743ab7]"
Interestingly the code generated by the C++ compiler is much better for both
ARM and PowerPC.
arm-eabi-g++ -march=armv7-a -std=c++11 -Wall -Wextra -O2 -S test.cc && cat
test.s
.arch armv7-a
.fpu softvfp
.eabi_attribute 20, 1
.eabi_attribute 21, 1
.eabi_attribute 23, 3
.eabi_attribute 24, 1
.eabi_attribute 25, 1
.eabi_attribute 26, 1
.eabi_attribute 30, 2
.eabi_attribute 34, 1
.eabi_attribute 18, 4
.file "test.cc"
.text
.align 2
.global _Z12test_and_setRSt11atomic_flag
.type _Z12test_and_setRSt11atomic_flag, %function
_Z12test_and_setRSt11atomic_flag:
.fnstart
.LFB326:
@ args = 0, pretend = 0, frame = 0
@ frame_needed = 0, uses_anonymous_args = 0
@ link register save eliminated.
mov r2, #1
.L2:
ldrexb r3, [r0]
strexb r1, r2, [r0]
cmp r1, #0
bne .L2
dmb sy
uxtb r0, r3
bx lr
.cantunwind
.fnend
.size _Z12test_and_setRSt11atomic_flag,
.-_Z12test_and_setRSt11atomic_flag
.ident "GCC: (GNU) 4.8.2 20130807 (prerelease) [master revision
022fc2d:bffd767:fd457cef14f3bc6673e90a2de80005feea743ab7]"
