Certain read accesses to objects marked as "volatile" are optimized away.
This happens when both "-O2" and "-fno-strict-aliasing" are specified.
This has been observed with the Linux kernel which is unusable in certain
configurations due to this problem. This used to work with 3.4.0.
Environment:
System: Linux macro 2.4.26 #3 Thu May 6 20:57:18 CEST 2004 i686 unknown unknown
GNU/Linux
Architecture: i686
host: i686-pc-linux-gnu
build: i686-pc-linux-gnu
target: mips64el-unknown-linux-gnu
configured with: ../configure --prefix=/usr --enable-languages=c --disable-libmudflap
--disable-shared --target=mips64el-linux
How-To-Repeat:
The following program can be used:
$ cat volatile.c
struct data {
volatile unsigned long *addr;
} *p;
int test()
{
*p->addr;
return 0;
}
$ mips64el-linux-gcc -mabi=32 -O2 -fno-strict-aliasing -mno-abicalls -S
volatile.c -o volatile.s
$ cat volatile.s
.file 1 "volatile.c"
.section .mdebug.abi32
.previous
.text
.align 2
.align 2
.globl test
.ent test
.type test, @function
test:
.frame $sp,0,$31 # vars= 0, regs= 0/0, args= 0, gp= 0
.mask 0x00000000,0
.fmask 0x00000000,0
.set noreorder
.set nomacro
j $31
move $2,$0
.set macro
.set reorder
.end test
.comm p,4,4
.ident "GCC: (GNU) 3.5.0 20040726 (experimental)"
After removing the "-fno-strict-aliasing" option, the output is correct:
$ mips64el-linux-gcc -mabi=32 -O2 -mno-abicalls -S volatile.c -o
volatile.s
$ cat volatile.s
.file 1 "volatile.c"
.section .mdebug.abi32
.previous
.text
.align 2
.align 2
.globl test
.ent test
.type test, @function
test:
.frame $sp,0,$31 # vars= 0, regs= 0/0, args= 0, gp= 0
.mask 0x00000000,0
.fmask 0x00000000,0
.set noreorder
.set nomacro
lui $2,%hi(p)
lw $4,%lo(p)($2)
move $2,$0
lw $3,0($4)
nop
lw $5,0($3)
j $31
nop
.set macro
.set reorder
.end test
.comm p,4,4
.ident "GCC: (GNU) 3.5.0 20040726 (experimental)"
------- Additional Comments From macro at linux-mips dot org 2004-07-26 18:11 -------
Fix:
None known. Using "-fstrict-aliasing" may be a workaround, but it is not
an option for Linux.
--
Summary: Accesses to volatile objects optimized away with -fno-
strict-aliasing
Product: gcc
Version: 3.5.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: macro at linux-mips dot org
CC: gcc-bugs at gcc dot gnu dot org
GCC build triplet: i686-pc-linux-gnu
GCC host triplet: i686-pc-linux-gnu
GCC target triplet: mips64el-unknown-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16721
