http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56476
Bug #: 56476
Summary: ARM volatile writes/str creates superfluous and
uncalled for read/ldr with -Os
Classification: Unclassified
Product: gcc
Version: 4.7.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: j...@embeddedarm.com
Gcc 4.7.1 crosscompiler ARM EABI target, x86 Linux host.
In the below code, there are only writes to the "regs" volatile unsigned int *.
Yet, compiling with "-Os", "-O2", or "-O -ftree-vrp" yields code that will do
an uncalled for ldrne in the assembler output. This is bad as this volatile
unsigned int * represents hardware registers where bus reads have side-effects
and reading a value and then writing the same value back to the same location
is not an equivalent operation to doing nothing at all.
Several gcc versions have been tested and all but some really old 3.3.4 gcc
toolchain exhibited this. (4.6.1 and a 4.4.4 were tested also and had the bug)
//Jesse Off
volatile unsigned int * regs;
void test(void)
{
int i, x;
regs[0x708 / 4] = 0xdeadbeef;
for (i = 0xbad; i >= 0; i--) {
if (i == 1)
regs[0x708 / 4] = 0xbeefdead;
for (x = 1; x >= 0; x--) {
regs[0x708 / 4] = 0xbadcab;
regs[0x704 / 4] = x;
}
}
}
/*
BROKEN ASM!!!! (Compiled with -Os)
00000000 <test>:
0: e59f3048 ldr r3, [pc, #72] ; 50 <test+0x50>
4: e59f2048 ldr r2, [pc, #72] ; 54 <test+0x54>
8: e5933000 ldr r3, [r3]
c: e92d4010 push {r4, lr}
10: e59f1040 ldr r1, [pc, #64] ; 58 <test+0x58>
14: e5832708 str r2, [r3, #1800] ; 0x708
18: e59f203c ldr r2, [pc, #60] ; 5c <test+0x5c>
1c: e3a04001 mov r4, #1
20: e3a0c000 mov ip, #0
24: e3520001 cmp r2, #1
28: 15930708 ldrne r0, [r3, #1800] ; 0x708
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^--- NOT SAFE!
2c: 059f002c ldreq r0, [pc, #44] ; 60 <test+0x60>
30: e2522001 subs r2, r2, #1
34: e5830708 str r0, [r3, #1800] ; 0x708
38: e5831708 str r1, [r3, #1800] ; 0x708
3c: e5834704 str r4, [r3, #1796] ; 0x704
40: e5831708 str r1, [r3, #1800] ; 0x708
44: e583c704 str ip, [r3, #1796] ; 0x704
48: 2afffff5 bcs 24 <test+0x24>
4c: e8bd8010 pop {r4, pc}
50: 00000000 .word 0x00000000
54: deadbeef .word 0xdeadbeef
58: 00badcab .word 0x00badcab
5c: 00000bad .word 0x00000bad
60: beefdead .word 0xbeefdead
RIGHT AND WORKING (Compiled with -O) (no superfluous ldr on a volatile)
00000000 <test>:
0: e52d4004 push {r4} ; (str r4, [sp, #-4]!)
4: e59f304c ldr r3, [pc, #76] ; 58 <test+0x58>
8: e5933000 ldr r3, [r3]
c: e59f2048 ldr r2, [pc, #72] ; 5c <test+0x5c>
10: e5832708 str r2, [r3, #1800] ; 0x708
14: e59f2044 ldr r2, [pc, #68] ; 60 <test+0x60>
18: e59f1044 ldr r1, [pc, #68] ; 64 <test+0x64>
1c: e3a0c001 mov ip, #1
20: e3a00000 mov r0, #0
24: e59f403c ldr r4, [pc, #60] ; 68 <test+0x68>
28: ea000001 b 34 <test+0x34>
2c: e3520001 cmp r2, #1
30: 05834708 streq r4, [r3, #1800] ; 0x708
34: e5831708 str r1, [r3, #1800] ; 0x708
38: e583c704 str ip, [r3, #1796] ; 0x704
3c: e5831708 str r1, [r3, #1800] ; 0x708
40: e5830704 str r0, [r3, #1796] ; 0x704
44: e2422001 sub r2, r2, #1
48: e3720001 cmn r2, #1
4c: 1afffff6 bne 2c <test+0x2c>
50: e8bd0010 pop {r4}
54: e12fff1e bx lr
58: 00000000 .word 0x00000000
5c: deadbeef .word 0xdeadbeef
60: 00000bad .word 0x00000bad
64: 00badcab .word 0x00badcab
68: beefdead .word 0xbeefdead
*/
