http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53842
Bug #: 53842
Summary: avr second write to volatile register gets dropped
Classification: Unclassified
Product: gcc
Version: 4.7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: j...@jtan.com
Hi,
I'm seeing volatile writes get dropped with avr-gcc 4.7.0 (as packaged in
Debian). This didn't happen with 4.5.3. Example:
------------
$ cat test.c
#define SPDR (*(volatile char *)(0x2e + 0x20))
#define SPSR (*(volatile char *)(0x2d + 0x20))
static inline char transfer(char tx) {
SPDR = tx;
while (!(SPSR & 0x80)) continue;
return SPDR;
}
void foo(void) {
transfer(0xff);
//asm("":::"memory");
transfer(0xff);
}
$ avr-gcc -c -mmcu=at90usb1287 -O1 test.c -o test.o
$ avr-objdump avr-objdump -d test.o
test.o: file format elf32-avr
Disassembly of section .text:
00000000 <foo>:
0: 8f ef ldi r24, 0xFF ; 255
2: 8e bd out 0x2e, r24 ; 46
4: 0d b4 in r0, 0x2d ; 45
6: 07 fe sbrs r0, 7
8: 00 c0 rjmp .+0 ; 0xa <foo+0xa>
a: 8e b5 in r24, 0x2e ; 46
c: 0d b4 in r0, 0x2d ; 45
e: 07 fe sbrs r0, 7
10: 00 c0 rjmp .+0 ; 0x12 <foo+0x12>
12: 8e b5 in r24, 0x2e ; 46
14: 08 95 ret
------------
Note only one "out" there. If I compile that same code on 4.5.3, or uncomment
that memory barrier and compile on 4.7.0, I get the correct code:
------------
0: 8f ef ldi r24, 0xFF ; 255
2: 8e bd out 0x2e, r24 ; 46
4: 0d b4 in r0, 0x2d ; 45
6: 07 fe sbrs r0, 7
8: 00 c0 rjmp .+0 ; 0xa <foo+0xa>
a: 8e b5 in r24, 0x2e ; 46
c: 8f ef ldi r24, 0xFF ; 255
e: 8e bd out 0x2e, r24 ; 46
10: 0d b4 in r0, 0x2d ; 45
12: 07 fe sbrs r0, 7
14: 00 c0 rjmp .+0 ; 0x16 <foo+0x16>
16: 8e b5 in r24, 0x2e ; 46
18: 08 95 ret
-------------
This seems similar to pr53085, but that bug was listed as existing on 4.5 as
well, while this one seems to be a regression.
