https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85805
Bug ID: 85805
Summary: Improper code generation for 64 bit comparisons on
avr-gcc
Product: gcc
Version: 4.8.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: sandor.zsuga at jubatian dot com
Target Milestone: ---
GCC version:
avr-gcc (GCC) 4.8.1
Compiling with either -O1 or -O2 optimizations enabled, tested with some ATMega
and XMega targets. Test case:
#include <stdint.h>
uint8_t volatile tmp;
__attribute__((noinline)) void test_64(uint64_t d64)
{
if ((d64 & 0xFF800000UL) == 0xFF800000UL){
tmp ++;
}
}
__attribute__((noinline)) void test_32(uint32_t d32)
{
if ((d32 & 0xFF800000UL) == 0xFF800000UL){
tmp ++;
}
}
int main(void)
{
test_64(0);
test_32(0);
while(1);
}
A cut from the output assembly, showing the critical part (the code generated
for test_64, test_32 and main):
00000228 <test_64>:
228: 08 95 ret
0000022a <test_32>:
22a: 66 27 eor r22, r22
22c: 77 27 eor r23, r23
22e: 80 78 andi r24, 0x80 ; 128
230: 61 15 cp r22, r1
232: 71 05 cpc r23, r1
234: 80 48 sbci r24, 0x80 ; 128
236: 9f 4f sbci r25, 0xFF ; 255
238: 09 f0 breq .+2 ; 0x23c <test_32+0x12>
23a: 08 95 ret
23c: 80 91 00 20 lds r24, 0x2000
240: 8f 5f subi r24, 0xFF ; 255
242: 80 93 00 20 sts 0x2000, r24
246: 08 95 ret
00000248 <main>:
248: 20 e0 ldi r18, 0x00 ; 0
24a: 30 e0 ldi r19, 0x00 ; 0
24c: 40 e0 ldi r20, 0x00 ; 0
24e: 50 e0 ldi r21, 0x00 ; 0
250: 60 e0 ldi r22, 0x00 ; 0
252: 70 e0 ldi r23, 0x00 ; 0
254: 80 e0 ldi r24, 0x00 ; 0
256: 90 e0 ldi r25, 0x00 ; 0
258: 0e 94 14 01 call 0x228 ; 0x228 <test_64>
25c: 60 e0 ldi r22, 0x00 ; 0
25e: 70 e0 ldi r23, 0x00 ; 0
260: cb 01 movw r24, r22
262: 0e 94 15 01 call 0x22a ; 0x22a <test_32>
266: ff cf rjmp .-2 ; 0x266 <main+0x1e>
It seems like the compiler incorrectly determines that the "if" is always false
in the 64 bit case, and produces a corresponding result (a function doing
nothing). A native version of GCC produces the expected code (the comparison
being performed).
