https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100622
Bug ID: 100622
Summary: Conversion to smaller unsigned type in loop
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: tkoenig at gcc dot gnu.org
Target Milestone: ---
Consider
unsigned int foo(unsigned int *a, int n)
{
int i;
unsigned int res = 0;
for (i=0; i<n; i++)
res += a[i];
return res;
}
unsigned int foo2 (unsigned int *a, int n)
{
int i;
unsigned long res = 0;
for (i=0; i<n; i++)
res += a[i];
return res;
}
Given modular 2^n arithmetic, these two functions are identical in effect.
On POWER with a reasonably recent trunk with -O1, this gets compiled to
(using -O1 in order to avoid loop unrolling for better visibility)
$ gcc -O1 -c add.c && objdump --disassemble add.o
add.o: file format elf64-powerpcle
Disassembly of section .text:
0000000000000000 <foo>:
0: 00 00 04 2c cmpwi r4,0
4: 30 00 81 40 ble 34 <foo+0x34>
8: 20 00 89 78 clrldi r9,r4,32
c: fc ff 43 39 addi r10,r3,-4
10: 00 00 60 38 li r3,0
14: a6 03 29 7d mtctr r9
18: 04 00 0a 85 lwzu r8,4(r10)
1c: 14 1a 68 7c add r3,r8,r3
20: 20 00 63 78 clrldi r3,r3,32
24: ff ff 29 39 addi r9,r9,-1
28: 20 00 29 79 clrldi r9,r9,32
2c: ec ff 00 42 bdnz 18 <foo+0x18>
30: 20 00 80 4e blr
34: 00 00 60 38 li r3,0
38: 20 00 80 4e blr
...
0000000000000048 <foo2>:
48: 00 00 04 2c cmpwi r4,0
4c: 30 00 81 40 ble 7c <foo2+0x34>
50: 20 00 89 78 clrldi r9,r4,32
54: fc ff 43 39 addi r10,r3,-4
58: 00 00 60 38 li r3,0
5c: a6 03 29 7d mtctr r9
60: 04 00 0a 85 lwzu r8,4(r10)
64: 14 42 63 7c add r3,r3,r8
68: ff ff 29 39 addi r9,r9,-1
6c: 20 00 29 79 clrldi r9,r9,32
70: f0 ff 00 42 bdnz 60 <foo2+0x18>
74: 20 00 63 78 clrldi r3,r3,32
78: 20 00 80 4e blr
7c: 00 00 60 38 li r3,0
80: f4 ff ff 4b b 74 <foo2+0x2c>
so there is an extra instruction to mask the result of the
addition in foo. This should not be needed.
