Hello!
GCC 4.1 seems to sometimes generate inefficient code when doing
structure assignments directly when compiling for MIPS1. When assigning
to structure members manually, it generates regular lw/sw sequences.
When assigning to the structure, you instead get lwl/lwr and swl/swr
pairs (for no reason, since the data is aligned).
I'm translating the MIPS code to Java bytecode, so the problem is even
worse for me as I then have to read four separate bytes and or
these together to implement a lwl/lwr pair.
I've attached a preprocessed file which exhibits this behavior. The two
functions below show this, the first doing a structure assignment and
the second member assignment.
void dummy1(NavigateSegment *segments,
int count,
RoadMapPosition *src_pos,
RoadMapPosition *dst_pos) {
int i;
int group_id = 0;
NavigateSegment *segment;
for (i=0; i < count; i++) {
set_from_pos (&segments[i].from_pos);
segments[i].shape_initial_pos = segments[i].from_pos;
}
}
void dummy2(NavigateSegment *segments,
int count,
RoadMapPosition *src_pos,
RoadMapPosition *dst_pos) {
int i;
int group_id = 0;
NavigateSegment *segment;
for (i=0; i < count; i++) {
set_from_pos (&segments[i].from_pos);
segments[i].shape_initial_pos.longitude = segments[i].from_pos.longitude;
segments[i].shape_initial_pos.latitude = segments[i].from_pos.latitude;
}
}
And if you disassemble it you see that they look like
00000014 <dummy1>:
14: 00003021 move a2,zero
18: 240804d2 li t0,1234
1c: 08000014 j 50 <dummy1+0x3c>
20: 2407162e li a3,5678
24: ac88001c sw t0,28(a0)
28: ac870020 sw a3,32(a0)
2c: 8882001c lwl v0,28(a0)
30: 88830020 lwl v1,32(a0)
...
00000064 <dummy2>:
64: 00001821 move v1,zero
68: 240604d2 li a2,1234
6c: 08000022 j 88 <dummy2+0x24>
70: 2407162e li a3,5678
74: ac870020 sw a3,32(a0)
78: ac86001c sw a2,28(a0)
7c: ac86002c sw a2,44(a0)
80: ac870030 sw a3,48(a0)
....
If the loop is removed, both functions generate the same code (with
regular loads/stores). The GCC version is
mips-linux-gnu-gcc (GCC) 4.1.2 20061115 (prerelease) (Debian 4.1.1-21)
Should I consider this a bug and report it to the bug tracking system?
I looked for similar problems, but couldn't find any matching bug report.
(Ehud Shabtai discovered this problem)
--
// Simon
Attachment:
main.i
Description: Binary data
