https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70946
Bug ID: 70946
Summary: Bad interaction between IVOpt and loop unrolling
Product: gcc
Version: 7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: wdijkstr at arm dot com
Target Milestone: ---
IVOpt chooses between using indexing for induction variables or incrementing
pointers. Due to way loop unrolling works, a decision that is optimal if
unrolling is disabled may become very non-optimal with unrolling.
Below are simple examples that show how the choice to use indexing can become a
very bad idea when unrolled, while using offset addressing leads to very decent
code. To improve this we either to teach IVOpt about unrolling (eg. prioritise
base+offset addressing) or add a tree unroller that unrolls small inner loops
before IVOpt.
void loop1 (int *p, int *q, int i)
{
for (i = 0; i < 1000; i++) p[i] = q[i] + 1;
}
void loop2 (int *p, int i)
{
for (i = 0; i < 1000; i++) p[i] = p[i] + 1;
}
On AArch64 with -O2 -funroll-loops this gives:
loop1:
mov x2, 0
.p2align 2
.L41:
ldr w4, [x1, x2]
add x3, x2, 4
add x10, x2, 8
add x9, x2, 12
add w5, w4, 1
str w5, [x0, x2]
add x8, x2, 16
add x7, x2, 20
add x6, x2, 24
add x11, x2, 28
ldr w12, [x1, x3]
add x2, x2, 32
cmp x2, 4000
add w13, w12, 1
str w13, [x0, x3]
ldr w14, [x1, x10]
add w15, w14, 1
str w15, [x0, x10]
ldr w16, [x1, x9]
add w17, w16, 1
str w17, [x0, x9]
ldr w18, [x1, x8]
add w4, w18, 1
str w4, [x0, x8]
ldr w3, [x1, x7]
add w10, w3, 1
str w10, [x0, x7]
ldr w9, [x1, x6]
add w5, w9, 1
str w5, [x0, x6]
ldr w8, [x1, x11]
add w7, w8, 1
str w7, [x0, x11]
bne .L41
ret
loop2:
add x6, x0, 4000
.p2align 2
.L51:
mov x1, x0
ldr w2, [x0]
add x0, x0, 32
add w3, w2, 1
cmp x0, x6
str w3, [x1], 4
ldr w4, [x0, -28]
add w5, w4, 1
str w5, [x0, -28]
ldr w7, [x1, 4]
add w8, w7, 1
str w8, [x1, 4]
ldp w9, w10, [x0, -20]
ldp w11, w12, [x0, -12]
add w14, w9, 1
ldr w13, [x0, -4]
add w15, w10, 1
add w16, w11, 1
add w17, w12, 1
add w18, w13, 1
stp w14, w15, [x0, -20]
stp w16, w17, [x0, -12]
str w18, [x0, -4]
bne .L51
ret
