https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98350
Bug ID: 98350
Summary: Reassociation breaks FMA chains
Product: gcc
Version: unknown
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: ktkachov at gcc dot gnu.org
Target Milestone: ---
Consider the testcase:
#define N 1024
double a[N];
double b[N];
double c[N];
double d[N];
double e[N];
double f[N];
double g[N];
double h[N];
double j[N];
double k[N];
double l[N];
double m[N];
double o[N];
double p[N];
void
foo (void)
{
for (int i = 0; i < N; i++)
{
a[i] += b[i]* c[i] + d[i] * e[i] + f[i] * g[i] + h[i] * j[i] + k[i] * l[i]
+ m[i]* o[i] + p[i];
}
}
For -Ofast --param=tree-reassoc-width=1 GCC generates the loop:
.L2:
ldr q1, [x1, x0]
ldr q0, [x12, x0]
ldr q3, [x14, x0]
fadd v0.2d, v0.2d, v1.2d
ldr q1, [x13, x0]
ldr q2, [x11, x0]
fmla v0.2d, v3.2d, v1.2d
ldr q1, [x10, x0]
ldr q3, [x9, x0]
fmla v0.2d, v2.2d, v1.2d
ldr q1, [x8, x0]
ldr q2, [x7, x0]
fmla v0.2d, v3.2d, v1.2d
ldr q1, [x6, x0]
ldr q3, [x5, x0]
fmla v0.2d, v2.2d, v1.2d
ldr q1, [x4, x0]
ldr q2, [x3, x0]
fmla v0.2d, v3.2d, v1.2d
ldr q1, [x2, x0]
fmla v0.2d, v2.2d, v1.2d
str q0, [x1, x0]
add x0, x0, 16
cmp x0, 8192
bne .L2
with --param=tree-reassoc-width=4 it generates:
.L2:
ldr q5, [x11, x0]
ldr q4, [x7, x0]
ldr q0, [x3, x0]
ldr q3, [x12, x0]
ldr q1, [x8, x0]
ldr q2, [x4, x0]
fmul v3.2d, v3.2d, v5.2d
fmul v1.2d, v1.2d, v4.2d
fmul v2.2d, v2.2d, v0.2d
ldr q16, [x1, x0]
ldr q18, [x14, x0]
ldr q17, [x13, x0]
ldr q0, [x2, x0]
ldr q7, [x10, x0]
ldr q6, [x9, x0]
ldr q5, [x6, x0]
ldr q4, [x5, x0]
fmla v3.2d, v18.2d, v17.2d
fadd v0.2d, v0.2d, v16.2d
fmla v1.2d, v7.2d, v6.2d
fmla v2.2d, v5.2d, v4.2d
fadd v0.2d, v0.2d, v3.2d
fadd v1.2d, v1.2d, v2.2d
fadd v0.2d, v0.2d, v1.2d
str q0, [x1, x0]
add x0, x0, 16
cmp x0, 8192
bne .L2
The reassociation is evident. The problem here is that the fmla chains are
something we'd want to preserve.
Is there a way we can get the reassoc pass to handle FMAs more intelligently?
