https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98598
Bug ID: 98598
Summary: Missed opportunity to optimize dependent loads in
loops
Product: gcc
Version: tree-ssa
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: hliu at amperecomputing dot com
Target Milestone: ---
As we know, dependent loads are not friendly to cache. Especially when in
nested loops, dependent loads such as pa->pb->pc->val may be repeated many
times. For example:
typedef struct C { int val; } C;
typedef struct B { C *pc; } B;
typedef struct A { B *pb; } A;
int foo (int n, int m, A *pa) {
int sum;
for (int i = 0; i < n; i++) {
for (int j = 0; j < m; j++) {
sum += pa[j].pb->pc->val; // each value is repeatedly loaded "n" times
// ...
}
// ...
}
return sum;
}
Such access pattern can be found in real applications and benchmarks, and this
can be critical to performance.
Can we cache the loaded value and avoid repeated dependent loads? E.g.
transform above case into following (suppose there is no alias issue or other
clobber, and "n" is big enough):
int foo (int n, int m, A *pa) {
int *cache = (int *) malloc(m * sizeof(int));
for (int j = 0; j < m; j++) {
cache[j] = pa[j].pb->pc->val;
}
int sum;
for (int i = 0; i < n; i++) {
for (int j = 0; j < m; j++) {
sum += cache[j]; // pa[j].pb->pc->val;
// ...
}
// ...
}
free(cache);
return sum;
}
This should improve performance a lot.
