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.