https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94580
Bug ID: 94580 Summary: missing warning accessing an interior flexible array member Product: gcc Version: 10.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: middle-end Assignee: unassigned at gcc dot gnu.org Reporter: msebor at gcc dot gnu.org Target Milestone: --- In the following (invalid) test case -Warray-bounds only diagnoses one of the four out-of-bounds references, although it eliminates the seemingly redundant stores on the (clearly incorrect in this case) assumption that they don't alias with the reads. It's fine to eliminate the stores (or take whatever other reasonable action) as long as the out-of-bounds accesses are diagnosed. GCC warns about declarations of interior flexible array members, but only with -Wpedantic, so this kind of bugs can easily go undetected. $ cat t.c && gcc -O2 -S -Wall -Wextra -fdump-tree-vrp=/dev/stdout t.c struct A { int n, a[]; }; struct B { struct A a; int x; }; struct B b; int f0 (void) { b.a.a[1] = 1; // missing -Warray-bounds (store eliminated) int t = b.x; b.a.a[1] = 0; // -Warray-bounds (good) return b.x - t; } int f1 (struct B *p) { p->a.a[1] = 1; // missing -Warray-bounds (store eliminated) int t = p->x; p->a.a[1] = 0; // missing -Warray-bounds return p->x - t; } ;; Function f0 (f0, funcdef_no=0, decl_uid=1937, cgraph_uid=1, symbol_order=1) ;; 1 loops found ;; ;; Loop 0 ;; header 0, latch 1 ;; depth 0, outer -1 ;; nodes: 0 1 2 ;; 2 succs { 1 } Value ranges after VRP: t.c: In function ‘f0’: t.c:10:8: warning: array subscript 1 is above array bounds of ‘int[0]’ [-Warray-bounds] 10 | b.a.a[1] = 0; // -Warray-bounds (good) | ~~~~~^~~ t.c:1:19: note: while referencing ‘a’ 1 | struct A { int n, a[]; }; | ^ f0 () { <bb 2> [local count: 1073741824]: b.a.a[1] = 0; return 0; } ;; Function f0 (f0, funcdef_no=0, decl_uid=1937, cgraph_uid=1, symbol_order=1) ;; 1 loops found ;; ;; Loop 0 ;; header 0, latch 1 ;; depth 0, outer -1 ;; nodes: 0 1 2 ;; 2 succs { 1 } Value ranges after VRP: f0 () { <bb 2> [local count: 1073741824]: b.a.a[1] = 0; return 0; } ;; Function f1 (f1, funcdef_no=1, decl_uid=1941, cgraph_uid=2, symbol_order=2) ;; 1 loops found ;; ;; Loop 0 ;; header 0, latch 1 ;; depth 0, outer -1 ;; nodes: 0 1 2 ;; 2 succs { 1 } Value ranges after VRP: f1 (struct B * p) { <bb 2> [local count: 1073741824]: p_2(D)->a.a[1] = 0; return 0; } ;; Function f1 (f1, funcdef_no=1, decl_uid=1941, cgraph_uid=2, symbol_order=2) ;; 1 loops found ;; ;; Loop 0 ;; header 0, latch 1 ;; depth 0, outer -1 ;; nodes: 0 1 2 ;; 2 succs { 1 } Value ranges after VRP: f1 (struct B * p) { <bb 2> [local count: 1073741824]: p_2(D)->a.a[1] = 0; return 0; }