| Message ID | 50d17a1faa652493a0fe0c303f9541b2cca4e68a.camel@med.uni-goettingen.de |
|---|---|
| State | New |
| Headers | show |
| Series | [C] Evaluate argument of sizeof that are structs of variable size. | expand |
On Mon, 9 Aug 2021, Uecker, Martin wrote: > Evaluate arguments of sizeof that are structs of variable size. > > Evaluate arguments of sizeof for all types of variable size > and not just for VLAs. This fixes some issues related to > [PR29970] where statement expressions need to be evaluated > so that the size is well defined. OK. It might be a good idea to check if something like this is also needed in c_expr_sizeof_type, since that's also using c_vla_type_p at present.
Am Montag, den 09.08.2021, 21:42 +0000 schrieb Joseph Myers: > On Mon, 9 Aug 2021, Uecker, Martin wrote: > > > Evaluate arguments of sizeof that are structs of variable size. > > > > Evaluate arguments of sizeof for all types of variable size > > and not just for VLAs. This fixes some issues related to > > [PR29970] where statement expressions need to be evaluated > > so that the size is well defined. > > OK. It might be a good idea to check if something like this is also > needed in c_expr_sizeof_type, since that's also using c_vla_type_p at > present. I am still trying to figure out what the branch guarded by c_vla_type_p is for in c_expr_sizeof_type. The comment talks about [*], but how would you apply sizeof to such an array? One can form something like int foo(int (*x)[*], int a[sizeof *x]); but only in prototypes and then it is never evaluated. I guess it is to avoid this being folded to: int foo(int (*x)[*], int a[0]); So the question is whether this can also happen with struct types. This seems possible: int foo(struct { int z[*]; } v, int (*y)[sizeof v]); So maybe we need the change there too. Martin
On Mon, 9 Aug 2021, Uecker, Martin wrote: > I am still trying to figure out what the branch guarded > by c_vla_type_p is for in c_expr_sizeof_type. The comment > talks about [*], but how would you apply sizeof to > such an array? One can form something like The comment describes a requirement on what happens for [*], but that branch in the code isn't restricted to cases involving [*]; it covers any case where (the type is a VLA and) groktypename has returned an expression to be evaluated before the type name. E.g. take your vla-stexp-1.c test but with sizeof(typeof(...)) instead of sizeof(...) - that aborts, but I'd hope such a fix in c_expr_sizeof_type would make it pass.
diff --git a/gcc/c/c-typeck.c b/gcc/c/c-typeck.c index 5d6565bdaa9..c5bf3372321 100644 --- a/gcc/c/c-typeck.c +++ b/gcc/c/c-typeck.c @@ -2992,7 +2992,7 @@ c_expr_sizeof_expr (location_t loc, struct c_expr expr) c_last_sizeof_loc = loc; ret.original_code = SIZEOF_EXPR; ret.original_type = NULL; - if (c_vla_type_p (TREE_TYPE (folded_expr))) + if (C_TYPE_VARIABLE_SIZE (TREE_TYPE (folded_expr))) { /* sizeof is evaluated when given a vla (C99 6.5.3.4p2). */ ret.value = build2 (C_MAYBE_CONST_EXPR, TREE_TYPE (ret.value), diff --git a/gcc/testsuite/gcc.dg/vla-stexp-01.c b/gcc/testsuite/gcc.dg/vla-stexp-01.c new file mode 100644 index 00000000000..97d66937e9a --- /dev/null +++ b/gcc/testsuite/gcc.dg/vla-stexp-01.c @@ -0,0 +1,18 @@ +/* PR29970*/ +/* { dg-do run } */ +/* { dg-options "-Wall -O0" } */ + +int foo(void) +{ + int n = 0; + return sizeof(*({ n = 10; struct foo { int x[n]; } x; &x; })); +} + + +int main() +{ + if (sizeof(struct foo { int x[10]; }) != foo()) + __builtin_abort(); + + return 0; +}
Here is a patch which changes the behavior of sizeof when applied to structs of variable size (a GNU extension) to evaluate its arguments as it does for VLAs. This is a breaking change, but it seems this is required if we want to fix [PR29970] (and it is also more consistent). Together with the patch I sent previously (which needs fixing as it breaks Ada) and another small change, this would fix most of the examples in PR29970. Comments? Evaluate arguments of sizeof that are structs of variable size. Evaluate arguments of sizeof for all types of variable size and not just for VLAs. This fixes some issues related to [PR29970] where statement expressions need to be evaluated so that the size is well defined. 2021-08-09 Martin Uecker <muecker@gwdg.de> gcc/ PR c/29970 * c-typeck.c (c_expr_sizeof_expr): Evaluate size expressions for structs of variable size. gcc/testsuite/ PR c/29970 * gcc.dg/vla-stexp-1.c: New test.