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improve the comments on integer overflow safety

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This commit is contained in:
Ariel Ben-Yehuda 2014-10-15 20:39:12 +03:00
parent 5bcd0a0b50
commit e74e059bd4
1 changed files with 6 additions and 3 deletions
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9
src/librustc/middle/trans/adt.rs
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@ -471,14 +471,16 @@ fn ensure_struct_fits_in_address_space(ccx: &CrateContext,
scapegoat: ty::t) {
let mut offset = 0;
for &llty in fields.iter() {
// Invariant: offset < ccx.max_obj_size() <= 1<<61
if !packed {
let type_align = machine::llalign_of_min(ccx, llty);
offset = roundup(offset, type_align);
}
// type_align is a power-of-2, so still offset < ccx.max_obj_size()
// llsize_of_alloc(ccx, llty) is also less than ccx.max_obj_size()
// so the sum is less than 1<<62 (and therefore can't overflow).
offset += machine::llsize_of_alloc(ccx, llty);
// We can get away with checking for overflow once per iteration,
// because field sizes are less than 1<<61.
if offset >= ccx.max_obj_size() {
ccx.report_overbig_object(scapegoat);
}
@ -498,7 +500,8 @@ fn ensure_enum_fits_in_address_space(ccx: &CrateContext,
let discr_size = machine::llsize_of_alloc(ccx, ll_inttype(ccx, discr));
let (field_size, field_align) = union_size_and_align(fields);
// This can't overflow because field_size, discr_size, field_align < 1<<61
// field_align < 1<<32, discr_size <= 8, field_size < MAX_OBJ_SIZE <= 1<<61
// so the sum is less than 1<<62 (and can't overflow).
let total_size = roundup(discr_size, field_align) + field_size;
if total_size >= ccx.max_obj_size() {