http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59719
Bug ID: 59719
Summary: Too much space allocated to unions containing variable
length arrays
Product: gcc
Version: 4.9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: nickc at gcc dot gnu.org
Created attachment 31771
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=31771&action=edit
test case to demonstrate the problem - effective on targets which use section
anchors
When gcc emits assembler directives to describe a union containing a variable
length array it tries to ensure that the end of the union is correctly aligned,
but it ends up emitting an incorrectly sized .zero pseudo-op.
For example in the uploaded testcase, when compiled with an x86 toolchain, the
variable first_var_union is described in the assembler output as:
.type first_var_union, @object
.size first_var_union, 20
first_var_union:
.zero 4
.string "12-octet-str"
.zero 16
Note how the size of the variable is set to 20, but the actual size is
4+13+16=33 bytes. Presumably the second .zero directive should have been
".zero 3".
This might just be a curiosity were it not for the fact that it makes any
following variables unaligned (unless the size of the union just happens to fir
into an alignment category). Plus for targets which use section anchors (eg
aarch64, arm, ppc, mips), the computation of the offset to variables beyond the
variable length union will be based upon the expected size (20) not the real
size (33).
