http://gcc.gnu.org/bugzilla/show_bug.cgi?id=61082
Bug ID: 61082
Summary: [x86-64 Itanium ABI] g++ uses wrong return location
for class with head padding
Product: gcc
Version: 4.9.0
Status: UNCONFIRMED
Severity: major
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: greened at obbligato dot org
Created attachment 32745
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=32745&action=edit
Testcase
gcc appears to use the wrong sequence for returning an object with head
padding.
For the attached testcase, gcc -O2 generates the following code for function
foo:
.globl _Z3fooR1Y
.type _Z3fooR1Y, @function
_Z3fooR1Y:
.LFB1019:
.cfi_startproc
movq 8(%rdi), %rax
ret
.cfi_endproc
The class Y should be classified INTEGER under the x86-64 ABI. It has eight
bytes of head padding because it inherits from an empty base class and has a
member that also inherits from the same empty base class. By my reading of the
ABI, foo should return the Y object in (RAX, RDX), with RDX containing the
meaningful bits (the long value).
The Intel compiler generates the code I would expect for this testcase.
