https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67283
Bug ID: 67283
Summary: GCC regression over inlining of returned structures
Product: gcc
Version: 5.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: roche at httrack dot com
Target Milestone: ---
Created attachment 36219
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=36219&action=edit
Sample test case (gcc -S -O3 -W -Wall)
An optimization bug regression appears to exist when dealing with structures
returned by inlined functions. This was working totally fine with GCC 4.4.7.
(see below)
A typical example is:
struct foo {
int flags;
/* Let it be enough NOT to be packed in registers */
void *opaque[2];
};
static __inline__ struct foo add_flag(struct foo foo, int flag) {
foo.flags |= flag;
return foo;
}
Calls to "add_flag" are inlined, but the stack usage increases with latest GCC
versions (the code should be almost identical, except the flag in place in the
stack).
Tested the following GCC versions: (grep "addq.*%rsp" to get stack usage for
each function) ; tested architecture: x86-64
GCC 4.4.7: OK
addq $72, %rsp # demo_1
addq $72, %rsp
addq $72, %rsp
addq $72, %rsp
addq $72, %rsp # demo_5
GCC 4.5.3 to 4.6.4: NOK (UNTESTED between 4.4.8 to 4.5.2)
addq $72, %rsp # demo_1
addq $136, %rsp
addq $168, %rsp
addq $200, %rsp
addq $232, %rsp # demo_5
GCC 4.7.3 to 5.2.0: NOK (UNTESTED between 4.6.5 to 4.7.2)
addq $72, %rsp # demo_1
addq $136, %rsp
addq $200, %rsp
addq $264, %rsp
addq $328, %rsp # demo_5
Therefore, the test case was fine in GCC 4.4.7, first degraded between 4.4.8
and 4.5.3, and then again between 4.6.5 and 4.7.3
Note: code produced with http://gcc.godbolt.org/ with -O3 -W -Wall flags.
(same results with -01)
