https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125596
Bug ID: 125596
Summary: i386: redundant register swapping with TImode values
Product: gcc
Version: 14.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: kim.kuparinen at tuni dot fi
Target Milestone: ---
After commit https://gcc.gnu.org/g:8911879415d6c2a7baad88235554a912887a1c5c,
code such as
typedef struct Point {
long x;
long y;
} Point;
long takes_point_struct(Point p);
long calls_point_struct(long x, long y)
{
return takes_point_struct((Point){.x = x, .y = y});
}
when compiled with -O2 -c on x86_64 produces
0000000000000000 <calls_point_struct>:
0: 48 87 fe xchg %rdi,%rsi
3: 48 89 f0 mov %rsi,%rax
6: 48 89 fe mov %rdi,%rsi
9: 48 89 c7 mov %rax,%rdi
c: e9 00 00 00 00 jmp 11 <calls_point_struct+0x11>
where the first four instructions just swap %rdi and %rsi with xchg and then
swap them back with regular moves. Without the above commit, GCC produces
0000000000000000 <calls_point_struct>:
0: e9 00 00 00 00 jmp 5 <calls_point_struct+0x5>
which is also what Debian clang 16.0.6 (15~deb12u1) produces. Initially found
on Debian gcc 14.2.0-19, but issue persists in current master,
https://gcc.gnu.org/g:0d82756a365ee6189af4490345bc0d2595007c84 as of reporting.
The commit in question references bug 88873 and only touches i386 code.
