https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93721
Bug ID: 93721
Summary: swapping adjacent scalars could be more efficient
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: msebor at gcc dot gnu.org
Target Milestone: ---
For an implementation of a swap function like this:
template <class T>
void swap (std::pair<T, T> &x)
{
T t = x.first;
x.first = x.second;
x.second = t;
}
GCC for x86 emits the ROL instruction for T=char:
_Z4swapIcEvRSt4pairIT_S1_E:
.LFB97:
.cfi_startproc
rolw $8, (%rdi)
ret
.cfi_endproc
but a series of MOV instructions for T=short and T=int:
_Z4swapIiEvRSt4pairIT_S1_E:
.LFB97:
.cfi_startproc
movl (%rdi), %eax
movl 4(%rdi), %edx
movl %eax, 4(%rdi)
movl %edx, (%rdi)
ret
.cfi_endproc
A hand-coded (but convoluted) implementation of the function like below lets
GCC for x86_64 emit the ROL instruction for both int and short:
void swap (std::pair<int, int> &x)
{
int y[2], t;
static_assert (sizeof x == sizeof y);
__builtin_memcpy (y, &x, sizeof x);
t = y[0]; y[0] = y[1]; y[1] = t;
__builtin_memcpy (&x, y, sizeof x);
}
_ZL4swapRSt4pairIiiE:
.LFB94:
.cfi_startproc
rolq $32, (%rdi)
ret
.cfi_endproc
Benchmarking it shows that the ROL form is measurably faster (at least on my
machine) than the MOV form.
