https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93737
Bug ID: 93737
Summary: inline memmove for insertion into small arrays
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: msebor at gcc dot gnu.org
Target Milestone: ---
GCC emits what's likely more efficient code for the insertion of elements into
the middle of small arrays by copying parts of the array into a temporary
buffer than it does for straight calls to memmove that achieve the same result
without the temporary buffer.
The example below shows the difference. Clang emits the same, presumably
optimally efficient, code for both functions as GCC does for f0.
$ cat t.c && gcc -DN=32 -O2 -S -Wall -o/dev/stdout t.c
int a[N];
void f0 (int x)
{
int b[N];
__builtin_memcpy (b, a + 1, sizeof a - sizeof *a);
__builtin_memcpy (a + 2, b, sizeof a - 2 * sizeof *a);
a[1] = x;
}
void f2 (int x)
{
__builtin_memmove (a + 2, a + 1, sizeof a - 2 * sizeof *a);
a[1] = x;
}
.file "t.c"
.text
.p2align 4
.globl f0
.type f0, @function
f0:
.LFB0:
.cfi_startproc
subq $16, %rsp
.cfi_def_cfa_offset 24
movdqu a+20(%rip), %xmm5
movdqu a+36(%rip), %xmm4
movdqu a+52(%rip), %xmm3
movdqu a+68(%rip), %xmm2
movdqu a+84(%rip), %xmm1
movdqu a+100(%rip), %xmm0
movups %xmm5, a+24(%rip)
movq a+116(%rip), %rax
movdqu a+4(%rip), %xmm6
movups %xmm4, a+40(%rip)
movl %edi, a+4(%rip)
movq %rax, a+120(%rip)
movups %xmm6, a+8(%rip)
movups %xmm3, a+56(%rip)
movups %xmm2, a+72(%rip)
movups %xmm1, a+88(%rip)
movups %xmm0, a+104(%rip)
addq $16, %rsp
.cfi_def_cfa_offset 8
ret
.cfi_endproc
.LFE0:
.size f0, .-f0
.p2align 4
.globl f2
.type f2, @function
f2:
.LFB1:
.cfi_startproc
pushq %rbx
.cfi_def_cfa_offset 16
.cfi_offset 3, -16
movl $120, %edx
movl %edi, %ebx
movl $a+4, %esi
movl $a+8, %edi
call memmove
movl %ebx, a+4(%rip)
popq %rbx
.cfi_def_cfa_offset 8
ret
.cfi_endproc
.LFE1:
.size f2, .-f2
.globl a
.bss
.align 32
.type a, @object
.size a, 128
a:
.zero 128
.ident "GCC: (GNU) 10.0.1 20200212 (experimental)"
.section .note.GNU-stack,"",@progbits
