SkipLocalsInit attribute
Use SkipLocalsInitAttribute to tell Burst that any stack allocations within a method don't have to be initialized to zero.
In C# all local variables are initialized to zero by default, which prevents a class of bugs related to undefined data. But this can impact runtime performance because initializing this data to zero takes work:
static unsafe int DoSomethingWithLUT(int* data);
static unsafe int DoSomething(int size)
{
int* data = stackalloc int[size];
// Initialize every field of data to be an incrementing set of values.
for (int i = 0; i < size; i++)
{
data[i] = i;
}
// Use the data elsewhere.
return DoSomethingWithLUT(data);
}
The X86 assembly for this is:
push rbp
.seh_pushreg rbp
push rsi
.seh_pushreg rsi
push rdi
.seh_pushreg rdi
mov rbp, rsp
.seh_setframe rbp, 0
.seh_endprologue
mov edi, ecx
lea r8d, [4*rdi]
lea rax, [r8 + 15]
and rax, -16
movabs r11, offset __chkstk
call r11
sub rsp, rax
mov rsi, rsp
sub rsp, 32
movabs rax, offset burst.memset.inline.X64_SSE4.i32@@32
mov rcx, rsi
xor edx, edx
xor r9d, r9d
call rax
add rsp, 32
test edi, edi
jle .LBB0_7
mov eax, edi
cmp edi, 8
jae .LBB0_3
xor ecx, ecx
jmp .LBB0_6
.LBB0_3:
mov ecx, eax
and ecx, -8
movabs rdx, offset __xmm@00000003000000020000000100000000
movdqa xmm0, xmmword ptr [rdx]
mov rdx, rsi
add rdx, 16
movabs rdi, offset __xmm@00000004000000040000000400000004
movdqa xmm1, xmmword ptr [rdi]
movabs rdi, offset __xmm@00000008000000080000000800000008
movdqa xmm2, xmmword ptr [rdi]
mov rdi, rcx
.p2align 4, 0x90
.LBB0_4:
movdqa xmm3, xmm0
paddd xmm3, xmm1
movdqu xmmword ptr [rdx - 16], xmm0
movdqu xmmword ptr [rdx], xmm3
paddd xmm0, xmm2
add rdx, 32
add rdi, -8
jne .LBB0_4
cmp rcx, rax
je .LBB0_7
.p2align 4, 0x90
.LBB0_6:
mov dword ptr [rsi + 4*rcx], ecx
inc rcx
cmp rax, rcx
jne .LBB0_6
.LBB0_7:
sub rsp, 32
movabs rax, offset "DoSomethingWithLUT"
mov rcx, rsi
call rax
nop
mov rsp, rbp
pop rdi
pop rsi
pop rbp
ret
In this example, the movabs rax, offset burst.memset.inline.X64_SSE4.i32@@32 line means that you've had to inject a memset to zero out the data. In the above example, you know that the array is entirely initialized in the following loop, but Burst doesn't know that.
To fix this problem, use Unity.Burst.CompilerServices.SkipLocalsInitAttribute, which tells Burst that any stack allocations within a method don't have to be initialized to zero.
Note
Only use this attribute if you're certain that you won't run into undefined behavior bugs.
For example:
using Unity.Burst.CompilerServices;
static unsafe int DoSomethingWithLUT(int* data);
[SkipLocalsInit]
static unsafe int DoSomething(int size)
{
int* data = stackalloc int[size];
// Initialize every field of data to be an incrementing set of values.
for (int i = 0; i < size; i++)
{
data[i] = i;
}
// Use the data elsewhere.
return DoSomethingWithLUT(data);
}
The assembly after adding the [SkipLocalsInit] on the method is:
push rbp
.seh_pushreg rbp
mov rbp, rsp
.seh_setframe rbp, 0
.seh_endprologue
mov edx, ecx
lea eax, [4*rdx]
add rax, 15
and rax, -16
movabs r11, offset __chkstk
call r11
sub rsp, rax
mov rcx, rsp
test edx, edx
jle .LBB0_7
mov r8d, edx
cmp edx, 8
jae .LBB0_3
xor r10d, r10d
jmp .LBB0_6
.LBB0_3:
mov r10d, r8d
and r10d, -8
movabs rax, offset __xmm@00000003000000020000000100000000
movdqa xmm0, xmmword ptr [rax]
mov rax, rcx
add rax, 16
movabs rdx, offset __xmm@00000004000000040000000400000004
movdqa xmm1, xmmword ptr [rdx]
movabs rdx, offset __xmm@00000008000000080000000800000008
movdqa xmm2, xmmword ptr [rdx]
mov r9, r10
.p2align 4, 0x90
.LBB0_4:
movdqa xmm3, xmm0
paddd xmm3, xmm1
movdqu xmmword ptr [rax - 16], xmm0
movdqu xmmword ptr [rax], xmm3
paddd xmm0, xmm2
add rax, 32
add r9, -8
jne .LBB0_4
cmp r10, r8
je .LBB0_7
.p2align 4, 0x90
.LBB0_6:
mov dword ptr [rcx + 4*r10], r10d
inc r10
cmp r8, r10
jne .LBB0_6
.LBB0_7:
sub rsp, 32
movabs rax, offset "DoSomethingWithLUT"
call rax
nop
mov rsp, rbp
pop rbp
ret
The call to memset is now gone, because you've told Burst that any stack allocations within a method don't have to be initialized to zero.