https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112584
Bug ID: 112584
Summary: Suboptimal stack usage on third memcpy
Product: gcc
Version: 13.2.1
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: antoshkka at gmail dot com
Target Milestone: ---
Consider the example:
struct string_view {
const char* data;
unsigned long size;
};
void AppendToCharArray(char*& data, string_view s1, string_view s2, string_view
s3) {
__builtin_memcpy(data, s1.data, s1.size);
data += s1.size;
__builtin_memcpy(data, s2.data, s2.size);
data += s2.size;
__builtin_memcpy(data, s3.data, s3.size);
data += s3.size;
}
With -O2 it generates an assembly with 6 push and 6 pop instructions. However,
there's a better assembly possible:
push r15
push r14
push r12
push rbx
push rax
mov rbx, r8
mov r14, rcx
mov r15, rdx
mov r12, rdi
mov rdi, qword ptr [rdi]
call memcpy
add r15, qword ptr [r12]
mov qword ptr [r12], r15
mov rdi, r15
mov rsi, r14
mov rdx, rbx
call memcpy
add rbx, qword ptr [r12]
mov qword ptr [r12], rbx
mov rsi, qword ptr [rsp + 48]
mov r14, qword ptr [rsp + 56]
mov rdi, rbx
mov rdx, r14
call memcpy
add qword ptr [r12], r14
add rsp, 8
pop rbx
pop r12
pop r14
pop r15
ret
Godbolt playground: https://godbolt.org/z/EY8E1GGPz
