https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82888
Bug ID: 82888
Summary: terrible code generation for initialization of POD
array members vs. clang
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: froydnj at gcc dot gnu.org
CC: jseward at acm dot org, mh+gcc at glandium dot org
Target Milestone: ---
Consider the testcase:
class A
{
public:
A();
private:
unsigned char mStorage[4096];
};
A::A()
: mStorage()
{}
gcc -O2 generates a loop that looks like:
.L2:
movb $0, (%rdi)
addq $1, %rdi
cmpq %rdi, %rax
jne .L2
which is terribly slow. (The original motivation for this bug report came from
an -Og compilation, where you get:
movl $4095, %eax
.L3:
testq %rax, %rax
js .L1
movb $0, (%rdi)
addq $1, %rdi
subq $1, %rax
jmp .L3
which is even worse.)
clang -O2 (or -O1), on the other hand, generates:
xorl %esi, %esi
movl $4096, %edx # imm = 0x1000
jmp memset # TAILCALL
which is ideal, and opens up opportunities for the backend to lower the memset
to something intelligent (e.g. rep stos)
