http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50384
Bug #: 50384
Summary: Copying a char array
Classification: Unclassified
Product: gcc
Version: 4.7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: other
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: marc.gli...@normalesup.org
#include <array>
typedef std::array<char,7> X;
X f(X,X);
X g(X a,X b){return f(a,b);}
I compiled this code on x86_64 with g++ -std=c++0x -Ofast and got:
_Z1gSt5arrayIcLm7EES0_:
.LFB837:
.cfi_startproc
pushq %rbx
.cfi_def_cfa_offset 16
subq $48, %rsp
.cfi_def_cfa_offset 64
.cfi_offset 3, -16
call _Z1fSt5arrayIcLm7EES0_
movzbl %ah, %edx
movb %al, (%rsp)
movb %dl, 1(%rsp)
movq %rax, %rdx
shrq $16, %rdx
movb %dl, 2(%rsp)
movq %rax, %rdx
shrq $24, %rdx
movb %dl, 3(%rsp)
movq %rax, %rdx
movl (%rsp), %ecx
shrq $32, %rdx
movb %dl, 4(%rsp)
movq %rax, %rdx
shrq $40, %rdx
movl %ecx, %esi
movzbl %cl, %ebx
movb %dl, 5(%rsp)
movl %ecx, %edx
movzwl 4(%rsp), %edi
shrl $16, %edx
movzbl %ch, %ecx
shrl $24, %esi
movzbl %dl, %edx
movb %cl, %bh
movzbl %sil, %esi
salq $16, %rdx
salq $24, %rsi
movabsq $-1095216660481, %rcx
orq %rbx, %rdx
movq %rdi, %rbx
addq $48, %rsp
.cfi_def_cfa_offset 16
orq %rsi, %rdx
movzbl %dil, %esi
salq $32, %rsi
andq %rcx, %rdx
movzbl %bh, %ecx
orq %rsi, %rdx
movabsq $-280375465082881, %rsi
salq $40, %rcx
andq %rsi, %rdx
orq %rcx, %rdx
movabsq $71776119061217280, %rcx
andq %rcx, %rax
movabsq $-71776119061217281, %rcx
andq %rcx, %rdx
orq %rax, %rdx
movq %rdx, %rax
popq %rbx
.cfi_def_cfa_offset 8
ret
.cfi_endproc
Ideally I would have liked a single jmp, but in any case this seems a bit
long... (the attribute((aligned)) in versions <= 4.2 did help)
Is that the best that can legally be done without alignment information?
