为什么加入装配意见导致产生code这样彻底的改变？ [英] Why does adding assembly comments cause such radical change in generated code?
问题描述
所以，我有这个code：
So, I had this code:
constexpr unsigned N = 1000;
void f1(char* sum, char* a, char* b) {
for(int i = 0; i < N; ++i) {
sum[i] = a[i] + b[i];
}
}
void f2(char* sum, char* a, char* b) {
char* end = sum + N;
while(sum != end) {
*sum++ = *a++ + *b++;
}
}
我想看看code的GCC 4.7.2将产生。于是我就 G ++ -march =本地-O3 -masm =英特尔-S变交流++ -std = C ++ 11 键，得到了以下的输出：
I wanted to see the code that GCC 4.7.2 would generate. So I ran g++ -march=native -O3 -masm=intel -S a.c++ -std=c++11 And got the following output:
.file "a.c++"
.intel_syntax noprefix
.text
.p2align 4,,15
.globl _Z2f1PcS_S_
.type _Z2f1PcS_S_, @function
_Z2f1PcS_S_:
.LFB0:
.cfi_startproc
lea rcx, [rdx+16]
lea rax, [rdi+16]
cmp rdi, rcx
setae r8b
cmp rdx, rax
setae cl
or cl, r8b
je .L5
lea rcx, [rsi+16]
cmp rdi, rcx
setae cl
cmp rsi, rax
setae al
or cl, al
je .L5
xor eax, eax
.p2align 4,,10
.p2align 3
.L3:
movdqu xmm0, XMMWORD PTR [rdx+rax]
movdqu xmm1, XMMWORD PTR [rsi+rax]
paddb xmm0, xmm1
movdqu XMMWORD PTR [rdi+rax], xmm0
add rax, 16
cmp rax, 992
jne .L3
mov ax, 8
mov r9d, 992
.L2:
sub eax, 1
lea rcx, [rdx+r9]
add rdi, r9
lea r8, [rax+1]
add rsi, r9
xor eax, eax
.p2align 4,,10
.p2align 3
.L4:
movzx edx, BYTE PTR [rcx+rax]
add dl, BYTE PTR [rsi+rax]
mov BYTE PTR [rdi+rax], dl
add rax, 1
cmp rax, r8
jne .L4
rep
ret
.L5:
mov eax, 1000
xor r9d, r9d
jmp .L2
.cfi_endproc
.LFE0:
.size _Z2f1PcS_S_, .-_Z2f1PcS_S_
.p2align 4,,15
.globl _Z2f2PcS_S_
.type _Z2f2PcS_S_, @function
_Z2f2PcS_S_:
.LFB1:
.cfi_startproc
lea rcx, [rdx+16]
lea rax, [rdi+16]
cmp rdi, rcx
setae r8b
cmp rdx, rax
setae cl
or cl, r8b
je .L19
lea rcx, [rsi+16]
cmp rdi, rcx
setae cl
cmp rsi, rax
setae al
or cl, al
je .L19
xor eax, eax
.p2align 4,,10
.p2align 3
.L17:
movdqu xmm0, XMMWORD PTR [rdx+rax]
movdqu xmm1, XMMWORD PTR [rsi+rax]
paddb xmm0, xmm1
movdqu XMMWORD PTR [rdi+rax], xmm0
add rax, 16
cmp rax, 992
jne .L17
add rdi, 992
add rsi, 992
add rdx, 992
mov r8d, 8
.L16:
xor eax, eax
.p2align 4,,10
.p2align 3
.L18:
movzx ecx, BYTE PTR [rdx+rax]
add cl, BYTE PTR [rsi+rax]
mov BYTE PTR [rdi+rax], cl
add rax, 1
cmp rax, r8
jne .L18
rep
ret
.L19:
mov r8d, 1000
jmp .L16
.cfi_endproc
.LFE1:
.size _Z2f2PcS_S_, .-_Z2f2PcS_S_
.ident "GCC: (GNU) 4.7.2"
.section .note.GNU-stack,"",@progbits
我吸在读装配，所以我决定加一些标记知道在哪里环路的机构去：
I suck at reading assembly, so I decided to add some markers to know where the bodies of the loops went:
constexpr unsigned N = 1000;
void f1(char* sum, char* a, char* b) {
for(int i = 0; i < N; ++i) {
asm("# im in ur loop");
sum[i] = a[i] + b[i];
}
}
void f2(char* sum, char* a, char* b) {
char* end = sum + N;
while(sum != end) {
asm("# im in ur loop");
*sum++ = *a++ + *b++;
}
}
和GCC吐出了这一点：
And GCC spat this out:
.file "a.c++"
.intel_syntax noprefix
.text
.p2align 4,,15
.globl _Z2f1PcS_S_
.type _Z2f1PcS_S_, @function
_Z2f1PcS_S_:
.LFB0:
.cfi_startproc
xor eax, eax
.p2align 4,,10
.p2align 3
.L2:
#APP
# 4 "a.c++" 1
# im in ur loop
# 0 "" 2
#NO_APP
movzx ecx, BYTE PTR [rdx+rax]
add cl, BYTE PTR [rsi+rax]
mov BYTE PTR [rdi+rax], cl
add rax, 1
cmp rax, 1000
jne .L2
rep
ret
.cfi_endproc
.LFE0:
.size _Z2f1PcS_S_, .-_Z2f1PcS_S_
.p2align 4,,15
.globl _Z2f2PcS_S_
.type _Z2f2PcS_S_, @function
_Z2f2PcS_S_:
.LFB1:
.cfi_startproc
xor eax, eax
.p2align 4,,10
.p2align 3
.L6:
#APP
# 12 "a.c++" 1
# im in ur loop
# 0 "" 2
#NO_APP
movzx ecx, BYTE PTR [rdx+rax]
add cl, BYTE PTR [rsi+rax]
mov BYTE PTR [rdi+rax], cl
add rax, 1
cmp rax, 1000
jne .L6
rep
ret
.cfi_endproc
.LFE1:
.size _Z2f2PcS_S_, .-_Z2f2PcS_S_
.ident "GCC: (GNU) 4.7.2"
.section .note.GNU-stack,"",@progbits
这是相当短，且具有一样缺乏SIMD指令一些显著差异。我期待的地方相同的输出，有一些评论在它的中间。我在做一些错误的假设吗？是gcc的优化由ASM意见阻碍？
This is considerably shorter, and has some significant differences like the lack of SIMD instructions. I was expecting the same output, with some comments somewhere in the middle of it. Am I making some wrong assumption here? Is GCC's optimizer hindered by asm comments?
推荐答案
与最佳化的相互中途解释有关下来的汇编指令用C防爆pression操作数文档页面。
The interactions with optimisations are explained about halfway down the "Assembler Instructions with C Expression Operands" page in the documentation.
GCC不试着去了解任何内部的实际装配的 ASM ;它知道有关内容的唯一的事情就是你（可选）告诉它的输出和输入操作规范和寄存器撞名单。
GCC doesn't try to understand any of the actual assembly inside the asm; the only thing it knows about the content is what you (optionally) tell it in the output and input operand specification and the register clobber list.
在特别注意：
这是 ASM 指令将被同等对待，以挥发性 ASM 指令。
An
asminstruction without any output operands will be treated identically to a volatileasminstruction.
和
的挥发性关键字表示指令有重要的副作用[...]
The
volatilekeyword indicates that the instruction has important side-effects [...]
做的presence的 ASM 里面的循环已经禁止的矢量图形优化，因为GCC假设它有副作用。
So the presence of the asm inside your loop has inhibited a vectorisation optimisation, because GCC assumes it has side effects.
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