考虑以下代码
for (unsigned x = 0; x < static_cast<unsigned>(bitmap->width); ++x)
{
*p++ = 0xAA;
*p++ = 0xBB;
*p++ = 0xCC;
}
是否有这样的情况可以通过以下方式产生更有效的结果:
unsigned width(static_cast<unsigned>(bitmap->width));
for (unsigned x = 0; x < width; ++x)
{
*p++ = 0xAA;
*p++ = 0xBB;
*p++ = 0xCC;
}
发布于 2018-03-26 14:20:06
来源unoptimized.cpp
struct bitmap_t
{
long long width;
} bitmap;
int main(int argc, char** argv)
{
for (unsigned x = 0 ; x < static_cast<unsigned>(bitmap.width) ; ++x)
{
argv[x][0] = '\0';
}
return 0;
}
optimized.cpp
struct bitmap_t
{
long long width;
} bitmap;
int main(int argc, char** argv)
{
const unsigned width = static_cast<unsigned>(bitmap.width);
for (unsigned x = 0 ; x < width ; ++x)
{
argv[x][0] = '\0';
}
return 0;
}
.file "unoptimized.cpp"
.text
.p2align 4,,15
.globl main
.type main, @function
main:
.LFB0:
.cfi_startproc
.cfi_personality 0x3,__gxx_personality_v0
movl bitmap(%rip), %eax
testl %eax, %eax
je .L2
xorl %eax, %eax
.p2align 4,,10
.p2align 3
.L3:
mov %eax, %edx
addl $1, %eax
movq (%rsi,%rdx,8), %rdx
movb $0, (%rdx)
cmpl bitmap(%rip), %eax
jb .L3
.L2:
xorl %eax, %eax
ret
.cfi_endproc
.LFE0:
.size main, .-main
.globl bitmap
.bss
.align 8
.type bitmap, @object
.size bitmap, 8
bitmap:
.zero 8
.ident "GCC: (GNU) 4.4.7 20120313 (Red Hat 4.4.7-16)"
.section .note.GNU-stack,"",@progbits
.file "optimized.cpp"
.text
.p2align 4,,15
.globl main
.type main, @function
main:
.LFB0:
.cfi_startproc
.cfi_personality 0x3,__gxx_personality_v0
movl bitmap(%rip), %eax
testl %eax, %eax
je .L2
subl $1, %eax
leaq 8(,%rax,8), %rcx
xorl %eax, %eax
.p2align 4,,10
.p2align 3
.L3:
movq (%rsi,%rax), %rdx
addq $8, %rax
cmpq %rcx, %rax
movb $0, (%rdx)
jne .L3
.L2:
xorl %eax, %eax
ret
.cfi_endproc
.LFE0:
.size main, .-main
.globl bitmap
.bss
.align 8
.type bitmap, @object
.size bitmap, 8
bitmap:
.zero 8
.ident "GCC: (GNU) 4.4.7 20120313 (Red Hat 4.4.7-16)"
.section .note.GNU-stack,"",@progbits
$ diff -uN unoptimized.s optimized.s
--- unoptimized.s 2015-11-24 16:11:55.837922223 +0000
+++ optimized.s 2015-11-24 16:12:02.628922941 +0000
@@ -1,4 +1,4 @@
- .file "unoptimized.cpp"
+ .file "optimized.cpp"
.text
.p2align 4,,15
.globl main
@@ -10,16 +10,17 @@
movl bitmap(%rip), %eax
testl %eax, %eax
je .L2
+ subl $1, %eax
+ leaq 8(,%rax,8), %rcx
xorl %eax, %eax
.p2align 4,,10
.p2align 3
.L3:
- mov %eax, %edx
- addl $1, %eax
- movq (%rsi,%rdx,8), %rdx
+ movq (%rsi,%rax), %rdx
+ addq $8, %rax
+ cmpq %rcx, %rax
movb $0, (%rdx)
- cmpl bitmap(%rip), %eax
- jb .L3
+ jne .L3
.L2:
xorl %eax, %eax
ret
https://stackoverflow.com/questions/-100007787
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