[英]Floating point number comparison trick: inline assembly
A long time ago, I've used this simple x86 assembler trick to obtain 0 or 1 as a result of floating point number comparison: 很久以前,由于浮点数比较的结果,我使用了这个简单的x86汇编技巧,以获得0或1:
fld [value1]
fcom [value2]
fnstsw ax
mov al, ah
and eax, 1
This trick allows to avoid branching if comparison result only affects selection of a value from a set of 2 values. 如果比较结果仅影响从一组2个值中选择一个值,则此技巧可以避免分支。 It was fast in Pentium days, now it may not be so much faster, but who knows.
在Pentium时代速度很快,现在可能没有那么快,但是谁知道呢。
Now I mainly use C++ and compile using Intel C++ Compiler or GCC C++ Compiler. 现在,我主要使用C ++,并使用Intel C ++编译器或GCC C ++编译器进行编译。
Can someone please help rewrite this code into 2 built-in assembler flavors (Intel and GCC). 有人可以帮忙将此代码重写为2种内置汇编程序样式(Intel和GCC)。
The required function prototype is: inline int compareDoublesIndexed( const double value1, const double value2 ) { ... } 所需的函数原型为:内联int compareDoublesIndexed(const double value1,const double value2){...}
Maybe using SSE2 operations could be even more efficient. 也许使用SSE2操作可能更加有效。 Your perspective?
你的观点?
I've tried this: 我已经试过了:
__asm__(
"fcomq %2, %0\n"
"fnstsw %ax\n"
"fsubq %2, %0\n"
"andq $L80, %eax\n"
"shrq $5, %eax\n"
"fmulq (%3,%eax), %0\n"
: "=f" (penv)
: "0" (penv), "F" (env), "r" (c)
: "eax" );
But I get error in Intel C++ Compiler: Floating point output constraint must specify a single register. 但是我在Intel C ++编译器中遇到错误:浮点输出约束必须指定一个寄存器。
As you mentioned, things have changed since the Pentium days: 正如您提到的,自奔腾时代以来,情况已经发生了变化:
Therefore first check what the compiler generates, you might be pleasantly surprised. 因此,首先检查编译器生成的内容,您可能会感到惊喜。 I tried g++ with
-O3
on the following code 我在以下代码上用
-O3
尝试了g ++
fcmp.cpp: fcmp.cpp:
int compareDoublesIndexed( const double value1, const double value2 ) {
return value1 < value2 ? 1 : 0;
}
This is what the compiler generated 这就是编译器生成的
0000000000400690 <_Z21compareDoublesIndexeddd>:
400690: 31 c0 xor %eax,%eax
400692: 66 0f 2e c8 ucomisd %xmm0,%xmm1
400696: 0f 97 c0 seta %al
400699: c3 retq
This is what it means 这是什么意思
xor %eax,%eax ; EAX = 0
ucomisd %xmm0,%xmm1 ; compare value2 (in %xmm1) with value1 (in %xmm0)
seta %al ; AL = value2 > value1 ? 1 : 0
So the compiler avoided the conditional branch by using the seta
instruction (set byte to '1' if result is above, to '0' otherwise). 因此,编译器通过使用
seta
指令来避免条件分支(如果结果高于字节,则将字节设置为“ 1”,否则,将字节设置为“ 0”)。
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