c++ - 双除以双和整数 : which one is better?

Question

我总是假设将 double 除以整数会导致更快的代码，因为编译器会选择更好的微码来计算:

double a;
double b = a/3.0;
double c = a/3; // will compute faster than b

对于单个操作来说无所谓，但是对于重复性操作来说就不同了。我的假设是否始终正确或取决于编译器或 CPU 或其他什么？

同样的问题适用于乘法；即 3 * a 会比 3.0 * a 更快吗？

Answer 1

您的假设不是正确的，因为您的两个除法操作都将使用两个double 操作数执行。在第二种情况下，c = a/3，整数文字将在生成任何代码之前由编译器转换为 double 值。

来自 this Draft C++ Standard :

8.3 Usual arithmetic conversions          [expr.arith.conv]

¹    Many binary operators that expect operands of arithmetic or enumeration type cause conversions and yield result types in a similar way. The purpose is to yield a common type, which is also the type of the result. This pattern is called the usual arithmetic conversions, which are defined as follows:
…
^(1.3) – Otherwise, if either operand is double, the other shall be converted to double.

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请注意，在 this Draft C11 Standard 中, §6.3.1.8(通常的算术转换)具有等效(实际上，几乎相同)的文本。