什么是有序和无序LLVM CmpInst比较指令

Question

"llvm/IR/InstrTypes.h"中CmpInst::Predicate类型的定义描述了LLVM中比较指令的类型:

enum Predicate {
  // Opcode              U L G E    Intuitive operation
  FCMP_FALSE =  0,  ///< 0 0 0 0    Always false (always folded)
  FCMP_OEQ   =  1,  ///< 0 0 0 1    True if ordered and equal
  FCMP_OGT   =  2,  ///< 0 0 1 0    True if ordered and greater than
  FCMP_OGE   =  3,  ///< 0 0 1 1    True if ordered and greater than or equal
  FCMP_OLT   =  4,  ///< 0 1 0 0    True if ordered and less than
  FCMP_OLE   =  5,  ///< 0 1 0 1    True if ordered and less than or equal
  FCMP_ONE   =  6,  ///< 0 1 1 0    True if ordered and operands are unequal
  FCMP_ORD   =  7,  ///< 0 1 1 1    True if ordered (no nans)
  FCMP_UNO   =  8,  ///< 1 0 0 0    True if unordered: isnan(X) | isnan(Y)
  FCMP_UEQ   =  9,  ///< 1 0 0 1    True if unordered or equal
  FCMP_UGT   = 10,  ///< 1 0 1 0    True if unordered or greater than
  FCMP_UGE   = 11,  ///< 1 0 1 1    True if unordered, greater than, or equal
  FCMP_ULT   = 12,  ///< 1 1 0 0    True if unordered or less than
  FCMP_ULE   = 13,  ///< 1 1 0 1    True if unordered, less than, or equal
  FCMP_UNE   = 14,  ///< 1 1 1 0    True if unordered or not equal
  FCMP_TRUE  = 15,  ///< 1 1 1 1    Always true (always folded)
  FIRST_FCMP_PREDICATE = FCMP_FALSE,
  LAST_FCMP_PREDICATE = FCMP_TRUE,
  BAD_FCMP_PREDICATE = FCMP_TRUE + 1,
  ICMP_EQ    = 32,  ///< equal
  ICMP_NE    = 33,  ///< not equal
  ICMP_UGT   = 34,  ///< unsigned greater than
  ICMP_UGE   = 35,  ///< unsigned greater or equal
  ICMP_ULT   = 36,  ///< unsigned less than
  ICMP_ULE   = 37,  ///< unsigned less or equal
  ICMP_SGT   = 38,  ///< signed greater than
  ICMP_SGE   = 39,  ///< signed greater or equal
  ICMP_SLT   = 40,  ///< signed less than
  ICMP_SLE   = 41,  ///< signed less or equal
  FIRST_ICMP_PREDICATE = ICMP_EQ,
  LAST_ICMP_PREDICATE = ICMP_SLE,
  BAD_ICMP_PREDICATE = ICMP_SLE + 1
};

我想知道什么是"有序"one_answers"无序"谓词(如"如果有序且相等"或"如果无序，大于或等于"与正常的"unsigned greater or equal"相比)。

Answer 1

如果你不知道NaN是什么，从最后一段开始:)

如果至少有一个操作数是NaN，则有序浮点数比较和无序浮点数比较的结果是不同的。有关更多细节，请参阅IR lang参考中的fcmp说明。特别是这句话很重要:"有序意味着两个操作数都不是QNAN，而无序意味着两个操作数都可能是QNAN"。注意LLVM (AFAIK)不支持SNaN，这就是为什么lang只谈论QNaN。

命名的原因是NaN不能与浮点数进行比较。你不能说NaN小于或大于0。所以NaN是无序的。因此，如果其中一个操作数是NaN，则无序比较返回true。有序比较要求两个操作数都是数字。

这是关于NaN的维基百科页面，如果你需要背景知识的话。简单地说，当某些浮点计算的结果不是一个数字时，就会生成一个特殊的结果，称为NaN (not a number)。例如，如果您要求负数的平方根，' std::sqrt'将生成NaN。NaN有两种变体。SNaN和QNan。维基百科是这样描述的。对于您的问题，您可以忽略差异，因为只有QNaN重要。LLVM AFAIK不支持SNaN