如何选择AVX比较谓词变体 [英] How to choose AVX compare predicate variants
问题描述
In the Advanced Vector Extensions (AVX) the compare instructions like _m256_cmp_ps, the last argument is a compare predicate. The choices for the predicate overwhelm me. They seem to be a tripple of type, ordering, signaling. E.g. _CMP_LE_OS is 'less than or equal, ordered, signaling.
对于初学者来说,是否出于性能原因选择信令或非信令, 同样,有序或无序比另一个更快?
For starters, is there a performance reason for selecting signaling or non signaling, and similarly, is ordered or unordered faster than the other?
不发信号"甚至意味着什么? 我根本找不到在文档中. 何时选择什么有什么经验法则吗?
And what does 'non signaling' even mean? I can't find this in the docs at all. Any rule of thumb on when to select what?
以下是来自avxintrin.h的谓词选择:
Here are the predicate choices from avxintrin.h:
/* Compare */
#define _CMP_EQ_OQ 0x00 /* Equal (ordered, non-signaling) */
#define _CMP_LT_OS 0x01 /* Less-than (ordered, signaling) */
#define _CMP_LE_OS 0x02 /* Less-than-or-equal (ordered, signaling) */
#define _CMP_UNORD_Q 0x03 /* Unordered (non-signaling) */
#define _CMP_NEQ_UQ 0x04 /* Not-equal (unordered, non-signaling) */
#define _CMP_NLT_US 0x05 /* Not-less-than (unordered, signaling) */
#define _CMP_NLE_US 0x06 /* Not-less-than-or-equal (unordered, signaling) */
#define _CMP_ORD_Q 0x07 /* Ordered (nonsignaling) */
#define _CMP_EQ_UQ 0x08 /* Equal (unordered, non-signaling) */
#define _CMP_NGE_US 0x09 /* Not-greater-than-or-equal (unord, signaling) */
#define _CMP_NGT_US 0x0a /* Not-greater-than (unordered, signaling) */
#define _CMP_FALSE_OQ 0x0b /* False (ordered, non-signaling) */
#define _CMP_NEQ_OQ 0x0c /* Not-equal (ordered, non-signaling) */
#define _CMP_GE_OS 0x0d /* Greater-than-or-equal (ordered, signaling) */
#define _CMP_GT_OS 0x0e /* Greater-than (ordered, signaling) */
#define _CMP_TRUE_UQ 0x0f /* True (unordered, non-signaling) */
#define _CMP_EQ_OS 0x10 /* Equal (ordered, signaling) */
#define _CMP_LT_OQ 0x11 /* Less-than (ordered, non-signaling) */
#define _CMP_LE_OQ 0x12 /* Less-than-or-equal (ordered, non-signaling) */
#define _CMP_UNORD_S 0x13 /* Unordered (signaling) */
#define _CMP_NEQ_US 0x14 /* Not-equal (unordered, signaling) */
#define _CMP_NLT_UQ 0x15 /* Not-less-than (unordered, non-signaling) */
#define _CMP_NLE_UQ 0x16 /* Not-less-than-or-equal (unord, non-signaling) */
#define _CMP_ORD_S 0x17 /* Ordered (signaling) */
#define _CMP_EQ_US 0x18 /* Equal (unordered, signaling) */
#define _CMP_NGE_UQ 0x19 /* Not-greater-than-or-equal (unord, non-sign) */
#define _CMP_NGT_UQ 0x1a /* Not-greater-than (unordered, non-signaling) */
#define _CMP_FALSE_OS 0x1b /* False (ordered, signaling) */
#define _CMP_NEQ_OS 0x1c /* Not-equal (ordered, signaling) */
#define _CMP_GE_OQ 0x1d /* Greater-than-or-equal (ordered, non-signaling) */
#define _CMP_GT_OQ 0x1e /* Greater-than (ordered, non-signaling) */
#define _CMP_TRUE_US 0x1f /* True (unordered, signaling) */
推荐答案
有序与无序有关,如果其中一个操作数包含NaN,则比较是否为真(请参阅
Ordered vs Unordered has to do with whether the comparison is true if one of the operands contains a NaN (see What does ordered / unordered comparison mean?). Signaling (S) vs non-signaling (Q for quiet?) will determine whether an exception is raised if an operand contains a NaN.
从性能的角度来看,这些都应该是相同的(假设当然不会引发异常).如果要在出现NaN时收到警报,则需要发信号.至于有序与无序,这完全取决于您要如何处理NaN.
From a performance perspective, these should all be the same (assuming of course no exceptions are raised). If you want to be alerted when there's a NaN, then you want signaling. As for ordered vs unordered, it all depends on how you want to deal with NaNs.
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