On Mon, 15 Jun 2015, Maciej W. Rozycki wrote: > operands negated. That negation, implemented with the IEEE Std 754-2008 > `negate' operation that you referred to, by definition is required to > operate on the sign of its operand in a specific way even if the operand > is a qNaN. > > So for example `fmsM4', that is specified at the RTL level as (fma:M OP1 > OP2 (neg:M OP3)) will not produce the correct result with the fused > version of the MIPS MSUB.fmt instruction in the case where OP1 and OP2 are > numeric data patterns and OP3 is a qNaN data pattern that has its sign bit > clear. As specified by IEEE Std 754-2008 the (neg:M OP3) operation is > required to invert the sign bit of the qNaN data pattern in calculating > TMP3, and then the (fma:M OP1 OP2 TMP3) operation is required to pass the > TMP3 qNaN data pattern unchanged in calculating the final result.
It is only required (well, recommended) to pass the *payload*. The sign bit is not part of the payload. "For all other operations, this standard does not specify the sign bit of a NaN result, even when there is only one input NaN, or when the NaN is produced from an invalid operation.". -- Joseph S. Myers jos...@codesourcery.com
