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xiaofeibao-xjtu avatar xiaofeibao-xjtu commented on September 25, 2024

这两种情况正确结果都应该是负无穷0xff800000,NEMU的判断可能有问题

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Jishuaishuai123 avatar Jishuaishuai123 commented on September 25, 2024

这两种情况正确结果都应该是负无穷0xff800000,NEMU的判断可能有问题

Hi 您好:我刚下载了2022.12.22版本,尝试运行了一下,这个问题还是存在的

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xiaofeibao-xjtu avatar xiaofeibao-xjtu commented on September 25, 2024

fnmadd.s计算-(rs1rs2)-rs3
现在我们把rs2符号反向,rs3符号反向,变成(rs1
(-rs2))+(-rs3)
构造新的fma操作数a=rs1,b=-rs2,c=-rs3,计算(ab)+c
a:0x4f057641
b:0xf0000000
c:0x7f057641
a
b是不会舍入的,是否溢出要看(ab)+c整体的值
以下编码不考虑溢出:
(a
b)=0xff857641,c=0x7f057641,(ab)是负数比负无穷还小,但是c是正数,
并且-2c=2(-c)=20xff057641=0xff857641=(a
b),
所以(ab)+c=-c=0xff057641,
结果是精确的,和舍入模式也无关。
原先我回复溢出是我理解错了fnmadd.s,以为计算的-(rs1
rs2)+rs3。

目前结论NEMU计算结果是正确的,也单独对南湖fudian里面的FCMA硬件模块进行了测试,结果和NEMU一致。问题还待进一步复现确定。。。

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