考虑以下具有五个指令I1至I5的代码序列。这些指令中的每一个都具有以下格式。
OP Ri, Rj, Rk 其中对寄存器Rj和Rk的内容执行操作OP，并将结果存储在寄存器Ri中。
I1 : ADD R1, R2, R3
I2 : MUL R7, R1, R3
I3 : SUB R4, R1, R5
I4 : ADD R3, R2, R4
I5 : MUL R7, R8, R9 考虑以下三个语句：
S1: There is an anti-dependence between instructions I2 and I5.
S2: There is an anti-dependence between instructions I2 and I4.
S3: Within an instruction pipeline an anti-dependence always
creates one or more stalls. 以上哪种说法是正确的?
(A)只有S1为真
(B)只有S2为真
(C)仅S1和S2为真
(D)只有S2和S3为真答案： (B)
说明：给出的指令可以编写如下：
I1: R1 = R2 + R3
I2: R7 = R1 * R3
I3: R4 = R1 - R5
I4: R3 = R2 + R4
I5: R7 = R8 * R9 当指令需要稍后更新的值时，就会发生反依赖关系，也称为读后写(WAR)。
S1: There is an anti-dependence between instructions I2 and I5.
False, I2 and I5 don't form any write after read situation.
They both write R7.
S2: There is an anti-dependence between instructions I2 and I4.
True, I2 reads R3 and I4 writes it.
S3: Within an instruction pipeline an anti-dependence always
creates one or more stalls.
Anti-dependency can be removed by renaming variables.
See following example.
1. B = 3
2. A = B + 1
3. B = 7
Renaming of variables could remove the dependency.
1. B = 3
N. B2 = B
2. A = B2 + 1
3. B = 7这个问题的测验