在 MIPS 中，I-Type 指令会导致危险吗？

Question

我知道连续的 R-Type 指令会造成危险，例如：

add $2, $2, $1
add $2, $2, $3

但是可以连续的I-Type指令吗？例如：

addi $2, $0, 10
addi $2, $0, 5

Answer 1

鉴于您的情况：

addi $2, $0, 10 
addi $2, $0, 5

您永远不会遇到数据危险，因为您永远不会在写入后读取值（写入后读取）

可以这样想：

$2 = $0 + 10
$2 = $0 + 5

可以看到，$2 并没有在第二次计算中使用，$0 也没有被改变，所以没有数据危害。

如果你这样做：

addi $2, $0, 10 # $2 = $0 + 10
addi $3, $2, 5  # $3 = $2 + 5

流水线无法保证在第二次计算期间读取 $2 是预期值。

认为 lw 和 sw 也是 I 型指令；

RAW
    A Read After Write hazard occurs when, in the code as written, one instruction
    reads a location after an earlier instruction writes new data to it, but in the
     pipeline the write occurs after the read (so the instruction doing the read gets stale data).
WAR
    A Write After Read hazard is the reverse of a RAW: in the code a write occurs after a read,
     but the pipeline causes write to happen first.
WAW
    A Write After Write hazard is a situation in which two writes occur out of order. We normally
    only consider it a WAW hazard when there is no read in between; if there is, then we have a RAW
    and/or WAR hazard to resolve, and by the time we've gotten that straightened out the WAW has 
    likely taken care of itself.

http://www.cs.nmsu.edu/~pfeiffer/classes/473/notes/hazards.html

鉴于读取和写入数据的操作是 I 型指令，并给出这些潜在数据危险的定义，是的，I 型指令仍然存在危险。