Approaches to data dependence relations:
- Address Based: Dependences which use the same address
- Value Based: Dependences which use the same value
Consider
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A = 0
B = A
A = B + 1
C = A |
| Address-based approach |
Value-Based Approach |
There is a flow dependence
between S1 and S4
S1 → S4
because S4 uses A |
In (4) value of A used is
defined in (3) and not in (1)
thus, there is no data dependence |
Value based dependence is a subset of Address based dependence. For Address-based dependence:
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| |
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 then ?
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This is written as
 and is used for cache optimizations
Basic Block Dependence
- Construct dependence graph for the instructions
- I1 and I2 may have flow, anti or output dependence
- Can not determine whether I1 can be moved beyond I2
- Suppose an instruction reads from [r11](4) and the next instruction writes to [r12+12](4)
- Unless we know r11 and r12+12 point to different locations assume a flow dependence
- I1 is a predecessor of I2 if I2 must not execute before some cycles of I1
- Type of dependency is not important
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