[LLVMdev] Memory Dependence Analysis: getNonLocalDependency()

[Owen Anderson]

On Jul 28, 2008, at 5:53 PM, Prakash Prabhu wrote:

> Thanks for the quick reply, Owen. I went through the code in  
> MemoryDependenceAnalysis.cpp again. Since DenseMap<BasicBlock*,  
> Value*> maps a BasicBlock to a single Value (Instruction) and it is  
> populated by a DFS on the reverse Control Flow Graph starting at the  
> query instruction's block, is it correct to say that the last  
> dependent instruction in each visited basic block is what is  
> mapped ? For example, if a basicblock B has two instructions:
>
> B:
>
> store i32 1, i32* %i, align 4                           (say inst1)
> ...
> load i32* %i, align 4           ; <i32>:39 [#uses=1]  (say inst2)
>
> and suppose i had one more Store instruction to %i outside B which  
> is the query, then when B is visited, in the DenseMap only the  
> mapping <B, inst2> is added and not <B, inst1> ?
>
> I am guessing that this information is sufficient for most code- 
> hoisting optimizations since just the knowledge that a basic block  
> has some load/store instruction accessing the same (modulo the  
> pointer analysis) address is sufficient to prevent the  
> transformation from causing an unsafe code-movement ?
>

Yup, that's exactly right.

--Owen
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