[llvm-dev] A problem with register allocator

[Ehsan Amiri via llvm-dev]

Hi



I have a code pattern like this:



Load value 1 from mem

Load value 2  from mem



For (…) {

  Use value 2

  Use value 1

}



During register allocations values loaded are spilled and I end up with
this pattern:



Load value 1 from mem

Store value 1 to stack

Load value 2  from mem

Store value 2 to stack



For (…) {

  Load value 2 from stack

  Use value 2

  Load value 1 from stack

  Use value 1

}



At least for my case, I prefer to load directly from memory inside the
loop. From what I have heard, this is a known issue with the register
allocator. I have a few questions:



1-      Has there been any attempt to fix this? If yes, what was the
experience when dealing with tradeoffs (for example if value 1 and value 2
are far apart in memory and loop trip count is high, the current codegen
might be better)


2-      If I want to experiment with some fixes for this, where in the
register allocator code I should look?



Thanks

Ehsan
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