llvm dev - Aug 2017 - A problem with register allocator

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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This is called rematerialization, and the register allocator can already do it
but there are various constraints.
For it happen:
* The instruction needs to be marked as isReMaterializable in the tablegen file
* TargetInstrInfo::isTriviallyReMaterializable needs to return true for the
instruction
* InlineSpiller and LiveRangeEdit in lib/Codegen are the things that actually do
the rematerialization, and it
    may be worthwhile looking there
* The base register of the load needs to be guaranteed to be live at the place
where it is rematerialized, e.g.
    in lib/Target/ARM/ARMInstrThumb.td the tLDRpci instruction is
rematerializable because the base in the
    pc register which is always available, but loads in general aren’t.

John

From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Ehsan
Amiri via llvm-dev
Sent: 17 August 2017 17:49
To: llvm-dev
Subject: [llvm-dev] A problem with register allocator


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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