RegAllocPBQP inaccessible

[Bob64375]

Hi,

I am having difficulty getting llvm to use the RegAllocPBQP algorithm in 19.1.0, not that that's an issue in 19, I had the same difficulty in 15 before upgrading.

I have examined these ways to choose RegAllocPBQP:

1. "-regalloc={greedy,default,fast,basic}" does not support pbqp.

2. std::unique_ptrllvm::legacy::FunctionPassManager fpm;
   fpm->add( llvm::createDefaultPBQPRegisterAllocator() );

   Asserts with "Pass [llvm::SlotIndexesWrapperPass] is not initialized"

Some thoughts:

A. Is RegAllocPBQP deprecated and I shouldn't bother?

B. Am I unaware of another way besides legacy FunctionPassManager to add the PBQP allocator? - which it seems would also require disabling "-regalloc" since otherwise it would seem two different allocators would be requested.

C. No one's gotten around to integrating RegAllocPBQP into the "-regalloc" methodology?

Notes:

• Target is x64
• Other than trying PBQP is there an option to block spilling?

[boomanaiden154]

What do you mean -regalloc=pbqp does not work? https://godbolt.org/z/5TczM34zM

[Bob64375]

Ah, yes, docs shows regalloc choices to be {basic,fast,greedy,pbqp}. However, I am not using clang, just calling cl::ParseCommandLineOptions with arg -regalloc=pbqp and that fails in CommandLine.h L859 because the Values set is {greedy,default,fast,basic}. There is no pbqp in the set to match.

Perhaps an oversight failed to replace default with pbqp?

[boomanaiden154]

It's definitely in there normally. It's the same -regalloc flag used between clang and llc. The logic for constructing the options looks like it requires some initialization to be done before it returns anything useful. Given it seems like you're working with the API, it's probably not unlikely you're missing some setup.

llvm-project/llvm/include/llvm/CodeGen/MachinePassRegistry.h

Line 150 in c676104

this->addLiteralOption(Node->getName(),

[Bob64375]

Issue explained and solved as a comment someone (not me) could add to file RegAllocPBQP.

// The below static constructor registers "pbqp" as a choice to command option "-regalloc=XXX".
// For that to happen, this object must be linked into the executable.
// If it is not, "pbqp" will not be a choice.
// The solution to this is simple, just include "...include/llvm/CodeGen/LinkAllCodegenComponents.h" in
// any file linked into the executable.
static RegisterRegAlloc RegisterPBQPRepAlloc("pbqp", "PBQP register allocator", createDefaultPBQPRegisterAllocator);

The other RegAllocXXXs are directly referenced from llvm code so they are already linked in.
While the LLVMCodeGen library contains RegAllocPBQP, the clever linker writing the executable could skip insertion of it, as in my case.

PS Thanks for pruning the number of directions I could have headed in!