Re: [PATCH v2 23/50] helper-to-tcg: PrepareForTcgPass, remove functions with cycles

[Alessandro Di Federico via qemu development <qemu-devel@nongnu.org>]

On Thu, 30 Jul 2026 05:09:57 +0200
Anton Johansson <anjo@rev.ng> wrote:

> Functions with cycles are removed for two primary reasons:
> 
>   * As a simplifying assumption for register allocation which occurs down
>     the line, and;
> 
>   * If a function contains cycles post-optimization neither unrolling or
>     loop vectorization were deemed beneficial, and the function _might_ be
>     better suited as a helper anyway.
> 
> Cycles are detected by iterating over Strongly Connected Components
> (SCCs) which imply the existence of cycles if a SCC contains more than one
> node, or it has a self-edge.

I'd move most of the commit message to the documentation of the
function.

> 
> Signed-off-by: Anton Johansson <anjo@rev.ng>
> ---
>  .../PrepareForTcgPass/PrepareForTcgPass.cpp   | 34 +++++++++++++++++--
>  1 file changed, 32 insertions(+), 2 deletions(-)
> 
> diff --git a/subprojects/helper-to-tcg/src/PrepareForTcgPass/PrepareForTcgPass.cpp b/subprojects/helper-to-tcg/src/PrepareForTcgPass/PrepareForTcgPass.cpp
> index cea6573e41..41f317ed0b 100644
> --- a/subprojects/helper-to-tcg/src/PrepareForTcgPass/PrepareForTcgPass.cpp
> +++ b/subprojects/helper-to-tcg/src/PrepareForTcgPass/PrepareForTcgPass.cpp
> @@ -17,9 +17,39 @@
>  
>  #include "PrepareForTcgPass.hpp"
>  
> +#include <llvm/ADT/SCCIterator.h>
> +#include <llvm/ADT/SmallPtrSet.h>
> +#include <llvm/IR/Function.h>
> +#include <llvm/IR/Module.h>
> +
>  using namespace llvm;
>  
> -PreservedAnalyses PrepareForTcgPass::run(Module &M, ModuleAnalysisManager &MAM)
> -{
> +static void removeFunctionsWithLoops(Module &M, ModuleAnalysisManager &MAM) {
> +    // Iterate over all Strongly Connected Components (SCCs), a SCC implies
> +    // the existence of loops if:
> +    //   - it has more than one node, or;
> +    //   - it has a self-edge.
> +    SmallPtrSet<Function *, 16> FunctionsToRemove;
> +    for (Function &F : M) {
> +        if (F.isDeclaration()) {
> +            continue;
> +        }
> +        for (auto It = scc_begin(&F); !It.isAtEnd(); ++It) {
> +            if (It.hasCycle()) {
> +                FunctionsToRemove.insert(&F);
> +                break;
> +            }
> +        }
> +    }
> +
> +    for (Function *F : FunctionsToRemove) {
> +        F->setComdat(nullptr);
> +        F->deleteBody();
> +    }
> +}
> +
> +PreservedAnalyses PrepareForTcgPass::run(Module &M,
> +                                         ModuleAnalysisManager &MAM) {
> +    removeFunctionsWithLoops(M, MAM);
>      return PreservedAnalyses::none();
>  }
> -- 
> 2.52.0

Reviewed-by: Alessandro Di Federico <ale@rev.ng>

-- 
Alessandro Di Federico
rev.ng Labs