yonghong-song added a comment.
I checked EmitAutoVarAlloca(). It emits the llvm.stacksave() due to
// If the type is variably-modified, emit all the VLA sizes for it.
if (Ty->isVariablyModifiedType())
EmitVariablyModifiedType(Ty);
Here, in order not to generate llvm.stacksave(), we need to evaluate the array
size expression for `Ty`.
But it is probably not a good idea to re-evaluate `Ty` during code generation
phase. If we really want to evaluate, we should evaluate earlier during
semantic analysis phase like this patch.
Another solution is to implement pruning llvm.stacksave/stackrestore in
backend. Currently InstCombine phase tries to remove some adjacent stacksave()
and stackrestore() instrinsics. We could try to enhance InstCombine to handle
more complicated cases, or considering this may be just a BPF use case (which
doesn't support dynamic memory allocation), we can implement a bpf backend IR
phase to remove it.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D107882/new/
https://reviews.llvm.org/D107882
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