compiler: avoid zero-sized alloca in channel operations

[aykevl]

This works around a bug in LLVM (https://bugs.llvm.org/show_bug.cgi?id=49916) but seems like a good change in general.

Fixes #2041.

[dgryski]

Are there any other places that zero-byte structs will be affected with a similar problem?

[deadprogram]

@aykevl not sure if you noticed that all of the CI builds are failing: https://app.circleci.com/pipelines/github/tinygo-org/tinygo/5375/workflows/4ce07cea-c635-4dee-8075-6546395e080c/jobs/31173?invite=true#step-117-660

[aykevl]

@aykevl not sure if you noticed that all of the CI builds are failing

Oops, yeah I just read the message from CircleCI. It should e fixed now.

[aykevl]

Are there any other places that zero-byte structs will be affected with a similar problem?

Yes. Some places have a zero sized alloca. This might be the last one remaining:

tinygo/compiler/compiler.go

Lines 1533 to 1537 in 485a928

	buf := llvmutil.CreateEntryBlockAlloca(b.Builder, typ, expr.Comment)
	if b.targetData.TypeAllocSize(typ) != 0 {
	b.CreateStore(llvm.ConstNull(typ), buf) // zero-initialize var
	}
	return buf, nil

If you want to catch them, you can build LLVM using ASSERTS=1 on the make command line (something like make llvm-build ASSERTS=1) and then build TinyGo using it.
I'm not sure I'm going to bother fixing this, unless it results in actual bugs: it will be fixed when we upgrade to LLVM 12 anyway. So I'd rather focus now on getting LLVM 12 to work properly (and after that, LLVM 13).

[deadprogram]

🚫 0️⃣

Thanks @aykevl now merging.