Disable preserve_none under ASAN due to bugs

[morrisonlevi]

See:

• Fully disable preserve_none under ASan llvm/llvm-project#190001, which merged just last week (April 1, 2026)
• Clang-19 crashed: fatal error: error in backend: Stack realignment in presence of dynamic allocas is not supported withthis calling convention. llvm/llvm-project#95928

[iluuu1994]

This is a bit unfortunate, as it makes it impossible to test the tailcall VM with ASAN. The error disappeared for me with Clang 21 (see llvm/llvm-project#95928 (comment)). Which Clang version are you running?

[morrisonlevi]

I had issues on clang 19 (to intentionally match Rust 1.84-1.88 for cross-langauge LTO and other reasons) on aarch64. I saw your comment when looking at the PRs! "Hey, that's a friendly face!" However, the fact that LLVM disabled it in main just a week ago does not give me confidence to enable this for any version, when ASAN is enabled, which is why I opened this PR. They cited issues on x86_64 as well. Thoughts?

[iluuu1994]

However, the fact that LLVM disabled it in main just a week ago does not give me confidence to enable this for any version

Yeah, that's fair. Though it's also possible this is only an issue with C++ in Clang 20+, but not C. Hence I'd prefer only enabling this if we run into an actual issue, especially considering this is exclusively a development configuration.

But ultimately, Arnaud should decide.

[iluuu1994]

Or maybe disable it only for Clang 19 and lower.

[arnaud-lb]

There appears to be two known issues with preserve_none:

• clang-19 crashes during compilation with ASan
• the issue mentioned in the LLVM commit ([clang / ASan] ASan doesn't respect preserve_none calling convention on ARMv8 llvm/llvm-project#177519)

For the first issue, I agree that we should disable preserve_none when using Clang 19 and lower + ASan.

I've investigated the second issue a bit, and found that it's likely a more general AArch64+preserve_none issue, unfortunately: llvm/llvm-project#177519 (comment).

This happens in specific conditions where the compiler can not use the stack pointer directly, and must address locals via x19, the "base pointer". The spilling logic in the AArch64 backend seems to ignore x19 in that case.

This affects functions that:

• Have over-aligned locals
• Use alloca
• Call a preserve_none function (not tailcall)
• Allow some locals to escape

I need to looks more into it, but I think that this doesn't affect php-src, or we could workaround the issue. The only function that calls a preserve_none functions is execute_ex (other call sites are tail calls), and it doesn't use alloca. ASan may use alloca, but even then the other criteria may not match. Otherwise, a workaround would be to wrap that call in a simpler function.