[llvm-dev] Avoiding alloca elision

[Sanjoy Das via llvm-dev]

Hi Samuel,

You can't expect alloca's to reliably lower to stack pointer
adjustments.  The semantics of alloca is more high level -- it give
you an abstract memory location that lives and dies with the function
frame, but there is no guarantee that it will actually "allocate"
memory from the stack frame.  The compiler may promote the memory to
registers or (theoretically speaking, we don't do this today) even
demote it to a heap allocation.

You'll have to phrase your stack copy coroutines implementation in
terms of the abstract alloca semantics without assuming that LLVM will
lower allocas in some specific manner (i.e. without relying on
"implementation details").

-- Sanjoy

On Sat, Jun 29, 2019 at 6:32 AM Samuel Williams via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> Hello
>
> I'm implementing stack copying coroutines. It's an interesting challenge. When we use setjmp/longjmp to move between stacks, we must make sure that restoring the stack doesn't blow away the current stack frame before calling longjmp.
>
> To do that, we check how big the stack we are restoring is, and use alloca to push the current stack frame beyond that. It works but when I add "-O3" the alloca is elided and it fails again.
>
> I managed to avoid this by using the resulting buffer in an operation the compiler can't elide. But I was wondering, is there an __attribute__ or other mechanism by which I can tell the compiler: Don't elide this statement even if the result is unused.
>
> Here is the code: https://github.com/ioquatix/ruby/blob/3fd9102cb885858038d6a29071acecef78e6d6a5/coroutine/copy/Context.c#L36-L80
>
> You can see on line 57, I forced compiler to use the result of the alloca. If I remove this, it fails because alloca is not invoked.
>
> Any help/suggestions appreciated.
>
> Kind regards,
> Samuel
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