[llvm-dev] why is llvm.stacksave() necessary?

[David Chisnall via llvm-dev]

> On 25 Jul 2017, at 10:04, alex via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Hi all,
> 
> the language reference for the alloca instruction states:
> The ‘alloca‘ instruction allocates memory on the stack frame of the
> currently executing function, to be automatically released when this
> function returns to its caller.
> 
> when using come c code like
> void myfunc(void){
> int i=4;
> double d[i];
> }
> 
> the ir shows enclosing llvm.stackSave & restore constructs, enclosing
> the alloca related to 'd’.

Clang does this because clang intentionally generates IR naïvely and relies on LLVM optimisation passes to clean it up.

In the case of C VLAs, the size of the alloca changes whenever the declaration goes into scope.  By emitting a stack save and restore for d, clang doesn’t need to have different IR generating code for your example and for this one:

void myfunc(void
{
	for (int i=0 ; i<4; i++)
	{
		double d[i];
		doSomethingWith(d);
	}
}

In this case, each time that the loop is entered you will get a new d with a different size.  Clang emits code that handles this case, and an early optimisation pass will remove the stack save and stack restore intrinsics if they’re not actually needed.

David