[LLVMdev] Stack alignment problem

Chris Lattner sabre at nondot.org
Tue Mar 22 09:34:48 PST 2005

On Tue, 22 Mar 2005, Vladimir Prus wrote:
> The PrologEpilogInserter.cpp file aligns the stack only if
> MachineFrameInfo::hasCalls returns true, which happens only if the function
> has "call frame setup instruction" which my backend does not generate.
> Chris suggested adding explicit MachineFrameInfo::setHasCalls call, which I've
> tried, but it does not help. The PrologEpilogInsert.cpp file always
> explicitly calls setHasCalls with the value it computes, so whatever value I
> set manually is just overwritten.
> So, what's the right approach? As usual, I'll be willing to code a 
> patch.

Can you explain the problem in more detail?  Specifically the LLVM code 
gneerator assumes that there is some alignment that the stack is required 
to have as part of its ABI.  For example, in X86 target machine, the stack 
is 8-byte aligned on entry to function calls.

What this means is that the frame info can assume that the stack pointer 
is 8-byte aligned on entry to every function, but that it has to preserve 
this alignment for any functions that call another function (oh and it 
also has to remember that the return address gets pushed as well).

This is why frame info works the way it does: it assumes that there is no 
reason to keep the outgoing stack pointer aligned unless there is a call.

How is your target different here?  Can you give an example of why this 
causes a problem?



More information about the llvm-dev mailing list