[cfe-dev] RFC: New IR attribute incoming-stack-align

[Huw Davies via cfe-dev]

Hi,

I'm trying to fix an issue with clang's __force_align_arg_pointer__
attribute.  The problem is described here:
https://llvm.org/bugs/show_bug.cgi?id=26662

The problem is affecting Wine (https://www.winehq.org/) where we have
a function that is an entry-point for the x86 Win32 abi.  That
function may then call functions in the host's (linux, OS/X, ...) abi.
The issue is that the Win32 abi guarantees stack alignment of four
bytes while the host abi is typically sixteen.

This is supposed to be solved by clang's __force_align_arg_pointer__
attribute.  What this does is to set the IR attribute alignstack = 16.
While this does correctly realign the stack it does not change the
assumption about the alignment of the incoming stack - which is still
assumed to have an alignment of sixteen.  We need a way to signal to
the stack layout generator that the incoming alignment can only be
assumed to be four.

In contrast, gcc realigns the stack but also reduces the assumed
alignment of the incoming stack to four.

To fix this I'm proposing a new IR attribute "incoming-stack-align"
that would be set by clang in addition to the alignstack attribute.
I'm attaching patches to llvm and clang that implement this.  The
patches are most certainly not correct, but I'd appreciate any
feedback about how to fix this.

Many thanks,
Huw.
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