Updated version of the patch described below.  Apart from fixing a
bug and adding a test, the new logic is now used always, for all
targets.  The discussion of the original patch starts here:

https://gcc.gnu.org/ml/gcc-patches/2015-11/msg03052.html

The new patch has been bootstrapped and regression tested on s390,
s390x and x86_64, but please check the questions/comments in the
follow up message.

On Wed, Nov 25, 2015 at 01:56:10PM +0100, Dominik Vogt wrote:
> The attached patch fixes a warning during Linux kernel compilation
> on S/390 due to -mwarn-dynamicstack and runtime alignment of stack
> variables with constant size causing cfun->calls_alloca to be set
> (even if alloca is not used at all).  The patched code places
> constant size runtime aligned variables in the "virtual stack
> vars" area instead of creating a "virtual stack dynamic" area.
> 
> This behaviour is activated by defining
> 
>   #define ALLOCATE_DYNAMIC_STACK_SPACE_IN_PROLOGUE 1
> 
> in the backend; otherwise the old logic is used.
> 
> The kernel uses runtime alignment for the page structure (aligned
> to 16 bytes), and apart from triggereing the alloca warning
> (-mwarn-dynamicstack), the current Gcc also generates inefficient
> code like
> 
>   aghi %r15,-160  # prologue: create stack frame
>   lgr %r11,%r15   # prologue: generate frame pointer
>   aghi %r15,-32   # space for dynamic stack
> 
> which could be simplified to
> 
>   aghi %r15,-192
> 
> (if later optimization passes are able to get rid of the frame
> pointer).  Is there a specific reason why the patched behaviour
> shouldn't be used for all platforms?
> 
> --
> 
> As the placement of runtime aligned stack variables with constant
> size is done completely in the middleend, I don't see a way to fix
> this in the backend.

Ciao

Dominik ^_^  ^_^

-- 

Dominik Vogt
IBM Germany