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.
gcc/ChangeLog:
2016-07-18 Dominik Vogt <vogt@linux.vnet.ibm.com>
* cfgexpand.c (expand_stack_vars): Implement synamic stack space
allocation in the prologue.
* explow.c (get_dynamic_stack_base): New function to return an address
expression for the dynamic stack base.
(get_dynamic_stack_size): New function to do the required dynamic stack
space size calculations.
(allocate_dynamic_stack_space): Use new functions.
(align_dynamic_address): Move some code from
allocate_dynamic_stack_space to new function.
* explow.h (get_dynamic_stack_base, get_dynamic_stack_size): Export.
gcc/testsuite/ChangeLog:
2016-07-18 Dominik Vogt <vogt@linux.vnet.ibm.com>
* gcc.target/s390/warn-dynamicstack-1.c: New test.
* gcc.dg/stack-usage-2.c (foo3): Adapt expected warning.
stack-layout-dynamic-1.c: New test.
From-SVN: r238432
