Fix alignment of dynamically allocated stack areas on ppc-vxworks

Fix alignment of dynamically allocated stack areas on ppc-vxworks

[Olivier Hainque]

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Hello,

The attached patch, provided by Eric Botcazou (thanks!), installs
definitions of RS6000_STARTING_FRAME_OFFSET and STACK_DYNAMIC_OFFSET
for PowerPC VxWorks, which has STACK_BOUNDARY set to 128
unconditionally.

AFAICS, this is the third OS config file doing this, after Darwin
and AIX. It seems like we could (should ?) account for STACK_BOUNDARY
or maybe PREFERRED_STACK_BOUNDARY in the common rs6000.h definition.

We have tested this successfully with a gcc-8 based compiler for
powerpc-wrs-vxworks, where this fixes SEGVs observed on Ada programs
performing dynamic stack allocations.

With Kind Regards,

Olivier

2018-12-11  Eric Botcazou  <botcazou@adacore.com>

	* config/rs6000/vxworks.h (RS6000_STARTING_FRAME_OFFSET): Define,
	accounting for STACK_BOUNDARY 128.
	(STACK_DYNAMIC_OFFSET): Likewise.


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diff --git a/gcc/config/rs6000/vxworks.h b/gcc/config/rs6000/vxworks.h
index d2033f6..24fe9ba 100644
--- a/gcc/config/rs6000/vxworks.h
+++ b/gcc/config/rs6000/vxworks.h
@@ -116,7 +116,7 @@ VXWORKS_ADDITIONAL_CPP_SPEC
 #undef SDATA_DEFAULT_SIZE
 #define SDATA_DEFAULT_SIZE (TARGET_VXWORKS_RTP ? 8 : 0)
 
-/* Enforce 16bytes alignment for the stack pointer, to permit general
+/* Enforce 16-byte alignment for the stack pointer, to permit general
    compliance with e.g. Altivec instructions requirements.  Make sure
    this isn't overruled by the EABI constraints.  */
 
@@ -128,6 +128,40 @@ VXWORKS_ADDITIONAL_CPP_SPEC
 
 #undef  ABI_STACK_BOUNDARY
 
+/* Offset within stack frame to start allocating local variables at.
+   If FRAME_GROWS_DOWNWARD, this is the offset to the END of the
+   first local allocated.  Otherwise, it is the offset to the BEGINNING
+   of the first local allocated.
+
+   On the RS/6000, the frame pointer is the same as the stack pointer,
+   except for dynamic allocations.  So we start after the fixed area and
+   outgoing parameter area.
+
+   If the function uses dynamic stack space (CALLS_ALLOCA is set), that
+   space needs to be aligned to STACK_BOUNDARY, i.e. the sum of the
+   sizes of the fixed area and the parameter area must be a multiple of
+   STACK_BOUNDARY.  */
+
+#undef RS6000_STARTING_FRAME_OFFSET
+#define RS6000_STARTING_FRAME_OFFSET					\
+  (cfun->calls_alloca							\
+   ? RS6000_ALIGN (crtl->outgoing_args_size + RS6000_SAVE_AREA, 16)	\
+   : (RS6000_ALIGN (crtl->outgoing_args_size, 16) + RS6000_SAVE_AREA))
+
+/* Offset from the stack pointer register to an item dynamically
+   allocated on the stack, e.g., by `alloca'.
+
+   The default value for this macro is `STACK_POINTER_OFFSET' plus the
+   length of the outgoing arguments.  The default is correct for most
+   machines.  See `function.c' for details.
+
+   This value must be a multiple of STACK_BOUNDARY (hard coded in
+   `emit-rtl.c').  */
+#undef STACK_DYNAMIC_OFFSET
+#define STACK_DYNAMIC_OFFSET(FUNDECL)					\
+   RS6000_ALIGN (crtl->outgoing_args_size.to_constant ()		\
+		 + STACK_POINTER_OFFSET, 16)
+
 #undef SUBSUBTARGET_OVERRIDE_OPTIONS
 #define SUBSUBTARGET_OVERRIDE_OPTIONS		\
   do {						\