Re: [PATCH, PowerPC] Fix PR57949 (ABI alignment issue)

[Alan Modra <amodra@gmail.com>]

On Wed, Sep 11, 2013 at 07:55:43AM -0500, Bill Schmidt wrote:
> On Wed, 2013-09-11 at 21:08 +0930, Alan Modra wrote:
> > On Wed, Aug 14, 2013 at 10:32:01AM -0500, Bill Schmidt wrote:
> > > This fixes a long-standing problem with GCC's implementation of the
> > > PPC64 ELF ABI.  If a structure contains a member requiring 128-bit
> > > alignment, and that structure is passed as a parameter, the parameter
> > > currently receives only 64-bit alignment.  This is an error, and is
> > > incompatible with correct code generated by the IBM XL compilers.
> > 
> > This caused multiple failures in the libffi testsuite:
> > libffi.call/cls_align_longdouble.c
> > libffi.call/cls_align_longdouble_split.c
> > libffi.call/cls_align_longdouble_split2.c
> > libffi.call/nested_struct5.c
> > 
> > Fixed by making the same alignment adjustment in libffi to structures
> > passed by value.  Bill, I think your patch needs to go on all active
> > gcc branches as otherwise we'll need different versions of libffi for
> > the next gcc releases.
> 
> Hm, the libffi case is unfortunate. :(
> 
> The alternative is to leave libffi alone, and require code that calls
> these interfaces with "bad" structs passed by value to be built using
> -mcompat-align-parm, which was provided for such compatibility issues.
> Hopefully there is a small number of cases where this can happen, and
> this could be documented with libffi and gcc.  What do you think?

We have precedent for compiling libffi based on gcc preprocessor
defines, eg. __NO_FPRS__, so here's a way of making upstream libffi
compatible with the various versions of gcc out there.  I've taken the
condition under which we align aggregates from
rs6000_function_arg_boundary, and defined a macro with a value of the
maximum alignment.

Bootstrapped and regression tested powerpc64-linux.  OK for mainline?

gcc/
	* config/rs6000/rs6000-c.c (rs6000_cpu_cpp_builtins): Define
	__STRUCT_PARM_ALIGN__.
libffi/
	* src/powerpc/ffi.c (ffi_prep_args64): Align FFI_TYPE_STRUCT.
	(ffi_closure_helper_LINUX64): Likewise.

Index: gcc/config/rs6000/rs6000-c.c
===================================================================
--- gcc/config/rs6000/rs6000-c.c	(revision 202428)
+++ gcc/config/rs6000/rs6000-c.c	(working copy)
@@ -473,6 +473,12 @@ rs6000_cpu_cpp_builtins (cpp_reader *pfile)
   if (TARGET_SOFT_FLOAT || !TARGET_FPRS)
     builtin_define ("__NO_FPRS__");
 
+  /* Whether aggregates passed by value are aligned to a 16 byte boundary
+     if their alignment is 16 bytes or larger.  */
+  if ((TARGET_MACHO && rs6000_darwin64_abi)
+      || (DEFAULT_ABI == ABI_AIX && !rs6000_compat_align_parm))
+    builtin_define ("__STRUCT_PARM_ALIGN__=16");
+
   /* Generate defines for Xilinx FPU. */
   if (rs6000_xilinx_fpu) 
     {
Index: libffi/src/powerpc/ffi.c
===================================================================
--- libffi/src/powerpc/ffi.c	(revision 202428)
+++ libffi/src/powerpc/ffi.c	(working copy)
@@ -462,6 +462,9 @@ ffi_prep_args64 (extended_cif *ecif, unsigned long
     double **d;
   } p_argv;
   unsigned long gprvalue;
+#ifdef __STRUCT_PARM_ALIGN__
+  unsigned long align;
+#endif
 
   stacktop.c = (char *) stack + bytes;
   gpr_base.ul = stacktop.ul - ASM_NEEDS_REGISTERS64 - NUM_GPR_ARG_REGISTERS64;
@@ -532,6 +535,12 @@ ffi_prep_args64 (extended_cif *ecif, unsigned long
 #endif
 
 	case FFI_TYPE_STRUCT:
+#ifdef __STRUCT_PARM_ALIGN__
+	  align = (*ptr)->alignment;
+	  if (align > __STRUCT_PARM_ALIGN__)
+	    align = __STRUCT_PARM_ALIGN__;
+	  next_arg.ul = ALIGN (next_arg.ul, align);
+#endif
 	  words = ((*ptr)->size + 7) / 8;
 	  if (next_arg.ul >= gpr_base.ul && next_arg.ul + words > gpr_end.ul)
 	    {
@@ -1349,6 +1358,9 @@ ffi_closure_helper_LINUX64 (ffi_closure *closure,
   long i, avn;
   ffi_cif *cif;
   ffi_dblfl *end_pfr = pfr + NUM_FPR_ARG_REGISTERS64;
+#ifdef __STRUCT_PARM_ALIGN__
+  unsigned long align;
+#endif
 
   cif = closure->cif;
   avalue = alloca (cif->nargs * sizeof (void *));
@@ -1399,6 +1411,12 @@ ffi_closure_helper_LINUX64 (ffi_closure *closure,
 	  break;
 
 	case FFI_TYPE_STRUCT:
+#ifdef __STRUCT_PARM_ALIGN__
+	  align = arg_types[i]->alignment;
+	  if (align > __STRUCT_PARM_ALIGN__)
+	    align = __STRUCT_PARM_ALIGN__;
+	  pst = ALIGN (pst, align);
+#endif
 #ifndef __LITTLE_ENDIAN__
 	  /* Structures with size less than eight bytes are passed
 	     left-padded.  */

-- 
Alan Modra
Australia Development Lab, IBM