Re: POWERPC64_TOC_POINTER_ALIGNMENT

[Michael Meissner <meissner at linux dot vnet dot ibm dot com>]

On Wed, Nov 18, 2015 at 09:52:41AM +1030, Alan Modra wrote:
> David noticed that gcc112 was generating gcc/auto-host.h with
> #define POWERPC64_TOC_POINTER_ALIGNMENT 32768
> 
> This is not the correct value of either 8 or 256 depending on how old
> ld is.  On investigating I found the cause is Fedora 21 modifying the
> toolchain to default to -z relro.  ld -z relro puts the relro gap just
> before .got (prior to my patches reordering sections for relro on
> powerpc64).  That unfortunately aligns .got, defeating the deliberate
> mis-alignment of .got in the testcase.
> 
> Fixed with the following obvious patch and committed to mainline.
> 
> Incidentally, bootstrap fails for me on powerpc64 due to "comparison
> is always true due to limited range of data type [-Wtype-limits]"
>         && GET_MODE_SIZE (mode) <= POWERPC64_TOC_POINTER_ALIGNMENT));
> since my POWERPC64_TOC_POINTER_ALIGNMENT is 256 and mode_size is an
> unsigned char array.  Grrr, so what code obfuscation do we use here to
> work around this annoying warning?

For the moment, I added the following to my local build.  However, I can't
build libgcc on an x86 cross compiler:

In file included from /home/meissner/fsf-src/ieee/libgcc/soft-fp/soft-fp.h:321:0,
                 from addkf3.c:33:
addkf3.c: In function â__addkf3â:
/home/meissner/fsf-src/ieee/libgcc/soft-fp/op-common.h:2057:6: internal compiler error: in convert_move, at expr.c:286
  (r) = __builtin_clzl (x);     \
  ~~~~^~~~~~~~~~~~~~~~~~~~

/home/meissner/fsf-src/ieee/libgcc/soft-fp/op-2.h:129:2: note: in expansion of macro â__FP_CLZâ
  __FP_CLZ ((R), X##_f1);   \
  ^~~~~~~~

/home/meissner/fsf-src/ieee/libgcc/soft-fp/op-common.h:827:8: note: in expansion of macro â_FP_FRAC_CLZ_2â
        _FP_FRAC_CLZ_##wc (_FP_ADD_INTERNAL_diff, R);  \
        ^~~~~~~~~~~~~

/home/meissner/fsf-src/ieee/libgcc/soft-fp/op-common.h:850:34: note: in expansion of macro â_FP_ADD_INTERNALâ
 #define _FP_ADD(fs, wc, R, X, Y) _FP_ADD_INTERNAL (fs, wc, R, X, Y, '+')
                                  ^~~~~~~~~~~~~~~~

/home/meissner/fsf-src/ieee/libgcc/soft-fp/quad.h:306:29: note: in expansion of macro â_FP_ADDâ
 # define FP_ADD_Q(R, X, Y)  _FP_ADD (Q, 2, R, X, Y)
                             ^~~~~~~

addkf3.c:48:3: note: in expansion of macro âFP_ADD_Qâ
   FP_ADD_Q (R, A, B);
   ^~~~~~~~

0x78432e convert_move(rtx_def*, rtx_def*, int)
        /home/meissner/fsf-src/ieee/gcc/expr.c:286
0x8753a6 expand_direct_optab_fn
        /home/meissner/fsf-src/ieee/gcc/internal-fn.c:2132
0x6686ca expand_call_stmt
        /home/meissner/fsf-src/ieee/gcc/cfgexpand.c:2565
0x6698e4 expand_gimple_stmt_1
        /home/meissner/fsf-src/ieee/gcc/cfgexpand.c:3525
0x6698e4 expand_gimple_stmt
        /home/meissner/fsf-src/ieee/gcc/cfgexpand.c:3688
0x66b2fe expand_gimple_basic_block
        /home/meissner/fsf-src/ieee/gcc/cfgexpand.c:5694
0x66f226 execute
        /home/meissner/fsf-src/ieee/gcc/cfgexpand.c:6309
Please submit a full bug report,

Here is the temporary patch I'm using to get past rs6000.c.  But I suspect the
TOC alignment should never be 256.

Index: gcc/config/rs6000/rs6000.c
===================================================================
--- gcc/config/rs6000/rs6000.c	(revision 230511)
+++ gcc/config/rs6000/rs6000.c	(working copy)
@@ -7992,14 +7992,21 @@ rs6000_cannot_force_const_mem (machine_m
    can be addressed relative to the toc pointer.  */
 
 static bool
-use_toc_relative_ref (rtx sym, machine_mode mode)
+use_toc_relative_ref (rtx sym, machine_mode mode ATTRIBUTE_UNUSED)
 {
   return ((constant_pool_expr_p (sym)
 	   && ASM_OUTPUT_SPECIAL_POOL_ENTRY_P (get_pool_constant (sym),
 					       get_pool_mode (sym)))
 	  || (TARGET_CMODEL == CMODEL_MEDIUM
 	      && SYMBOL_REF_LOCAL_P (sym)
-	      && GET_MODE_SIZE (mode) <= POWERPC64_TOC_POINTER_ALIGNMENT));
+	      /* If the linker says that TOC alignment is 256 bits, this test
+		 will always be true, since GET_MODE_SIZE returns an unsigned
+		 char on the PowerPC.  Prevent an warning/error in this
+		 case.  */
+#if POWERPC64_TOC_POINTER_ALIGNMENT < 256
+	      && GET_MODE_SIZE (mode) <= POWERPC64_TOC_POINTER_ALIGNMENT
+#endif
+	      ));
 }
 
 /* Our implementation of LEGITIMIZE_RELOAD_ADDRESS.  Returns a value to

-- 
Michael Meissner, IBM
IBM, M/S 2506R, 550 King Street, Littleton, MA 01460-6245, USA
email: meissner@linux.vnet.ibm.com, phone: +1 (978) 899-4797