IBM Z: Handle unaligned symbols

The IBM Z ELF ABI mandates every symbol to reside on a 2 byte boundary
in order to be able to use the larl instruction. However, in some
situations it is difficult to enforce this, e.g. for common linker
scripts as used in the Linux kernel. This patch introduces the
-munaligned-symbols option. When that option is used, external symbols
without an explicit alignment are considered unaligned and its address
will be pushed into GOT or the literal pool.

If the symbol in the final linker step turns out end up on a 2 byte
boundary the linker is able to take this back and replace the indirect
reference with larl again. This should minimize the effect to symbols
which are actually unaligned in the end.

gcc/ChangeLog:

	* config/s390/s390.cc (s390_encode_section_info): Assume external
	symbols without explicit alignment to be unaligned if
	-munaligned-symbols has been specified.
	* config/s390/s390.opt (-munaligned-symbols): New option.

gcc/testsuite/ChangeLog:

	* gcc.target/s390/aligned-1.c: New test.
	* gcc.target/s390/unaligned-1.c: New test.

gcc/config/s390/s390.cc

@@ -13709,8 +13709,13 @@ s390_encode_section_info (tree decl, rtx rtl, int first)
 	 a larl/load-relative instruction.  We only handle the cases
 	 that can go wrong (i.e. no FUNC_DECLs).
 	 All symbols without an explicit alignment are assumed to be 2
-	 byte aligned as mandated by our ABI.  */
-      if (DECL_USER_ALIGN (decl) && DECL_ALIGN (decl) % 16)
+	 byte aligned as mandated by our ABI.  This behavior can be
+	 overridden for external symbols with the -munaligned-symbols
+	 switch.  */
+      if (DECL_ALIGN (decl) % 16
+	  && (DECL_USER_ALIGN (decl)
+	      || (!SYMBOL_REF_LOCAL_P (XEXP (rtl, 0))
+		  && s390_unaligned_symbols_p)))
 	SYMBOL_FLAG_SET_NOTALIGN2 (XEXP (rtl, 0));
       else if (DECL_ALIGN (decl) % 32)
 	SYMBOL_FLAG_SET_NOTALIGN4 (XEXP (rtl, 0));

gcc/config/s390/s390.opt

@@ -329,3 +329,10 @@ Target Undocumented Var(unroll_only_small_loops) Init(0) Save
 mpreserve-args
 Target Var(s390_preserve_args_p) Init(0)
 Store all argument registers on the stack.
+
+munaligned-symbols
+Target Var(s390_unaligned_symbols_p) Init(0)
+Assume external symbols to be potentially unaligned.  By default all
+symbols without explicit alignment are assumed to reside on a 2 byte
+boundary as mandated by the IBM Z ABI.
+

gcc/testsuite/gcc.target/s390/aligned-1.c

@@ -0,0 +1,20 @@
+/* Even symbols without explicite alignment are assumed to reside on a
+   2 byte boundary, as mandated by the IBM Z ELF ABI, and therefore
+   can be accessed using the larl instruction.  */
+
+/* { dg-do compile } */
+/* { dg-options "-O3 -march=z900 -fno-section-anchors" } */
+
+extern unsigned char extern_implicitly_aligned;
+extern unsigned char extern_explicitly_aligned __attribute__((aligned(2)));
+unsigned char aligned;
+
+unsigned char
+foo ()
+{
+  return extern_implicitly_aligned + extern_explicitly_aligned + aligned;
+}
+
+/* { dg-final { scan-assembler-times "larl\t%r\[0-9\]*,extern_implicitly_aligned\n" 1 } } */
+/* { dg-final { scan-assembler-times "larl\t%r\[0-9\]*,extern_explicitly_aligned\n" 1 } } */
+/* { dg-final { scan-assembler-times "larl\t%r\[0-9\]*,aligned\n" 1 } } */

gcc/testsuite/gcc.target/s390/unaligned-1.c

@@ -0,0 +1,20 @@
+/* With the -munaligned-symbols option all external symbols without
+   explicite alignment are assumed to be potentially unaligned and
+   therefore cannot be accessed with larl.  */
+
+/* { dg-do compile } */
+/* { dg-options "-O3 -march=z900 -fno-section-anchors -munaligned-symbols" } */
+
+extern unsigned char extern_unaligned;
+extern unsigned char extern_explicitly_aligned __attribute__((aligned(2)));
+unsigned char aligned;
+
+unsigned char
+foo ()
+{
+  return extern_unaligned + extern_explicitly_aligned + aligned;
+}
+
+/* { dg-final { scan-assembler-times "larl\t%r\[0-9\]*,extern_unaligned\n" 0 } } */
+/* { dg-final { scan-assembler-times "larl\t%r\[0-9\]*,extern_explicitly_aligned\n" 1 } } */
+/* { dg-final { scan-assembler-times "larl\t%r\[0-9\]*,aligned\n" 1 } } */