rs6000 small data: add note on sdata2/r2

for  gcc/ChangeLog

	* config/rs6000/rs6000.c (SMALL_DATA_RELOC, SMALL_DATA_REG): Add
	a comment about how uses of r2 for .sdata2 come about.

From-SVN: r263760

gcc/ChangeLog

+2018-08-22  Alexandre Oliva <oliva@adacore.com>
+
+	* config/rs6000/rs6000.c (SMALL_DATA_RELOC, SMALL_DATA_REG): Add
+	a comment about how uses of r2 for .sdata2 come about.
+
 2018-08-22  Alexandre Oliva <aoliva@redhat.com>
 
 	* tree-ssa-reassoc.c (is_reassociable_op): Fix cut&pasto.

gcc/config/rs6000/rs6000.c

@@ -20349,6 +20349,9 @@ rs6000_output_function_entry (FILE *file, const char *fname)
 /* Print an operand.  Recognize special options, documented below.  */
 
 #if TARGET_ELF
+/* Access to .sdata2 through r2 (see -msdata=eabi in invoke.texi) is
+   only introduced by the linker, when applying the sda21
+   relocation.  */
 #define SMALL_DATA_RELOC ((rs6000_sdata == SDATA_EABI) ? "sda21" : "sdarel")
 #define SMALL_DATA_REG ((rs6000_sdata == SDATA_EABI) ? 0 : 13)
 #else