MSP430: Support relocations for subtract expressions in .uleb128 directives

Link-time relaxations of branches are common for MSP430, given that GCC
can generate pessimal branch instructions, and the
-mcode-region=either/-mdata-region=either options to shuffle sections
can further change the type of branch instruction required.

These relaxations can result in invalid code when .uleb128
directives, used in the .gcc_except_table section, are used to calculate
the distance between two labels. A value for the .uleb128 directive is
calculated at assembly-time, and can't be updated at link-time, even if
relaxation causes the distance between the labels to change.

This patch adds relocations for subtract expressions in .uleb128
directives, to allow the linker to re-calculate the value of these
expressions after relaxation has been performed.

bfd/ChangeLog:
	* bfd-in2.h (bfd_reloc_code_real): Add
	BFD_RELOC_MSP430_{SET,SUB}_ULEB128.
	* elf32-msp430.c (msp430_elf_ignore_reloc): New.
	(elf_msp430_howto_table): Add R_MSP430{,X}_GNU_{SET,SUB}_ULEB128.
	(msp430_reloc_map): Add R_MSP430_GNU_{SET,SUB}_ULEB128.
	(msp430x_reloc_map): Add R_MSP430X_GNU_{SET,SUB}_ULEB128.
	(write_uleb128): New.
	(msp430_final_link_relocate): Handle R_MSP430{,X}_GNU_{SET,SUB}_ULEB128.
	* libbfd.c (_bfd_write_unsigned_leb128): New.
	* libbfd.h (_bfd_write_unsigned_leb128): New prototype.
	Add BFD_RELOC_MSP430_{SET,SUB}_ULEB128.
	* reloc.c: Document BFD_RELOC_MSP430_{SET,SUB}_ULEB128.

binutils/ChangeLog:
	* readelf.c (target_specific_reloc_handling): Handle
	R_MSP430{,X}_GNU_{SET,SUB}_ULEB128.

gas/ChangeLog:
	* config/tc-msp430.c (msp430_insert_uleb128_fixes): New.
	(msp430_md_end): Call msp430_insert_uleb128_fixes.

include/ChangeLog:
	* elf/msp430.h (elf_msp430_reloc_type): Add
	R_MSP430_GNU_{SET,SUB}_ULEB128.
	(elf_msp430x_reloc_type): Add R_MSP430X_GNU_{SET,SUB}_ULEB128.

ld/ChangeLog:
	* testsuite/ld-msp430-elf/msp430-elf.exp: Run new tests.
	* testsuite/ld-msp430-elf/uleb128.s: New test.
	* testsuite/ld-msp430-elf/uleb128_430.d: New test.
	* testsuite/ld-msp430-elf/uleb128_430x.d: New test.

1 files changed, 53 insertions, 1 deletions

diff --git a/gas/config/tc-msp430.c b/gas/config/tc-msp430.c
index 2738937..6d18032 100644
--- a/gas/config/tc-msp430.c
+++ b/gas/config/tc-msp430.c
@@ -5048,8 +5048,56 @@ msp430_fix_adjustable (struct fix *fixp ATTRIBUTE_UNUSED)
   return FALSE;
 }
 
-/* Set the contents of the .MSP430.attributes and .GNU.attributes sections.  */
+/* Scan uleb128 subtraction expressions and insert fixups for them.
+   e.g., .uleb128 .L1 - .L0
+   Because relaxation may change the value of the subtraction, we
+   must resolve them at link-time.  */
 
+static void
+msp430_insert_uleb128_fixes (bfd *abfd ATTRIBUTE_UNUSED,
+			    asection *sec, void *xxx ATTRIBUTE_UNUSED)
+{
+  segment_info_type *seginfo = seg_info (sec);
+  struct frag *fragP;
+
+  subseg_set (sec, 0);
+
+  for (fragP = seginfo->frchainP->frch_root;
+       fragP; fragP = fragP->fr_next)
+    {
+      expressionS *exp, *exp_dup;
+
+      if (fragP->fr_type != rs_leb128  || fragP->fr_symbol == NULL)
+	continue;
+
+      exp = symbol_get_value_expression (fragP->fr_symbol);
+
+      if (exp->X_op != O_subtract)
+	continue;
+
+      /* FIXME: Skip for .sleb128.  */
+      if (fragP->fr_subtype != 0)
+	continue;
+
+      exp_dup = xmemdup (exp, sizeof (*exp), sizeof (*exp));
+      exp_dup->X_op = O_symbol;
+      exp_dup->X_op_symbol = NULL;
+
+      /* Emit the SUB relocation first, since the SET relocation will write out
+	 the final value.  */
+      exp_dup->X_add_symbol = exp->X_op_symbol;
+      fix_new_exp (fragP, fragP->fr_fix, 0,
+		   exp_dup, 0, BFD_RELOC_MSP430_SUB_ULEB128);
+
+      exp_dup->X_add_symbol = exp->X_add_symbol;
+      /* Insert relocations to resolve the subtraction at link-time.  */
+      fix_new_exp (fragP, fragP->fr_fix, 0,
+		   exp_dup, 0, BFD_RELOC_MSP430_SET_ULEB128);
+
+    }
+}
+
+/* Called after all assembly has been done.  */
 void
 msp430_md_end (void)
 {
@@ -5065,6 +5113,10 @@ msp430_md_end (void)
 	as_warn (_(WARN_NOP_AT_EOF));
     }
 
+  /* Insert relocations for uleb128 directives, so the values can be recomputed
+     at link time.  */
+  bfd_map_over_sections (stdoutput, msp430_insert_uleb128_fixes, NULL);
+
   /* We have already emitted an error if any of the following attributes
      disagree with the attributes in the input assembly file.  See
      msp430_object_attribute.  */