MIPS/GAS: Warn about unmatched REL HI16 relocations

Ever since commit 749b8d9d455c ("PATCH: ld/4208: `final link failed: Bad
value' when building Linux MIPS kernels."), which went in back in 2007,
<https://inbox.sourceware.org/binutils/20070320043504.GA7735@lucon.org/>,
we've been warning about unmatched HI16 REL relocations in the linker,
as it has been concluded in the discussion on PR ld/4208 that we have no
need to be forgiving about compiler bugs that lead to object files to be
made that are not compliant with the psABI.

However no corresponding update has ever been made to GAS, despite that
it's always helpful to issue a warning as early as possible in the build
process.  In this particular case GAS also has more information to hand
as it always has the full original addend available that has been used
for both the high-part and the low-part relocation, while the linker has
to rely on the in-place value stored in the relocatable field by earlier
processing, which has necessarily been truncated.  Therefore the linker
may miss cases where unrelated high-part and low-part relocations have
been put next to each other that had significantly different addends at
assembly time (small differences are deliberately permitted where they
are known not to affect any borrow from the high part).

Additionally GAS is able to point at the exact place in sources where
any offending orphan relocations have been used.

Update GAS to issue the warning then where applicable and remove an old
comment referring to GCC producing orphan HI16 relocations.

Test cases will be added with a separate change.

diff --git a/gas/config/tc-mips.c b/gas/config/tc-mips.c
index 5a5e6b8cdaa28c0ca5e25c0e13dca23792ec2623..4440fb7cafbf218540814ddab5c7964297c2260c 100644 (file)
--- a/gas/config/tc-mips.c
+++ b/gas/config/tc-mips.c
@@ -7677,12 +7677,11 @@ micromips_add_label (void)
   S_SET_OTHER (s, ELF_ST_SET_MICROMIPS (S_GET_OTHER (s)));
 }
 
-/* If assembling microMIPS code, then return the microMIPS reloc
-   corresponding to the requested one if any.  Otherwise return
-   the reloc unchanged.  */
+/* Return the microMIPS relocation corresponding to RELOC if there is any
+   and MICROMIPS_P is true.  Otherwise return RELOC unchanged.  */
 
 static bfd_reloc_code_real_type
-micromips_map_reloc (bfd_reloc_code_real_type reloc)
+micromips_map_reloc (bfd_reloc_code_real_type reloc, bool micromips_p)
 {
   static const bfd_reloc_code_real_type relocs[][2] =
     {
@@ -7718,7 +7717,7 @@ micromips_map_reloc (bfd_reloc_code_real_type reloc)
   bfd_reloc_code_real_type r;
   size_t i;
 
-  if (!mips_opts.micromips)
+  if (!micromips_p)
     return reloc;
   for (i = 0; i < ARRAY_SIZE (relocs); i++)
     {
@@ -8199,7 +8198,8 @@ append_insn (struct mips_cl_insn *ip, expressionS *address_expr,
       /* Perform any necessary conversion to microMIPS relocations
         and find out how many relocations there actually are.  */
       for (i = 0; i < 3 && reloc_type[i] != BFD_RELOC_UNUSED; i++)
-       final_type[i] = micromips_map_reloc (reloc_type[i]);
+       final_type[i] = micromips_map_reloc (reloc_type[i],
+                                            mips_opts.micromips);
 
       /* In a compound relocation, it is the final (outermost)
         operator that determines the relocated field.  */
@@ -14955,6 +14955,24 @@ parse_relocation (char **str, bfd_reloc_code_real_type *reloc)
   return false;
 }
 
+/* Return a relocation operator corresponding to RELOC.  */
+
+static const char *
+mips_op_for_relocation (bfd_reloc_code_real_type reloc)
+
+{
+  size_t i;
+
+  for (i = 0; i < ARRAY_SIZE (mips_percent_op); i++)
+    if (reloc == micromips_map_reloc (mips_percent_op[i].reloc,
+                                     micromips_reloc_p (reloc)))
+      return mips_percent_op[i].str;
+  for (i = 0; i < ARRAY_SIZE (mips16_percent_op); i++)
+    if (reloc == mips16_percent_op[i].reloc)
+      return mips16_percent_op[i].str;
+
+  abort ();
+}
 
 /* Parse string STR as a 16-bit relocatable operand.  Store the
    expression in *EP and the relocations in the array starting
@@ -15816,11 +15834,7 @@ mips_frob_file (void)
         fixup_has_matching_lo_p() will return true.  For PC-relative
         relocations the distance between the offsets is retained
         according to expectations in `fixup_has_matching_lo_p',
-        `_bfd_mips_elf_lo16_reloc' and `mips_elf_add_lo16_rel_addend'.
-
-        We don't warn about unmatched high-part relocations since some
-        versions of gcc have been known to emit dead "lui ...%hi(...)"
-        instructions.  */
+        `_bfd_mips_elf_lo16_reloc' and `mips_elf_add_lo16_rel_addend'.  */
       if (lo_pos != NULL)
        {
          if (l->fixp->fx_r_type != BFD_RELOC_HI16_S_PCREL)
@@ -15832,6 +15846,11 @@ mips_frob_file (void)
              *lo_pos = l->fixp;
            }
        }
+      else
+       as_warn_where
+         (l->fixp->fx_file, l->fixp->fx_line,
+          _("can't find matching low-part relocation for %s operator"),
+          mips_op_for_relocation (l->fixp->fx_r_type));
     }
 }