MIPS: Add options to control branch ISA checks

Complement commit 9d862524f6ae ("MIPS: Verify the ISA mode and alignment
of branch and jump targets") and add GAS and LD options to control the
checks for invalid branches between ISA modes introduced there, to help
with some handwritten code lacking `.insn' annotation for labels used as
branch targets and code produced by older versions of GCC which suffers
from the issue with branches to code that has been optimized away,
addressed with GCC commit 242424 ("MIPS/GCC: Mark trailing labels with
`.insn'"), <https://gcc.gnu.org/ml/gcc-patches/2016-11/msg01061.html>.

	bfd/
	* elfxx-mips.h (_bfd_mips_elf_insn32): Rename prototype to...
	(_bfd_mips_elf_linker_flags): ... this.  Add another parameter.
	* elfxx-mips.c (mips_elf_link_hash_table): Add
	`ignore_branch_isa' member.
	(mips_elf_perform_relocation): Do not treat an ISA mode mismatch
	in branch relocation calculation as an error if
	`ignore_branch_isa' has been set.
	(_bfd_mips_elf_insn32): Rename to...
	(_bfd_mips_elf_linker_flags): ... this.  Rename the `on'
	parameter to `insn32' and add an `ignore_branch_isa' parameter.
	Handle the new parameter.

	gas/
	* config/tc-mips.c (mips_ignore_branch_isa): New variable.
	(options): Add OPTION_IGNORE_BRANCH_ISA and
	OPTION_NO_IGNORE_BRANCH_ISA enum values.
	(md_longopts): Add "mignore-branch-isa" and
	"mno-ignore-branch-isa" options.
	(md_parse_option): Handle OPTION_IGNORE_BRANCH_ISA and
	OPTION_NO_IGNORE_BRANCH_ISA.
	(fix_bad_cross_mode_branch_p): Return FALSE if
	`mips_ignore_branch_isa' has been set.
	(md_show_usage): Add `-mignore-branch-isa' and
	`-mno-ignore-branch-isa'.

	* doc/as.texinfo (Target MIPS options): Add
	`-mignore-branch-isa' and `-mno-ignore-branch-isa' options.
	(-mignore-branch-isa, -mno-ignore-branch-isa): New options.
	* doc/c-mips.texi (MIPS Options): Add `-mignore-branch-isa' and
	`-mno-ignore-branch-isa' options.

	* testsuite/gas/mips/branch-local-ignore-2.d: New test.
	* testsuite/gas/mips/branch-local-ignore-3.d: New test.
	* testsuite/gas/mips/branch-local-ignore-n32-2.d: New test.
	* testsuite/gas/mips/branch-local-ignore-n32-3.d: New test.
	* testsuite/gas/mips/branch-local-ignore-n64-2.d: New test.
	* testsuite/gas/mips/branch-local-ignore-n64-3.d: New test.
	* testsuite/gas/mips/mips.exp: Run the new tests.

	ld/
	* emultempl/mipself.em (ignore_branch_isa): New variable.
	(mips_create_output_section_statements): Rename
	`_bfd_mips_elf_insn32' called to `_bfd_mips_elf_linker_flags',
	add `ignore_branch_isa' argument.
	(PARSE_AND_LIST_PROLOGUE): Add OPTION_IGNORE_BRANCH_ISA and
	OPTION_NO_IGNORE_BRANCH_ISA enum values.
	(PARSE_AND_LIST_LONGOPTS): Add "ignore-branch-isa" and
	"no-ignore-branch-isa" options.
	(PARSE_AND_LIST_OPTIONS): Add `--ignore-branch-isa' and
	`--no-ignore-branch-isa'.
	(PARSE_AND_LIST_ARGS_CASES): Handle OPTION_IGNORE_BRANCH_ISA and
	OPTION_NO_IGNORE_BRANCH_ISA.

	* ld.texinfo (Options specific to MIPS targets): Add
	`--ignore-branch-isa' and `--no-ignore-branch-isa' options.
	(ld and the MIPS family): Likewise.

	* testsuite/ld-mips-elf/bal-jalx-pic-ignore.d: New test.
	* testsuite/ld-mips-elf/bal-jalx-pic-ignore-n32.d: New test.
	* testsuite/ld-mips-elf/bal-jalx-pic-ignore-n64.d: New test.
	* testsuite/ld-mips-elf/unaligned-branch-ignore-2.d: New test.
	* testsuite/ld-mips-elf/unaligned-branch-ignore-r6-1: New test.
	* testsuite/ld-mips-elf/unaligned-branch-ignore-mips16: New
	test.
	* testsuite/ld-mips-elf/unaligned-branch-ignore-micromips: New
	test.
	* testsuite/ld-mips-elf/mips-elf.exp: Run the new tests.

2 files changed, 43 insertions, 0 deletions

diff --git a/gas/doc/as.texinfo b/gas/doc/as.texinfo
index 5e30fc4..007332e 100644
--- a/gas/doc/as.texinfo
+++ b/gas/doc/as.texinfo
@@ -416,6 +416,7 @@ gcc(1), ld(1), and the Info entries for @file{binutils} and @file{ld}.
    [@b{-mips32r3}] [@b{-mips32r5}] [@b{-mips32r6}] [@b{-mips64}] [@b{-mips64r2}]
    [@b{-mips64r3}] [@b{-mips64r5}] [@b{-mips64r6}]
    [@b{-construct-floats}] [@b{-no-construct-floats}]
+   [@b{-mignore-branch-isa}] [@b{-mno-ignore-branch-isa}]
    [@b{-mnan=@var{encoding}}]
    [@b{-trap}] [@b{-no-break}] [@b{-break}] [@b{-no-trap}]
    [@b{-mips16}] [@b{-no-mips16}]
@@ -1546,6 +1547,17 @@ The @samp{--relax-branch} option enables the relaxation of out-of-range
 branches.  By default @samp{--no-relax-branch} is selected, causing any
 out-of-range branches to produce an error.
 
+@item -mignore-branch-isa
+@itemx -mno-ignore-branch-isa
+Ignore branch checks for invalid transitions between ISA modes.  The
+semantics of branches does not provide for an ISA mode switch, so in
+most cases the ISA mode a branch has been encoded for has to be the
+same as the ISA mode of the branch's target label.  Therefore GAS has
+checks implemented that verify in branch assembly that the two ISA
+modes match.  @samp{-mignore-branch-isa} disables these checks.  By
+default @samp{-mno-ignore-branch-isa} is selected, causing any invalid
+branch requiring a transition between ISA modes to produce an error.
+
 @item -mnan=@var{encoding}
 Select between the IEEE 754-2008 (@option{-mnan=2008}) or the legacy
 (@option{-mnan=legacy}) NaN encoding format.  The latter is the default.
diff --git a/gas/doc/c-mips.texi b/gas/doc/c-mips.texi
index 04666f4..d9a09f1 100644
--- a/gas/doc/c-mips.texi
+++ b/gas/doc/c-mips.texi
@@ -472,6 +472,37 @@ Also no MIPS16 branches are ever relaxed.
 By default @samp{--no-relax-branch} is selected, causing any out-of-range
 branches to produce an error.
 
+@item -mignore-branch-isa
+@itemx -mno-ignore-branch-isa
+Ignore branch checks for invalid transitions between ISA modes.
+
+The semantics of branches does not provide for an ISA mode switch, so in
+most cases the ISA mode a branch has been encoded for has to be the same
+as the ISA mode of the branch's target label.  If the ISA modes do not
+match, then such a branch, if taken, will cause the ISA mode to remain
+unchanged and instructions that follow will be executed in the wrong ISA
+mode causing the program to misbehave or crash.
+
+In the case of the @code{BAL} instruction it may be possible to relax
+it to an equivalent @code{JALX} instruction so that the ISA mode is
+switched at the run time as required.  For other branches no relaxation
+is possible and therefore GAS has checks implemented that verify in
+branch assembly that the two ISA modes match, and report an error
+otherwise so that the problem with code can be diagnosed at the assembly
+time rather than at the run time.
+
+However some assembly code, including generated code produced by some
+versions of GCC, may incorrectly include branches to data labels, which
+appear to require a mode switch but are either dead or immediately
+followed by valid instructions encoded for the same ISA the branch has
+been encoded for.  While not strictly correct at the source level such
+code will execute as intended, so to help with these cases
+@samp{-mignore-branch-isa} is supported which disables ISA mode checks
+for branches.
+
+By default @samp{-mno-ignore-branch-isa} is selected, causing any invalid
+branch requiring a transition between ISA modes to produce an error.
+
 @cindex @option{-mnan=} command line option, MIPS
 @item -mnan=@var{encoding}
 This option indicates whether the source code uses the IEEE 2008