Convert microMIPS branches and jumps whose delay slot would be filled by
a generated NOP instruction to the corresponding compact form where one
exists, in a manner similar to MIPS16 JR->JRC and JALR->JALRC swap.
Do so even where the transformation switches from a 16-bit to a 32-bit
branch encoding for no benefit in code size reduction, as this is still
advantageous. This is because a branch/NOP pair takes 2 pipeline slots
or a 2-cycle completion latency except in superscalar implementations.
Whereas a compact branch may or may not stall on its target fetch, so it
will at most have a 2-cycle completion latency and may have only 1 even
in scalar implementations, and in superscalar implementations it is
expected to have no worse latency as a branch/NOP pair has. Also it
won't stall and therefore take the extra latency cycle in the not-taken
case.
Technically this is the same as MIPS16 compaction: for the qualifying
instruction encodings the APPEND_ADD_COMPACT machine code generation
method is selected where APPEND_ADD_WITH_NOP otherwise would and tells
the code generator in `append_insn' to convert the regular form of an
instruction to its corresponding compact form. For this the opcode is
tweaked as necessary and the microMIPS opcode table is scanned for the
matching updated instruction. A non-$0 `rt' operand to BEQ and BNE
instructions is moved to the `rs' operand field of BEQZC and BNEZC
encodings as required.
Unlike with MIPS16 compaction however we need to handle out-of-distance
branch relaxation as well. We do this by deferring the generation of
any delay-slot NOP required to relaxation made in `md_convert_frag', by
converting the APPEND_ADD_WITH_NOP machine code generation to APPEND_ADD
where a relaxed instruction is recorded. Relaxation then, depending on
actual code produced, chooses between either using a compact branch or
jump encoding and emitting the NOP outstanding if no compact encoding is
possible.
For code simplicity's sake the relaxation pass is retained even if the
principle of preferring a compact encoding to a 16-bit branch/NOP pair
means, in the absence of out-of-range branch relaxation, that a single
compact branch machine code instruction will eventually be produced from
a given assembly source instruction.
gas/
* config/tc-mips.c (RELAX_MICROMIPS_ENCODE): Add `nods' flag.
(RELAX_MICROMIPS_RELAX32, RELAX_MICROMIPS_TOOFAR16)
(RELAX_MICROMIPS_MARK_TOOFAR16, RELAX_MICROMIPS_CLEAR_TOOFAR16)
(RELAX_MICROMIPS_TOOFAR32, RELAX_MICROMIPS_MARK_TOOFAR32)
(RELAX_MICROMIPS_CLEAR_TOOFAR32): Shift bits.
(get_append_method): Also return APPEND_ADD_COMPACT for
microMIPS instructions.
(find_altered_mips16_opcode): Exclude macros from matching.
Factor code out...
(find_altered_opcode): ... to this new function.
(find_altered_micromips_opcode): New function.
(frag_branch_delay_slot_size): Likewise.
(append_insn): Handle microMIPS branch/jump compaction.
(macro_start): Likewise.
(relaxed_micromips_32bit_branch_length): Likewise.
(md_convert_frag): Likewise.
* testsuite/gas/mips/micromips.s: Add conditional explicit NOPs
for delay slot filling.
* testsuite/gas/mips/micromips-b16.s: Add explicit NOPs for
delay slot filling.
* testsuite/gas/mips/micromips-size-1.s: Likewise.
* testsuite/gas/mips/micromips.l: Adjust line numbers.
* testsuite/gas/mips/micromips-warn.l: Likewise.
* testsuite/gas/mips/micromips-size-1.l: Likewise.
* testsuite/gas/mips/micromips.d: Adjust padding.
* testsuite/gas/mips/micromips-trap.d: Likewise.
* testsuite/gas/mips/micromips-insn32.d: Likewise.
* testsuite/gas/mips/micromips-noinsn32.d: Likewise.
* testsuite/gas/mips/micromips@beq.d: Update patterns for
branch/jump compaction.
* testsuite/gas/mips/micromips@bge.d: Likewise.
* testsuite/gas/mips/micromips@bgeu.d: Likewise.
* testsuite/gas/mips/micromips@blt.d: Likewise.
* testsuite/gas/mips/micromips@bltu.d: Likewise.
* testsuite/gas/mips/micromips@branch-misc-4.d: Likewise.
* testsuite/gas/mips/micromips@branch-misc-4-64.d: Likewise.
* testsuite/gas/mips/micromips@branch-misc-5.d: Likewise.
* testsuite/gas/mips/micromips@branch-misc-5pic.d: Likewise.
* testsuite/gas/mips/micromips@branch-misc-5-64.d: Likewise.
* testsuite/gas/mips/micromips@branch-misc-5pic-64.d: Likewise.
* testsuite/gas/mips/micromips@jal-svr4pic-local.d: Likewise.
* testsuite/gas/mips/micromips@jal-svr4pic-local-n32.d:
Likewise.
* testsuite/gas/mips/micromips@jal-svr4pic-local-n64.d:
Likewise.
* testsuite/gas/mips/micromips@loc-swap.d: Likewise.
* testsuite/gas/mips/micromips@loc-swap-dis.d: Likewise.
* testsuite/gas/mips/micromips@relax.d: Likewise.
* testsuite/gas/mips/micromips@relax-at.d: Likewise.
* testsuite/gas/mips/micromips@relax-swap3.d: Likewise.
* testsuite/gas/mips/branch-extern-2.d: Likewise.
* testsuite/gas/mips/branch-extern-4.d: Likewise.
* testsuite/gas/mips/branch-section-2.d: Likewise.
* testsuite/gas/mips/branch-section-4.d: Likewise.
* testsuite/gas/mips/branch-weak-2.d: Likewise.
* testsuite/gas/mips/branch-weak-5.d: Likewise.
* testsuite/gas/mips/micromips-branch-absolute.d: Likewise.
* testsuite/gas/mips/micromips-branch-absolute-n32.d: Likewise.
* testsuite/gas/mips/micromips-branch-absolute-n64.d: Likewise.
* testsuite/gas/mips/micromips-branch-absolute-addend.d:
Likewise.
* testsuite/gas/mips/micromips-branch-absolute-addend-n32.d:
Likewise.
* testsuite/gas/mips/micromips-branch-absolute-addend-n64.d:
Likewise.
* testsuite/gas/mips/micromips-compact.d: New test.
* testsuite/gas/mips/mips.exp: Run the new test.
ld/
* testsuite/ld-mips-elf/micromips-branch-absolute.d: Update
patterns for branch compaction.
* testsuite/ld-mips-elf/micromips-branch-absolute-addend.d:
Likewise.
opcodes/
* micromips-opc.c (micromips_opcodes): Reorder "bc" next to "b",
"beqzc" next to "beq", "bnezc" next to "bne" and "jrc" next to
"j".
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README for LD
This is the GNU linker. It is distributed with other "binary
utilities" which should be in ../binutils. See ../binutils/README for
more general notes, including where to send bug reports.
There are many features of the linker:
* The linker uses a Binary File Descriptor library (../bfd)
that it uses to read and write object files. This helps
insulate the linker itself from the format of object files.
* The linker supports a number of different object file
formats. It can even handle multiple formats at once:
Read two input formats and write a third.
* The linker can be configured for cross-linking.
* The linker supports a control language.
* There is a user manual (ld.texinfo), as well as the
beginnings of an internals manual (ldint.texinfo).
Installation
============
See ../binutils/README.
If you want to make a cross-linker, you may want to specify
a different search path of -lfoo libraries than the default.
You can do this by setting the LIB_PATH variable in ./Makefile
or using the --with-lib-path configure switch.
To build just the linker, make the target all-ld from the top level
directory (one directory above this one).
Porting to a new target
=======================
See the ldint.texinfo manual.
Reporting bugs etc
===========================
See ../binutils/README.
Known problems
==============
The Solaris linker normally exports all dynamic symbols from an
executable. The GNU linker does not do this by default. This is
because the GNU linker tries to present the same interface for all
similar targets (in this case, all native ELF targets). This does not
matter for normal programs, but it can make a difference for programs
which try to dlopen an executable, such as PERL or Tcl. You can make
the GNU linker export all dynamic symbols with the -E or
--export-dynamic command line option.
HP/UX 9.01 has a shell bug that causes the linker scripts to be
generated incorrectly. The symptom of this appears to be "fatal error
- scanner input buffer overflow" error messages. There are various
workarounds to this:
* Build and install bash, and build with "make SHELL=bash".
* Update to a version of HP/UX with a working shell (e.g., 9.05).
* Replace "(. ${srcdir}/scripttempl/${SCRIPT_NAME}.sc)" in
genscripts.sh with "sh ${srcdir}..." (no parens) and make sure the
emulparams script used exports any shell variables it sets.
Copyright (C) 2012-2016 Free Software Foundation, Inc.
Copying and distribution of this file, with or without modification,
are permitted in any medium without royalty provided the copyright
notice and this notice are preserved.